Jukuri, open repository of the Natural Resources Institute Finland (Luke) 
   
 
   
All material supplied via Jukuri is protected by copyright and other intellectual property rights. Duplication 
or sale, in electronic or print form, of any part of the repository collections is prohibited. Making electronic 
or print copies of the material is permitted only for your own personal use or for educational purposes.  For 
other purposes, this article may be used in accordance with the publisher’s terms. There may be 
differences between this version and the publisher’s version. You are advised to cite the publisher’s 
version. 
 
This is an electronic reprint of the original article.  
This reprint may differ from the original in pagination and typographic detail. 
 
Author(s): Jan Heggenes, Morten Stickler, Knut Alfredsen, John E. Brittain, Ana Adeva-Bustos 
and Ari Huusko 
Title: Hydropower-driven thermal changes, biological responses and mitigating measures 
in northern river systems 
Year: 2021 
Version: Published version 
Copyright:   The Author(s) 2021 
Rights: CC BY-NC-ND 4.0 
Rights url: http://creativecommons.org/licenses/by-nc-nd/4.0/ 
 
Please cite the original version: 
Heggenes, J, Stickler, M, Alfredsen, K, Brittain, JE, Adeva-Bustos, A, Huusko, A. Hydropower-driven 
thermal changes, biological responses and mitigating measures in northern river systems. River 
Res Applic. 2021; 37: 743– 765. https://doi.org/10.1002/rra.3788 
R E V I EW PA P E R
Hydropower-driven thermal changes, biological responses and
mitigating measures in northern river systems
Jan Heggenes1,2 | Morten Stickler1,3 | Knut Alfredsen4 | John E. Brittain2,3 |
Ana Adeva-Bustos4,5 | Ari Huusko6
1Department of Natural Sciences and
Environmental Health, University of South-
Eastern Norway, Bø, Norway
2Natural History Museum, University of Oslo,
Oslo, Norway
3Norwegian Water Resources & Energy
Directorate, Oslo, Norway
4Department of Hydraulic and Environmental
Engineering, Norwegian University of Science
and Technology, Trondheim, Norway
5SINTEF Energy Research, Trondheim, Norway
6Natural Resources Institute Finland, Kainuu
Fisheries Research Station, Paltamo, Finland
Correspondence
Jan Heggenes, Department of Natural
Sciences and Environmental Health, University
of South-Eastern Norway, Campus Bø,
Gullbringvegen 36, 3800 Bø I Telemark
Norway.
Email: jan.heggenes@usn.no
Funding information
Statkraft; University of South Eastern Norway
Abstract
Water temperatures control life histories and diversity of aquatic species. Hydropower
regulation, particularly in high head systems, alters natural water temperature regimes,
which may have profound and long-term impacts on aquatic environments. Tempera-
tures in by-pass sections and reaches affected by residual/environmental minimum flows
fluctuate more than in natural flow regimes, driven more by influence of air tempera-
tures. Reaches downstream of power plant outlets tend to become warmer in winter and
colder in summer, driven by stratification behind the reservoir dam. In hydro-peaked sys-
tems high-low temperature effects may thus be aggravated. We review alterations of
hydropower to natural thermal regimes, impacts on key organisms in terms of survival,
development and behavioral thresholds, and potential mitigation measures, with focus on
Atlantic salmon and brown trout in high northern latitude stream systems. Previous syn-
theses have focused mainly on flow changes and ecological impacts. Temperature effects
may not always be correlated with flow changes, although there are some unique chal-
lenges with temperature changes in far northern latitudes, for example, related to the
seasonal and colder climates. To help knowledge-based management and identify poten-
tial knowledge gaps, we review how hydropower regulation may impact seasonal water
temperatures, what impacts changes to stream system temperature regimes may have to
key organisms, for example, Atlantic salmon and brown trout, and what adaptations and
behavioral variations they may exhibit to respond to changed temperature regimes, and
finally what good practices can be recommended for mitigating temperature impacts.
This synthesis indicates that there are impacts to the fish and their supporting food webs,
in particular related to growth and development, and the potential for negative impacts
seems higher, and better studied, than positive impacts in northern river systems. Some
of these impacts may be modified by directed hydropower regulation practices, but here
effect studies and knowledge are limited.
K E YWORD S
biological impacts, hydropower regulation, mitigation, rivers, temperature
Received: 9 December 2019 Revised: 12 January 2021 Accepted: 15 February 2021
DOI: 10.1002/rra.3788
This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any
medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.
© 2021 The Authors. River Research and Applications published by John Wiley & Sons Ltd.
River Res Applic. 2021;37:743–765. wileyonlinelibrary.com/journal/rra 743
1 | INTRODUCTION
Water temperature is a controlling factor in aquatic species' life cycle,
growth, and diversity in lotic environments (e.g., Caissie, 2006; Gilles-
pie, Desmet, Kay, Tillotson, & Brown, 2015; Hynes, 1970; Ward &
Stanford, 1982; Webb & Walling, 1993). In northern cold climates, the
balance between length and intensity of low and high temperature
seasons (Peel, Finlayson, & McMahon, 2007) is pervasively important
in aquatic lotic systems. This balance will be affected by the 21st cen-
tury predicted climate change (e.g., Graham & Harrod, 2009; Heino,
Erkinaro, Huusko, & Luoto, 2016; Jonsson & Jonsson, 2009). How-
ever, many of these small-intermediate stream systems are already
affected by hydropower regulation and associated water temperature
changes.
In such northern temperate streams extensively regulated for
hydropower, environmental impacts are likely to be complex and site-
specific (Gillespie et al., 2015; Poff & Zimmerman, 2010) and depend
on spatial scale (“large” vs. “small” river systems, typically indicated
by, for example, higher channel slope, smaller drainage area, higher
relative roughness and shear stress, and more cascade, step-pool,
plane bed habitat in ‘small’ river systems [Montgomery &
Buffington, 1997]). A decrease in downstream inter-annual and intra-
annual variability is typical for the hydropower regulated flow regime,
notably for water flow and temperature (Geris, Tetzlaff, Seibert, Vis, &
Soulsby, 2015; Renofalt, Jansson, & Nilsson, 2010; Ward &
Stanford, 1979). Hydropower systems can typically be divided into
high head systems comprising reservoirs at high elevation and long
transfer tunnels to the power plant (Figure 1 top) and low head sys-
tems or run-of-the-river plants (Figure 1 bottom). With the limited
storage capacity in river reservoirs, low head hydropower regulation
systems tend to have limited effects on stream water temperatures
(Figure 1) (Asvall, 2008; Dickson, Carrivick, & Brown, 2012; Olden &
Naiman, 2010). In contrast, with the large storage capacity in reser-
voirs, a high head system, common in northern river systems, may
lead to major changes in water temperatures, in particular when there
is a direct path from a high-altitude reservoir to a lowland river. Partic-
ularly in systems with large reservoirs, thermal regimes may be altered
toward cooling in summer and warming in winter, including changes
in ice formation, effected by the typical reservoir bottom intakes
which tend to draw from the hypolimnion, where water temperatures
are often stratified into a winter ‘warm’ layer and a summer cold layer
(Figure 5). The impacts on aquatic life may be profound and long-term,
potentially altering life histories and biological diversity.
Water temperature is, therefore, an important water quality indi-
cator and parameter for potential mitigation measures aimed at
restoring and managing water bodies. Recent reviews of ecological
responses in regulated river systems have, however, focused more on
the obvious direct impacts from flow changes (Gillespie et al., 2015;
Olden et al., 2014; Poff & Zimmerman, 2010);(but see Austin, Bradley,
Steward-Rousson, & Milner, 2015). Water temperature effects have
attracted less attention, and it remains difficult to isolate the contribu-
tions of water temperature and water flow in biotic responses
(Olden & Naiman, 2010), particularly in empirical studies (details in
Heggenes, Alfredsen, Brittain, et al., 2017). We focus on temperature
in this review, and with emphasis on the more numerous small-
intermediate scale hydropower developments, because less synthesis
of the impacts of water temperature changes on such systems has
been provided.
F IGURE 1 The two typical hydropower regulation systems. Top: a high head system with a high elevation reservoir, tributary intakes, and
transfer tunnels to the power plant, which could also include a pumped storage facility. Bottom: a low head or run-of-the-river mainstem system
with intake and power station in the river dam. Output from generation station equals input from the river upstream. (After Heggenes
et al., 2017; Heggenes, Alfredsen, Bustos, Huusko, & Stickler, 2017; Toffolon, Siviglia, & Zolezzi, 2010) [Color figure can be viewed at
wileyonlinelibrary.com]
744 HEGGENES ET AL.
In ectothermic freshwater animals, seasonal water temperatures
may be the most pervasive environmental factor determining their
lives (Angilletta, Steury, & Sears, 2004; Hynes, 1970). The rates of bio-
chemical reactions, ontological development, and behavioral
responses depend on water temperature (e.g., Angilletta,
Niewiarowski, & Navas, 2002; Dell, Pawar, & Savage, 2011; Elliott &
Elliott, 2010) and sublethal effects such as life history traits, egg
development, and hatching cues. Temperature extremes, such as
extraordinarily high summer or low winter temperatures, may gener-
ate short-term thermal stress and challenge physiological tolerances
(Beitinger, Bennett, & McCauley, 2000; Elliott & Elliott, 2010;
Gunderson & Stillman, 2015; Sunday et al., 2019). Thermal stress may
be mitigated by basal thermo-tolerance or adaptive plastic responses
involving acclimation and/or local behavioral adjustments
(Gunderson & Stillman, 2015; Hutchison & Maness, 1979;
Pörtner, 2002). In seasonal climates, long-term fitness may reflect the
ability of ectothermic organisms to exploit the favorable summer sea-
son for temperature-dependent growth and recruitment. Develop-
mental, physiological, and behavioral strategies may also mitigate the
effects of the unfavorable low temperature season (Bradshaw, Zani, &
Holzapfel, 2004; Hedger et al., 2013), which may include freezing
(≤0
C) and ice formation (Prowse, 2001; Turcotte & Morse, 2013).
In this review, we question how northern latitude hydropower
regulation impact seasonal water temperatures in reaches down-
stream hydropower plants, including ice phenomena, and using Nordic
river systems as a focus. How do impacted thermal regimes impact
the ecology of selected key organisms, considering their survival
thresholds, adaptations, and behavioral variations? And what potential
mitigation measures could be applied to mitigate such impacts, and
how effective are they? The widely distributed and both ecologically
and culturally important fish species, Atlantic salmon (Salmo salar L.)
and brown trout (Salmo trutta L.) are used as model fish species, but
we also draw on results from other salmonid fishes, while benthic
macroinvertebrates examples represent the lower trophic level
supporting much of theses species' production in lotic environments.
2 | HOW CAN HYDROPOWER
REGULATION ALTER NATURAL THERMAL
REGIMES?
Life in northern rivers has adapted to natural thermal regimes for
millennia, before being disrupted by substantial hydropower develop-
ment and associated thermal alterations only during the last hundred
years or so.
2.1 | Water temperature processes in natural
northern river systems
In northern environments, rivers exhibit seasonal variations in ambient
temperatures. Water temperatures are low in winter, typically close to
zero degrees with ice formation. This may determine survival. During
summer high temperatures determine growth and biological produc-
tion. Within seasons, the thermal regime is largely controlled by tem-
peratures in amount of water contributed from upstream lakes and
reservoirs, tributaries, and groundwater (Figure 2). In addition, a
watershed's heat radiation regime and subsoil environment, broadly
determined by atmospheric conditions, topography, streambed, and
stream discharge (Caissie, 2006), also affect water temperatures
(Figure 2). Rivers per se typically heat or cool primarily by radiation to
or from the surface (e.g., Caissie, 2006; Rishel, Lynch, &
Corbett, 1982), but this may be modified by the surrounding environ-
ment, including the hyporheic zone (Caissie, 2006; Poole &
Berman, 2001). Microgradients in stream temperature (cm to meters;
Webb, Hannah, Moore, Brown, & Nobilis, 2008) tend to be limited,
and if found, are related to strong solar heating and low flows, inflows
from sources with different thermal characteristics, notably meltwa-
ter, groundwater, tributaries, lake outflows, and associated vegetation
(e.g., Dugdale, Malcolm, Kantola, & Hannah, 2018; Nuhfer, Zorn, &
Wills, 2017). Thus, thermal complexity in northern rivers may be con-
siderable and may vary between day and night, particularly in small,
alpine streams largely affected by short wave radiation and groundwa-
ter fluxes (Figure 2) (Brown, Hannah, & Milner, 2006; Cadbury,
Hannah, Milner, Pearson, & Brown, 2008; Ward, 1994). On a temporal
scale, diel, annual and interannual temperature variation tends to be
more pronounced in smaller compared to larger streams (Figure 1). On
a larger spatial scale, northern stream temperatures tend to increase
with the lower altitudes downstream, but may be modified by for
example groundwater and tributary inflows (Figure 2). In winter, sub-
zero air temperatures may induce ice formation when water tempera-
tures fall below zero, that is, supercooled water. Supercooled water
occurs both on short temporal (minutes, hours) and spatial scales
(Twinter: -0.06– -0.001
C) (Stickler & Alfredsen, 2009) and is therefore
sensitive to even small changes in temperature.
F IGURE 2 Schematic representation of mean daily and diel water
temperature variability as a function of spatial scale, that is,
downstream direction, which also generally translates to stream order.
(Modified after Caissie, 2006; Austin et al., 2015)
HEGGENES ET AL. 745
2.2 | Hydropower regulation impacts on water
temperatures in river systems
The extent of hydropower generated changes on the water tempera-
ture regime depends primarily on the hydropower regulation system,
reservoir capacity, inflow and outflow valve position and capacity, and
operational strategy, that is, origin, timing, and amount of water
released for hydropower production (Archer, 2008; Hamblin &
McAdam, 2003; Webb & Walling, 1997). Typically in high-head regu-
lation systems, facilities draw reservoir water from the hypolimnion
(“bottom layer”) and transfer it directly to the power plant
(Figures 1 & 5), bypassing downstream river reaches and leaving resid-
ual or environmental flows. Because water density is the highest
around 4
C, temperature stratified reservoirs flip from warmest water
in the hypolimnion (bottom layer) in the winter to coldest water on
the bottom in summer. Therefore, in summer, the water downstream
of hydropower outlet is cooled by release of hypolimnic water from
the reservoir, but warmed in winter (Figures 3 & 4). In broad terms,
this leads to reduced diurnal temperature fluctuations and changes in
seasonal temperature regimes, notably winter warming and summer
cooling (Figure 3 & 4) (Austin et al., 2015; Poff & Zimmerman, 2010;
Poole & Berman, 2001), with associated important ecological effects,
noting likely site-specificity and complexity. Unfortunately, the num-
ber of studies that quantify the water temperature (and chemistry)
effects of dam-induced change are limited (Archer, 2008; Austin
et al., 2015; Gillespie et al., 2015; Haxton & Findlay, 2008).
For northern streams, impacts of hydropower regulation on water
temperature are neither well published nor quantified (Ellis &
Jones, 2013) (but see e.g., Tvede, 1994; Kvambekk, 2004). In winter,
the naturally near 0
C water temperatures and stable mid-winter
periods with surface ice cover may be replaced by warmer water and
repeated unstable transition periods in regulated systems (Figures 3 &
4) (Gebre, Alfredsen, Lia, Stickler, & Tesaker, 2013; Prowse
et al., 2011; Stickler & Alfredsen, 2009; Weber, Nilsson, Lind,
Alfredsen, & Polvi, 2013). Water drawn from the reservoir hypolim-
nion will in most cases raise downstream winter water temperatures
above the natural level, from around 0
 to maybe 2
C or more
(Halleraker, Sundt, Alfredsen, & Dangelmaier, 2007; Ugedal
et al., 2008) (Figure 3a & 4). Additional heat flux from transfer systems
due to frictional heat from moving water in tunnels, may occur, but
are expected to be limited, and local flow pressure changes may be
more relevant (Ettema, Kirkil, & Daly, 2009). Importantly, release of
warm water during winter may also alter downstream ice dynamics,
even with relatively small water temperature changes (Asvall, 2008).
The natural, typically stable mid-winter surface ice period may after
regulation be replaced by prolonged unstable periods with dynamic
ice formation and the absence of surface ice cover, exposing the river
to frazil ice (Timalsina, Alfredsen, & Killingtveit, 2015; Timalsina,
Charmasson, & Alfredsen, 2013; Ugedal et al., 2008). On the other
hand, warm water releases may reduce the possibility of large ice runs
(Gebre et al., 2013) due to periodic and controlled ice runs throughout
the winter. Bypass river sections with reduced/required flow will
experience higher water temperature variability during fall and spring
and potentially more freezing in winter, but this may be modified par-
ticularly by groundwater flows (Asvall, 1977; Constantz, 1998;
Osterkamp & Gosink, 1983). Unfortunately, most previous impact
studies are rather qualitative, with little quantitative data on natural
versus regulated temperature differences in winter, and effects of
alternative production regimes (Austin et al., 2015; Olden &
Naiman, 2010) .
Drawing water from the reservoir hypolimnion and releasing it
downstream of the hydropower plant may result in complex thermal
and seasonal longitudinal effects (Tvede, 1994; Webb et al., 2008;
Webb & Walling, 1997), largely depending on the balance between
F IGURE 3 (a) Mean monthly water temperatures for Alta River,
North-Norway, were raised 1–2
C primarily in fall–winter from before
to after regulation. The effect was reduced somewhat after
restoration/mitigation, primarily to obtain more surface ice cover in
winter. (b) Mean monthly water temperatures for Lærdalselva River,
West-Norway, were raised 1–2
C primarily in spring–summer from
before to after regulation. (Data source: Norwegian Water and Energy
Directorate)
746 HEGGENES ET AL.
residual and regulated flow. Although water temperature changes
have been reported for tens of kilometers downstream or more in
larger systems (Austin et al., 2015; Webb & Walling, 1993, 1988), for
example, 10–60 km downstream a hydropower outlet (Archer, 2008;
Ellis & Jones, 2013; Halleraker et al., 2007; Webb & Walling, 1988),
more in situ studies are needed to determine the extent of impacts
Although models suggest thermal alterations due to hydropeaking/
thermopeaking (Toffolon et al., 2010; Vanzo, Siviglia, Carolli, &
Zolezzi, 2016), there are few studies that empirically illustrate such
alterations (Bakken, King, & Alfredsen, 2016; Bruno, Siviglia, Carolli, &
Maiolini, 2013; Casas-Mulet, Alfredsen, Brabrand, & Saltveit, 2015;
Casas-Mulet, Alfredsen, Hamududu, & Timalsina, 2015). Empirical vali-
dations will be important in the future as short-term operational
regimes are to be expected (e.g., Catrinu-Renström & Knudsen, 2011).
3 | IMPACTS OF ALTERED THERMAL
REGIMES ON THE ECOLOGY OF KEY
ORGANISMS
Low temperatures in northern climates are a controlling factor for
production and natural selection for fish and macroinvertebrates
(e.g., Elliott & Elliott, 2010; Huryn & Wallace, 2000; Ward, 1994).
Responses to seasonal temperature variation constitute part of natu-
ral thermal strategies (e.g., Crozier & Hutchings, 2014; Danks, 2008;
Olsson, 1981). The thermal regime may shape important ecological
events at different trophic levels (e.g., Barneche & Allen, 2018; Wood-
ward, Perkins, & Brown, 2010). Thus, the life histories of northern fish
species like brown trout and Atlantic salmon are largely controlled by
water temperature in terms of the basic life history characteristics of
feeding and growth and survival, and certain life stages may be
particularly sensitive to the thermal regime, for example, egg and ale-
vin development, and the associated timing of spawning migration
and spawning itself (e.g., Solomon & Lightfoot, 2008) (Table 1). At a
lower trophic level, benthic macroinvertebrate responses to tempera-
ture are variable and species specific, and with more pronounced life
history adaptations compared to stream-living fish species (Bale &
Hayward, 2010; Brittain, 1991; Dallas & Ross-Gillespie, 2015;
Lencioni, 2004).
3.1 | Water temperatures and fish: Feeding,
growth, and survival of Atlantic salmon and brown
trout
Water temperature and food availability are key factors influencing
growth and life history in salmon and trout in lotic environments
(Table 1) (e.g., Budy et al., 2013; Forseth, Letcher, &
Johansen, 2010; Logez & Pont, 2011). The summer period is the
principal feeding and growing season for river fish (Elliott, 2009;
Elliott & Elliott, 2010; Forseth, Hurley, Jensen, & Elliott, 2001). Win-
ter necessitates energy saving and survival strategies, for example,
quiescence, sheltering, nocturnal behavior, and starvation (Crozier &
Hutchings, 2014; Heggenes, Alfredsen, Bustos, et al., 2017; Shuter,
Finstad, Helland, Zweimuller, & Holker, 2012). Thus, in regulated
systems, the modified temperature regime will directly affect fish
growth and fitness (Tables 1–3). The build-up of larger fat reserves
in summer and lower depletion rates in winter may, to some extent,
compensate for longer winters in natural northern systems
(e.g., Shuter et al., 2012). Indeed, juvenile northern Atlantic salmon
populations show lower lipid-depletion rates during winter than
southern populations, and storage lipid levels cluster close to critical
F IGURE 4 Typical winter (bottom dashed lines) and summer (top solid lines) water temperature changes effected by high head hydropower
regulation (January and July 2009, Vallaråi R., South-East Norway). Winter: Upstream residual flow water with temperature about 0
C (lower
dashed dark grey line) is mixed with hydropower reservoir water with temperature about 3.2
C from the station outlet (top dashed grey line),
resulting in a downstream temperature of about 2.5
C (middle dotted black line). Summer: Upstream residual flow water with temperatures
around 20
C (top solid dark grey line) is mixed with hydropower reservoir water with temperature about 13
C from the station outlet (lower solid
grey line), resulting in a downstream reduction in temperature by about 5
C (middle solid black line)
HEGGENES ET AL. 747
TABLE 1 An overview of thermal biology in the life histories of wild brown trout and Atlantic salmon
Life stage Temeperature effect Brown trout Atlantic salmon Note Selected references
Egg incubation
period
Determined by
degree-days (DD).
At 5
C ≈ 100 days.
At 2
C ≈ 160 days.
At 5
C ≈ 100 days.
At 2
C ≈ 160 days.
Varies somewhat
with ambient
temperature.
Days to median
hatch = 746/
(Temp-
0.5323)1.2233
Embody (1934),
Jungwirth and
Winkler (1984),
Kane (1988).
Formulae from
Jungwirth and
Winkler (1984)
Egg survival Depends on
temperature and
oxygen level.
0–13
C
Optimum at 8
C?
0–16
C
Optimum at 6–8
C?
May increase if raised
above 0
C.
decrease markedly
at higher
temperatures.
Suboptimal
temperatures
reduce alevin size.
Low interpopulation
variability.
Some genetical
adaptation?
Gunnes (1979), Reiser
and Wesche (1979),
Jungwirth and
Winkler (1984),
Stonecypher, Hubert,
and Gern (1994);
Ojanguren, Reyes-
Gavilan, and
Munoz (1999);
Ojanguren and
Brana (2003),
Ornsrud, Gil, and
Waagbo (2004),
Ornsrud, Wargelius,
Saele, Pittman, and
Waagbo (2004),
Takle, Baeverfjord,
and Andersen (2004),
Takle, Baeverfjord,
Helland, Kjorsvik, and
Andersen (2006),
Syrjanen, Kiljunen,
Karjalainen, Eloranta,
and Muotka (2008)
Alevin
development
hatching to first
feeding
Influenced by
temperature,
similar to eggs.
# of
days = 193 T−0.83
(r2 = .970).
At 7.5
C ≈ 100 days.
# of
days = 472 T−1.27
(r2 = .956).
At 7.5
C ≈ 100 days.
Kane (1988), Jensen,
Johnsen, and
Saksgard (1989),
Ojanguren and
Brana (2003)
Alevin survival Depends on
temperature.
<22
C Survive higher
temperatures than
eggs.
Lower survival during
low temperatures
and high flows.
Jensen and
Johnsen (1999),
Ojanguren
et al. (1999), Syrjanen
et al. (2008)
Fry to adult feeding Temperature
dependent activity.
4–18
C 4–22
C Depends on
acclimation.
Indications of genetic
adaptation.
Elliott, Hurley, and
Fryer (1995), Elliott
and Hurley (1997),
Solomon and
Lightfoot (2008)
Growth Determined by
temperature and
food ration.
GW = cWt
−b (T–Tlim)/
(TM–T)
GW = cWt
−b (T–Tlim)/
(TM–T)
Interpopulation
variation.
Genetic adaptation?
Jensen (1990), Forseth
and Jonsson (1994),
Elliott et al. (1995),
Elliott and
Hurley (1997),
Lobon-Cervia and
Rincon (1998), Elliott
and Elliott (2010)
Maximum growth ≈13
C ≈16
C Interpopulation
variation.
Genetic adaptation?
Above
748 HEGGENES ET AL.
TABLE 1 (Continued)
Life stage Temeperature effect Brown trout Atlantic salmon Note Selected references
Avoided
temperatures
Cease feeding and
growth, induce
thermal stress.
<3–4
C and
>19–20
C at
acclimation 15
C.
<5–6
C
<6
C and >22
C at
acclimation 15
C.
Depends on
acclimation, few
data for Atlantic
salmon.
Elliott (1981),
Elliott (1994), Elliott
and Hurley (1997),
Elliott and
Elliott (2010)
7 days lethal
temperature
Mortality. ≈25
C ≈28
C Influenced by
acclimation.
Genetical adaptation?
Above
Smolt age Temperature via
growth/size.
Depends primarily on
fish size.
Influenced by several
factors, including
temperature.
Metcalfe and
Thorpe (1990),
Saltveit (1990),
Jonsson and Labée-
Lund (1993)
Development of
smolt
characteristic
Primarily
photoperiod,
influenced by
temperature.
Restricted <3
C,
faster with
increasing
temperature (up to
10
C, and more?)
Staurnes, Sigholt, and
Gulseth (1994),
McCormick, Hansen,
Quinn, and
Saunders (1998)
Smolt migration Several factors (e.g.,
flow, photo-
period), including
temperature.
Lower trigger
temperature
(≈ 7
C), less
temperature
dependent?
Triggered ≈ 10
C? Interpopulation
variation re. Factor
importance.
Genetic adaptation?
Hansen and
Jonsson (1985),
Jonsson and Ruud-
Hansen (1985),
McCormick
et al. (1998), Byrne,
Poole, Dillane, Rogan,
and Whelan (2004),
Stewart, Middlemas,
and
Youngson (2006),
Solomon and
Lightfoot (2008),
Otero et al. (2014)
Swimming ability
(burst swimming)
Limited by low
temperatures.
Much reduced
<≈5
C, in particular
for smaller fish.
Much reduced
<≈5
C, in particular
for smaller fish.
Beamish (1979),
Beach (1984)
Spawning migration Several factors (e.g.,
flow), including
temperature.
Reduced <≈5
C and
>≈16
C, very little
>20–23
C.
Juanes, Gephard, and
Beland (2004),
Solomon and
Sambrook (2004),
Saraniemi, Huusko,
and Tahkola (2008),
Quinn, McGinnity,
and Reed (2016)
Gamete quality Reduced by high
temperatures.
?? >≈18
C High temperatures
reduce egg size,
fertility and
viability, certain
time periods more
sensitive.
King, Pankhurst, Watts,
and
Pankhurst (2003),
Anderson, Swanson,
Pankhurst, King, and
Elizur (2012)
Ovulation Reduced by high
temperatures.
>14–16
C Low temperatures
(<7–3
C) may
increase embryo
survival.
Vikingstad et al. (2008),
Vikingstad
et al. (2016)
Spawning time Temperature link via
DD/egg
development.
1–8
C Timed to optimize
early survival
(above), much
Webb and
McLay (1996),
Shields, Stubbing,
Summers, and
(Continues)
HEGGENES ET AL. 749
limits for survival (Finstad, Berg, Forseth, Ugedal, & Naesje, 2010).
Thus, Atlantic salmon and brown trout tend to exhibit a tempera-
ture dependent north–south gradient in winter lipid storage (Berg
et al., 2009; Berg, Rod, Solem, & Finstad, 2011). Northern
populations may also show a stronger positive scaling of feeding
activity with decreasing energy levels, that is, presumably compen-
satory adaptive differences in state-dependent feeding motivation
(Finstad et al., 2010). This may explain the observed variation in
lower temperatures for feeding activity in Atlantic salmon and
brown trout (Tables 2 & 3). Temperature-dependent tolerances and
changes in performance for Atlantic salmon and brown trout are
well established in general (Tables 2 & 3) (Elliott, 1994; Fry, 1971;
Larsson & Berglund, 2006). There is, however, also substantial ther-
mal plasticity to be considered, and less is known about possible
adaptations, both important with respect to potential responses to
thermal regimes altered by hydropower.
TABLE 1 (Continued)
Life stage Temeperature effect Brown trout Atlantic salmon Note Selected references
interpopulation
variation.
Genetic adaptation?
Giles (2005), Jonsson
and Jonsson (2009)
Spawning Limited temperature
range.
1–8
C?
<11.5
C
Webb and
McLay (1996),
Solomon and
Lightfoot (2008)
Thermal stress
limits restricting
long term
population
survival
High temperatures. >19.5
C >22.5
C Regular occurrence of
high temperatures
causing thermal
stress.
Solomon and
Lightfoot (2008)
Note: Question marks denote uncertainty about temperature range given. (Adapted from Solomon & Lightfoot, 2008).
TABLE 2 Temperature tolerances (
C) for survival of Atlantic salmon and brown trout
Atlantic salmon Brown trout
Life stage Lower Upper Lower Upper
Eggs 0 16 0 13
Alevins
Long terma 0–2 23–24 0–1 20–22
Short termb 0–1 24–25 0 22–24
Parr and smolt
Incipient 0–2 22–28 0–0.7 22–25
Ultimate −0.8 30–33 −0.8 26–30
Feeding 0–7 22–28 0.4–4 19–26
Note: (After Elliott & Elliott, 2010).
aIncipient Lethal Temperature (ILT): Tolerance for a long time period, usually 7 days.
bUltimate Lethal Temperature (ULT): Tolerance for a short time period, usually 10 min.
TABLE 3 Temperature limits (
C) for
growth range, optimum growth (on
maximum rations), and maximum growth
efficiency for Atlantic salmon and brown
trout
Species Lower Upper Optimum Growth efficiency
Atlantic salmon
UK 6.0 22.5 15.9 c. 13
Norway 1.0–7.7 23.3–26.7 16.3–20.0 12–18
Brown trout
Invertebrate food 2.9–3.6 18.2–19.5 13.1–14.1 8.9
Fish food c. 2.0 c. 19.5 16.6–17.4 9.3
Pelleted food 1.2–6.1 19.4–26.8 11.6–19.1
Note: (After Elliott & Elliott, 2010).
750 HEGGENES ET AL.
TABLE 4 Summary of hydropower regulation water temperature impacts on life stages of brown trout and Atlantic salmon, with likely
biological responses, ecological consequences, and potential mitigation measures
Life stage
(Potential) Temperature
regulation impact
(Potential) Biological
response Ecological consequences
Potential mitigation
measures
Egg incubation
period
Determined by degree-days
(DD).
Warmer winter water
shortens incubation
period.
Earlier hatching, later
spawning.
Mismatch of hatching with
environmental conditions
(flow, temperature, food
availability).***
Reduce winter water
temperatures via flexible
water intakes and
adaptive flow regime.
Egg survival Depends on temperature.
Regulation changes typically
within egg survival
temperature range.
Changed ice phenomena
may influence survival.
?
Does a temperature increase
for example, 0! 3
C
affect egg survival?
Or changed ice phenomena?
?
Changed population
recruitment?*
Reestablish surface ice
during winter via lower
water temperatures via
flexible water intakes and
adaptive flow regime?
Increase egg survival via
stable flow of 2–3
C
water, avoiding ice
phenomena?
Alevin
development,
hatching to first
feeding
Influenced by temperature,
similar to eggs.
Slower spring warming. (but
also unpredictable
transient temperatures).
Slower alevin development,
smaller alevins.
Mismatch of alevin
development with
environmental conditions,
for example, natural spring
flows, food availability.
Shorter growing season.***
Reduce regulated spring
flow.
Increase spring water
temperatures via flexible
water intakes and
adaptive flow regime.
Alevin survival Depends on temperature.
Slower spring warming.
Lower survival during low
temperatures and high
flows.
Prolonged alevin stage and
reduced size lead to
reduced survival?
Reduced population
recruitment?*
Higher spring temperatures
via flexible water intakes
and adaptive flow regime.
Fry to SMOLT/
adult feeding
Temperature dependent
activity and metabolism.
Slower spring warming,
lower summer
temperatures.
Typically reduced feeding.
Potentially increased
daytime feeding during
summer in warm, wide-
shallow streams receiving
high solar radiation.
Lower production.
Lower juvenile survival.*
Higher spring and early
summer temperatures via
flexible water intakes and
adaptive flow regime.
Avoid high summer
temperatures via drawing
cooler reservoir water.
Growth Determined by temperature
and food ration.
Slower spring warming,
lower summer
temperatures.
Reduced growth.
During high summer in
warmer climates, growth
may increase.
Smaller fish. Lower
production.***
Lower juvenile survival.**
Higher summer
temperatures via flexible
water intakes and
adaptive flow regime.
Avoid high summer
temperatures via drawing
cooler reservoir water.
Maximum growth ≈ 13–16
C Suitable summer
temperatures via flexible
water intakes and
adaptive flow regime.
Avoided
temperatures
Cease feeding and growth,
induce thermal stress.
Stabilize temperatures
within suitable
temperature range.
<3–6
C and >19–22
C at
acclimation 15
C.
Reduced growth and lower
survival**
Smolt migration Several factors (e.g., flow,
photoperiod), including
temperature.
Slower spring warming.
Lower trigger temperature
(≈ 7–10
C).
Delayed smolt migration?
Lower ocean survival?
Reduced population
recruitment?*
Increase spring water
temperatures via flexible
water intakes and
adaptive flow regime.
Spawning
migration
Several factors (e.g., flow),
including temperature.
Higher early fall
temperatures.
Delayed? Delayed?**
(Continues)
HEGGENES ET AL. 751
3.1.1 | Thermal plasticity, but little adaptation
Variation in thermal tolerance throughout an Atlantic salmon or
brown trout life cycle may be substantial and depends on develop-
mental stage (Tables 1–3). The egg stage has the narrowest tempera-
ture tolerance (Table 2) (Elliott & Elliott, 2010), and the young-of-the-
year, including the yolk sac stage, are less tolerant than later life
stages (Ayllon, Nicola, Elvira, Parra, & Almodovar, 2013; Breau,
Cunjak, & Peake, 2011; Elliott & Elliott, 2010). Atlantic salmon has
generally greater tolerance of high water temperatures in all life cycle
stages compared to brown trout, whereas brown trout is slightly more
cold adapted (Table 2). Interestingly, temperature tolerances remain
similar over a wide geographical range with negligible indications of
regional adaptations (Elliott & Elliott, 2010), suggesting that observed
variation in performance limits (Table 3) is primarily due to substantial
phenotypic plasticity. Such plasticity enhance ecological resilience to
thermal stress effected by hydropower development (Tables 4 & 5).
Feeding and growth in northern populations appear to some
extent to be adapted to the longer and more intense winters. A north-
ern Atlantic salmon population fed and grew better under the ice in
the dark than their southern cousins, under benign laboratory tank
conditions (Finstad, Forseth, Faenstad, & Ugedal, 2004; Finstad,
Naesje, & Forseth, 2004). They in turn, grew better in the light with-
out surface ice (Finstad & Forseth, 2006). This is important per se as
hydropower may reduce surface ice formation, and this should there-
fore be more explored also outside the laboratory. Trout appear to
grow better than expected in colder rivers (Jensen, Forseth, &
Johnsen, 2000), suggesting adaptations. In species like Atlantic salmon
and brown trout with a wide geographical distribution, and constraints
on gene flow imposed by watershed isolation and restricted spatial
dimensionality, intra-specific variation together with local adaptation,
may be expected. Indeed, differences in warm and cold-water thermal
performance exist among Atlantic salmon and brown trout
populations (Anttila et al., 2013; Forseth et al., 2009; Hartman &
Porto, 2014). However, whereas intraspecific phenotypic plasticity is
important in providing ecological resilience, there is little indication of
local thermal adaptation (Finstad & Jonsson, 2012; Jensen
et al., 2008; Meier et al., 2014; Skoglund, Einum, Forseth, &
Barlaup, 2011). This may seem surprising, but adaptations of enzyme
systems to different temperatures appear to come at a high cost
TABLE 4 (Continued)
Life stage
(Potential) Temperature
regulation impact
(Potential) Biological
response Ecological consequences
Potential mitigation
measures
Spawning time Temperature link via
DD/egg development.
Warmer winter water
shorten incubation period.
1–8
C.
Later spawning.
?*
See egg incubation
Timed to optimize early
survival in natural streams.
Note: Question marks indicate knowledge gaps and important research areas. Effects will depend on local conditions and ramping regime. Increasing
number of asterisks indicate increasing level of authors' confidence.
TABLE 5 Suggested future water temperature and hydropower
regulation research and knowledge needs
Hydro-physical conditions
Seasonal and thermal longitudinal effects are neither well studied
nor quantified. How extensive are they, and what local conditions
(e.g., groundwater, tributaries, catchment run of) affect the
changes?
How do different production regimes (e.g., peaking or seasonal) affect
longitudinal downstream water temperatures?
Replacement of the naturally stable mid-winter surface ice period
with prolonged transition periods may lead to more frazil ice
formation and correspondingly unstable or no surface ice. This may
expose fish and macroinvertebrates to more open water and frazil
ice, but also to increased winter temperatures. How does this affect
fish and macroinvertebrates?
To what extent do different macroinvertebrates survive
encasement in ice?
Is egg survival and invertebrate production higher in “winter-warm”
regulated rivers than in the natural state?
In regulated rivers, mechanistic ice breakup may occur several times
and at any time during winter. To what extent does this cause
higher winter mortality in fish?
Biological responses
Longitudinal water temperature impacts on fish populations and
invertebrate communities at the river scale?
To what extent are there differences in both warm and cold water
thermal performance among salmon and trout populations, and
are such differences indicative of local thermal adaptations?
Mitigating measures
Have restored/increased flows in regulated rivers resulted in higher
and/or more natural salmonid production? Also, has higher
minimum discharge in winter after hydropower regulation, with
associated higher water temperatures, resulted in higher parr
survival rates and smolt production?
What are the effects of hydropower regulation-induced
temperature changes/increases in early spring on egg and alevin
development and survival?
A suite of numerical models for water temperature calculations in
combination with models of effects on changing physical and
thermal habitat on fish is available. There is, however, a scarcity
of empirical examples?
Regarding models, development and integration of small-scale
thermal (fish) habitat models and validation against fish data, are
also remaining challenges.
752 HEGGENES ET AL.
(Portner, 2006; Shuter et al., 2012). In a few cases, there appear to be
some genetically based local adaptation to winter climates, notably
energy storage (Alvarez, Cano, & Nicieza, 2006; Berg et al., 2011;
Crespel, Bernatchez, Garant, & Audet, 2013; Finstad et al., 2010). The
maximum growth capacity in brown trout and Atlantic salmon may
vary among populations, but neither correlate with local natural tem-
perature optima nor indicate counter gradient variation in growth
(Baerum, Vollestad, Kiffney, Remy, & Haugen, 2016; Forseth
et al., 2009; Jonsson, Forseth, Jensen, & Naesje, 2001; Larsson &
Berglund, 2006). Instead, adaptive variation in growth potential may
be related to factors affecting reproductive success (Elliott &
Elliott, 2010; Forseth et al., 2009; Jonsson & Jonsson, 2009). It has
been suggested that thermal adaptations may occur as counter gradi-
ent adaptations in (very) cold environments (Elliott & Elliott, 2010;
Finstad, Naesje, & Forseth, 2004; Jensen et al., 2000; Nicola &
Almodovar, 2004), which would, in case, be particularly important in
northern rivers influenced by hydropower development (Tables 4 &
5). Limited local thermal adaptation in Atlantic salmon has tentatively
been attributed to the fact that they experience a common thermal
environment in the North Atlantic Ocean which may perhaps be more
important than freshwater growth in locally different temperature
regimes (Forseth et al., 2009). For the more resident brown trout, dif-
ferent population-specific thermal regimes throughout its life history
are more likely, in particular in colder streams (Alvarez et al., 2006;
Jensen et al., 2000; Nicola & Almodovar, 2004), although environmen-
tal variability may account for most of the observed variation in
annual growth rates.
Thus, salmonid species appear to be physiologically bound for
optimal performance and growth to a rather specific range of summer
temperatures (Tables 2 & 3) (Clarke & Portner, 2010; Shuter
et al., 2012). The notion of “species-specific preferred temperature”
±2
C seems to work quite well (Magnuson, Crowder, &
Medvick, 1979), although the optimal growth and preferred water
temperature may be shifted toward the upper end of this window
(Portner, 2010; Shuter et al., 2012). This “fundamental thermal niche”
may explain population productivity and northern zoogeographic
boundaries (Shuter et al., 2012). Interestingly, most freshwater fish,
including the cold-water Atlantic salmon and brown trout, have pre-
ferred temperatures well above 4–5
C (Tables 2 & 3) (Elliott &
Elliott, 2010; Shuter et al., 2012), whereas ambient winter tempera-
tures are well below that. On a cautionary note, optimum growth tem-
perature decreases with decreasing energy intake (Elliott &
Elliott, 2010). Therefore, optimum temperatures in nature may be
lower than values reported in Table 3, which in part are based on max-
imum rations in the laboratory.
Consequently, even apparently moderate changes in natural
water temperatures caused by hydropower regulation (e.g., Figure 1,
Table 4) may have important effects on Atlantic salmon and brown
trout growth and performance (Tables 1 & 4). Temperature is the
most important factor in their growth models. Although originally
based on relatively benign laboratory conditions with maximum
rations, they may be good approximations of growth patterns in natu-
ral streams, demonstrating the importance of temperature (Elliott &
Elliott, 2010; Elliott & Hurley, 1997; Forseth et al., 2001; Jonsson
et al., 2001). On a cautionary note, if growth during (early) summer is
hampered for some reason, for example, by limited habitat during low
flows and droughts, trout are capable of compensatory growth
(Elliott, 2009, 2015), which may modify such models, and should
therefore be explored more in a hydropower context in which change
thermal regimes seasonally. Still, such growth models are robust with
respect to assumed base growth temperatures (Chezik, Lester, &
Venturelli, 2014) and are versatile tools that may be used in compara-
tive studies (Hayes, 2013), for example, exploring potential fish pro-
duction under alternative thermal regimes effected by different
tapping and release strategies in a high-head hydropower system
(Figure 1). Growth models may also be used to control for influence of
other environmental factors, such as food availability (Sanchez-
Hernandez, Gabler, Elliott, & Amundsen, 2016).
Foraging during low temperatures in winter is rare for salmon
and trout, and winter is primarily about survival. The common strat-
egy is energy storage (see review Heggenes, Alfredsen, Bustos,
et al., 2017). Additional strategies are reduced metabolism, toler-
ance, and starvation effected by quiescence (Shuter et al., 2012)
and shorter behavioral movements or migration to more suitable
deep and slow flow winter habitat (Linnansaari et al., 2009; Rimmer,
Paim, & Saunders, 1984; Saraniemi et al., 2008). Activity may result
from lack of available refuge habitat and predation risk, but also
easily available food, or perhaps stronger feeding motivation as
energy stores become low (Heggenes, Alfredsen, Bustos,
et al., 2017; Metcalfe, Fraser, & Burns, 1999). Brown trout may feed
throughout the year, but little and with no apparent growth, or even
shrinking, in winter (Elliott, 2009; Huusko, Maki-Petays, Stickler, &
Mykra, 2011; Lien, 1978). Atlantic salmon and brown trout become
less aggressive and less efficient foragers at lower light levels and
temperatures (Elliott, 2011; Valdimarsson & Metcalfe, 2001). Down-
stream winter temperature warming due to river regulation tends to
be in a range of 0–3
C for high-head systems (e.g., Halleraker
et al., 2007; Kvambekk, 2012). Such range is below the thermal
thresholds for growth and foraging for Atlantic salmon and brown
trout (Tables 1–3), suggesting that direct effects on growth may be
limited.
3.1.2 | Autumnal fish migration, spawning, and egg
development
For autumn spawning and downstream spring smolt migrations, the
larger time window is regulated by photoperiod (McCormick
et al., 1998; Moore et al., 2012; Robards & Quinn, 2002). However,
water temperature and/or flow levels may control local migrations,
although many other in situ factors may contribute (Table 1) (Cunjak,
Linnansaari, & Caissie, 2013; Milner, Solomon, & Smith, 2012;
Tetzlaff, Gibbins, Bacon, Youngson, & Soulsby, 2008; Thorstad,
Okland, Aarestrup, & Heggberget, 2008). Spawning time appears to
be population specific and locally adapted to winter temperature
regimes targeting hatching and first feeding at the most opportune
HEGGENES ET AL. 753
time in spring (Jensen, Johnsen, & Heggberget, 1991; Shields
et al., 2005). Thus, spawning is linked to autumnal water temperature
(1–6
C for peak spawning) (Jonsson & Jonsson, 2009; Riedl &
Peter, 2013; Taranger & Hansen, 1993), with earlier spawning in col-
der rivers. Consequently, a modified winter thermal regime due to
hydropower regulation is also likely to modify timing of spawning
(Table 4). The duration of embryonic development primarily depends
on the number of degree-days (DD) from spawning to hatching
(Elliott & Hurley, 1998b; Embody, 1934; Jungwirth & Winkler, 1984;
Kane, 1988). In brown trout DD is around 320 at low water tempera-
tures of 2
C, but increasing to about 400 DD with increasing water
temperatures up to 10
C. Post-regulation elevated winter water tem-
peratures in regulated rivers will result in earlier hatching (Syrjanen
et al., 2008) and may disrupt the delicate balance and link between
timing of spawning (Jonsson & Jonsson, 2009; Shuter et al., 2012) and
larval spring emergence. This timing maximizes growth performance
the following summer season, but presumably also reduces mortality
risk associated with spring runoff and wash-out of larvae (Einum &
Fleming, 2000; Jensen et al., 1991; Jensen & Johnsen, 1999; Letcher
et al., 2004; Skoglund, Einum, Forseth, & Barlaup, 2012).
Natural timing of emergence vary among years within a popula-
tion, mainly due to variations in rapidly rising water temperatures in
spring, and with spawning date as a secondary factor (Cunjak
et al., 2013; Elliott & Elliott, 2010). The number of egg incubation DD
may, like optimal growth temperature, show little adaptive variation,
for example, among natural Atlantic salmon populations
(Heggberget & Wallace, 1984; Wallace & Heggberget, 1988). Adapta-
tion may rather be reflected in varying spawning times, but studies
are limited with respect to population gradients investigated and
results are somewhat ambiguous (Jonsson & Jonsson, 2009). Egg sur-
vival is typically high (Barlaup, Gabrielsen, Skoglund, & Wiers, 2008;
Casas-Mulet, Saltveit, & Alfredsen, 2015; Elliott, 1994; Saltveit &
Brabrand, 2013), but variable (Cunjak et al., 2013; Sear &
DeVries, 2008) depending on a range of biotic and abiotic factors
(Gibbins, Shellberg, Moir, & Soulsby, 2008; Greig, Sear, &
Carling, 2007; Malcolm, Gibbins, Soulsby, Tetzlaff, & Moir, 2012;
Saltveit & Brabrand, 2013; Stonecypher et al., 1994), including
groundwater combined with local hyporheic environments and water
temperatures. In the absence of groundwater with sufficient oxygen,
egg mortality is likely to be higher with more shallow burial, that is,
more exposure to dewatering and sub-zero temperatures (Casas-
Mulet, Alfredsen, Brabrand, & Saltveit, 2015; Casas-Mulet, Alfredsen,
Hamududu, & Timalsina, 2015; Casas-Mulet, Saltveit, &
Alfredsen, 2015), an important consideration in regulated rivers and
winter temperatures and flow (Tables 4 & 5). Altered thermal regimes
linked to low flows may be a particular challenge. Increased tempera-
tures due to regulation in winter may be a driver for early hatching,
but translate into increased mortality if hatching occurs prior to the
spring floods in areas of the river where redds are not fully covered
with water. If hatching occurred later, eggs would survive, but not the
alevins (Casas-Mulet, Alfredsen, Brabrand, & Saltveit, 2016; Casas-
Mulet, Saltveit, & Alfredsen, 2016; Vanzo, Tancon, Zolezzi,
Alfredsen, & Siviglia, 2016).
3.1.3 | Spring emergence and early mortality
The early stage related to the alevin emergence is an important regu-
latory period with natural high mortality in salmonid populations
(Table 1) (Bret, Bergerot, Capra, Gouraud, & Lamouroux, 2016;
Jonsson & Jonsson, 2009; Milner et al., 2003), at least in more benign,
temperate streams, and high-density populations (Elliott, 2009;
Jonsson, Jonsson, & Hansen, 1998) where competition for space and
density-dependent mortality is important. Dominance is determined
by size and aggressiveness, and the more aggressive brown trout
dominates even larger Atlantic salmon (Einum & Fleming, 2000;
Einum, Robertsen, Nislow, McKelvey, & Armstrong, 2011; Skoglund
et al., 2012). Smaller individuals may be forced to move (Einum
et al., 2012) incurring potentially high mortalities (Einum &
Fleming, 2000; Elliott, 1994). In low-density (below carrying capacity)
populations in more challenging environments, density-independent
abiotic regulating factors such as flow and temperature may be more
important (Bret et al., 2016; Cunjak et al., 2013; Elliott &
Hurley, 1998a; Elliott, 1989; Lobon-Cervia & Mortensen, 2005;
Lobon-Cervia, 2014). Higher temperatures may benefit survival, but
not extreme temperatures (Gibson & Myers, 1988). Correspondingly,
low water temperatures may negatively affect survival (Elliott, 1985;
Jensen & Johnsen, 1999), especially combined with high flows (Bret
et al., 2016). Experimental studies on brown trout and Atlantic salmon
indicate that low incubation temperatures produce smaller fry with
larger yolk sacs, relative to higher incubation temperatures (Skoglund
et al., 2011; Syrjanen et al., 2008). The experimental fry started active
feeding with growth even at the lowest temperatures of 2
C, and
increasing with temperature, but temperature at first feeding is likely
higher in nature, for example, 8
C for Atlantic salmon (Jensen
et al., 1991). Smaller size or larger energy reserves may be beneficial
under low spring temperature conditions because they may lower
activity, which in turn increases sheltering and enables more cryptic
feeding strategies. Alternatively, larger size resulting from warmer
incubation temperatures may confer a competitive advantage. Thus,
this should be explored in natura if increased downstream tempera-
tures in winter typical of northern high-head hydropower regulation
(above), do indeed result in larger fry with smaller yolk sacs
(Tables 4 & 5).
3.2 | Water temperatures and the supporting food
web: Macroinvertebrates
For benthic macroinvertebrates as a group, different species show
considerable morphological and ecological variation, permitting adap-
tation to a wide range of aquatic thermal environments as well as
notably widespread species with flexible life cycle length
(Brittain, 1991; Brittain & Saltveit, 1989; Raddum & Fjellheim, 1993).
Abiotic factors such as temperature, habitat, and water chemistry
affect benthic macroinvertebrate development and production
(Huryn & Wallace, 2000). Like for fish, seasonally low temperatures, in
addition to food limitation, rather than specific modes of feeding or
754 HEGGENES ET AL.
life-history attributes, constrain macroinvertebrate production, which
is lowest in cool-temperate and arctic streams (Huryn &
Wallace, 2000).
Benthic macroinvertebrates use diverse winter strategies to cope
with and even take advantage of winter conditions. Many species
undergo cold-tolerant quiescence or diapause, most commonly in the
egg stage (Brittain, 1990; Danks, 2008; Harper & Hynes, 1970;
Lencioni, 2004), and this is usually cued by photoperiod. Movement,
into the substrate/hyporheic or tributaries/deeper water, is a freeze
avoiding strategy, which can be coupled with diapause/quiescence
(Bale & Hayward, 2010; Lencioni, 2004; Olsson, 1981, 1983). Alterna-
tively, certain species (Diptera, Plecoptera) stay-put and display freez-
ing tolerance (Bale & Hayward, 2010; Lencioni, 2004;
Lillehammer, 1987). Still, severe winters may reduce, at least in the
short term, both benthic abundance and taxonomic richness
(Hoffsten, 2003), and the majority of insects die from the effects of
cold water rather than freezing (Bale & Hayward, 2010). This has
important implications for vulnerability to winter warming in regulated
rivers.
Modified water temperatures, for example, by hydropower reg-
ulation, may have major effects on normal rates of development
and life cycles, as many facets of growth and emergence are
affected and even cued by water temperature (Brittain, 1982; Dal-
las & Ross-Gillespie, 2015; Lillehammer, Brittain, Saltveit, &
Nielsen, 1989; Raddum, 1985), also in the Southern Hemisphere
(Dallas & Ross-Gillespie, 2015; Ross-Gillespie, Picker, Dallas, &
Day, 2018). This facilitates modeling of potential impacts (Rivers-
Moore, Dallas, & Ross-Gillespie, 2013). Moreover, since photoperiod
is the dominant diapause-inducing cue and unaffected by hydro-
power regulation, unless the surface ice cover is changed, a modi-
fied temperature regime may lead to decoupling of synchrony
between diapause-sensitive life stages and thermally challenging
environments. Univoltine and longer life cycles are most at risk,
while flexible, short life cycles will be favored (Brittain, 1991; Petrin,
Brittain, & Saltveit, 2013). A reduction in the temperature range
between winter and summer will lead to a reduction in species
diversity (Vannote & Sweeney, 1980), again favoring the wide-
spread, ubiquitous species (Brittain, 1991; Saltveit, Bremnes, &
Brittain, 1994; Saltveit, Brittain, & Lillehammer, 1987). Reduced
snow and ice cover, which may result from hydropower regulation,
may expose macroinvertebrates to the more severe consequences
of low air temperatures (Raddum, 1985), and increased frequency of
freeze–thaw cycles may increase risks of ice encasement (Bale &
Hayward, 2010). Frazil ice events, which may increase after hydro-
power regulation, may increase post-event drift (Martin, Brown, Bar-
ton, & Power, 2001; Sertic Peric & Robinson, 2015), and
supercooled water may reduce macroinvertebrate abundance
(Hoffsten, 2003; Martin et al., 2001), although benthic organisms
may also survive entrapment in anchor ice (Brown, Clothier, &
Alvord, 1953; Benson, 1955; Oswood, Miller, & Irons, 1991; Irons,
Miller, & Oswood, 1993; but see Frisbie & Lee, 1997). The scarcity
of Ephemeroptera and Plecoptera in frozen substrates suggests that
these taxa either remain in habitats that do not freeze or move to
deeper water (Olsson, 1983; Oswood et al., 1991). Although most
aquatic insects emerge during the ice-free period, some species
emerge during winter, for example, in Chironomidae, Trichoptera,
and Plecoptera (Brinck, 1949; Hågvar & Østbye, 1973;
Lencioni, 2004).
During winter, allochthonous organic plant material (leaf litter)
provides food utilized by winter growing shredder species
(Brittain, 1983; Haapala & Muotka, 1998; Lillehammer et al., 1989),
raising their abundance in winter (Haapala, Muotka, &
Markkola, 2001). However, results are ambiguous, and winter reduc-
tions are also reported (Clifford, 1972; Martin et al., 2001; Morin,
Rodriguez, & Nadon, 1995), perhaps because of reduced water flow
(Waringer, 1992), reduced invertebrate activity (Ferreira &
Canhoto, 2014; Martin et al., 2001), or increased food availability in
the hyporheic zone (Dekar, Magoulick, & Huxel, 2009; Haapala &
Muotka, 1998; Hildebrand, 1974; Vannote, Minshall, Cummins,
Sedell, & Cushing, 1980). In general, mechanical disruption of the sub-
strate, for example, by ice in winter, may dislodge benthic inverte-
brates (Butler & Hawthorne, 1979) and lead to increased downstream
drift (Brittain & Eikeland, 1988; Finni & Chandler, 1979) and even fau-
nal depletion in some cases (Colbo, 1979), although not in others
(Brown et al., 1953).
We suggest that increased downstream winter temperatures may
lead to increased growth rates in some macroinvertebrates (Table 5)
(Nebeker, 1971; Raddum & Fjellheim, 1993; Raddum, Fjellheim, &
Velle, 2008; Ward & Stanford, 1982), as growth and development in
macroinvertebrates may continue in many species even at low winter
temperatures (Huryn & Wallace, 2000; Svensson, 1966; Vannote &
Sweeney, 1980), even in extreme Arctic environments that do not
freeze solid (e.g., Coulson et al., 2014). Potentially higher invertebrate
production in winter-warmer regulated rivers may precipitate more
opportunistic feeding in fish and a potentially earlier start of the
growth season. Unfortunately, such beneficial effects are not well
studied (Table 5) and may be confounded with simultaneous changes
in summer temperature and flow regime (Bruno et al., 2013; Bruno,
Maiolini, Carolli, & Silveri, 2009; Cereghino, Cugny, &
Lavandier, 2002; Jackson, Gibbins, & Soulsby, 2007; Miller &
Judson, 2014).
4 | WHAT GOOD PRACTICES CAN BE
RECOMMENDED FOR MITIGATING
TEMPERATURE IMPACTS?
Hydropower regulation may increase downstream winter water tem-
peratures up to 2–3
C or more, and mitigation measures reduce regu-
lated winter water temperatures by perhaps 0.1–1
C (Halleraker
et al., 2007; Kvambekk, 2012). Mitigation measures may increase
summer temperatures by 0.1–2
C (Halleraker et al., 2007). However,
the mitigation potential may be larger, depending on local/regional
conditions (Kvambekk, 2012).
Environmental design of water temperature regimes in regulated
rivers is challenging and complex, mostly due to various
HEGGENES ET AL. 755
environmental factors that influence and control water temperature in
nature and associated physical-ecological interactions (Table 4).
4.1 | Provide baseline and monitoring data
Baseline data for natural conditions are frequently scarce or lacking.
This is a main factor for mitigation, as is just obtaining adequate moni-
toring data. Recent technological advances have enabled high-
resolution GSM/satellite and cellular-based options for monitoring
and logging water temperature, light and chemical data in rivers and
reservoirs (e.g., Adu-Manu, Tapparello, Heinzelman, Katsriku, &
Abdulai, 2017; Myers, 2019; Tauro et al., 2018). These systems should
be implemented on regulated rivers, dams/reservoirs, and water trans-
fer schemes, including thermal and hydrological regime monitoring
and baselining in environmental flow studies. .
Empirical studies are few (Gillespie et al., 2015; Olden &
Naiman, 2010; Poff & Zimmerman, 2010), as are lessons learned from
implementation of different concrete mitigation measures in regulated
systems in northern temperate regions (e.g., Bartholow, Hanna, Saito,
Lieberman, & Horn, 2001; Bennet et al., 2011; Gillespie et al., 2015)
and much is based on modelling exercises and theory (e.g., Hanna,
Saito, Bartholow, & Sandelin, 1999; Bartholow et al., 2001; Sherman,
Todd, Koehn, & Ryan, 2007). In particular, there islittle empirical data
about the long-term biotic responses to environmental variables mod-
ified by hydropower flow regulation and water temperature (White
et al., 2017). Such studies are necessary to facilitate ecological learn-
ing (Table 5) (Roberts, Anderson, & Angermeier, 2016), but unfortu-
nately, too often compromised or omitted alltogether from
restoration programs (Bash & Ryan, 2002). This may be caused by
inadequate funding, deficiencies in the planning process, project time-
line, or a mismatch between temporal scales of hydropower flow reg-
ulation impacts and monitoring of responses. The lack of pre-project
monitoring data, and in many cases, even operational monitoring data
and inadequate consideration of appropriate response variables are
more serious (Roberts et al., 2016; Rolls, Leigh, & Sheldon, 2012). In
addition, the global climate change predictions and the consequences
when combined with thermal regime alterations due to hydropower
should be considered (e.g., Benjankar et al., 2018; Cheng, Voisin,
Yearsley, & Nijssen, 2020; Palmer et al., 2009) There is also the funda-
mental question whether one should design mitigation measures to
simulate natural conditions, or, if there is a documented potential for
it, to “improve” survival/production of economically important, har-
vestable fish. The precautionary principle suggests the former,
although much management has focused on the latter.
Overall, the main mitigation measures depend on reservoir supply
and operational strategies, but are basically two: (a) flexible and sec-
ondary water release strategies from reservoir to mimic natural water
temperature conditions (Figure 5) and (b) methods to increase mixing
of surface and bottom water in the reservoir to smoothen water tem-
perature differences with depth (Kvambekk, 2012; Sherman, 2000).
4.2 | Flexible water release
Flexible water release from reservoirs (selective withdrawal) uses a
multilevel reservoir intake structure. It is the most effective way of
controlling the water temperature of reservoir releases (Figure 5)
(Sherman, 2000), providing the opportunity to mimic seasonal varia-
tions similar to natural conditions. However, some water temperature
differences will remain, depending on release volumes, reservoir strat-
ification, and available water depth intakes. In the hydropower dam
on the Alta River (1987), Norway, a secondary upper intake was
intended for both summer and winter temperature control releases
(Asvall & Kvambekk, 2001). After hydropower regulation, a decline in
both Atlantic salmon parr densities (>1+) and adult catch in the upper
part of the river (Sautso) was suspected to be caused by the release of
warmer water in winter and associated reduced surface ice cover
(Ugedal et al., 2008). A trial temperature-flow release program was
established (2001–2002) using water from the upper intake to lower
the downstream water temperature and to reestablish surface ice
cover. This measure theoretically lowered temperatures near to
preregulation conditions, but also reduced annual discharge and
hydropower production (Asvall & Kvambekk, 2001; Kvambekk, 2012;
F IGURE 5 Intake control of temperature: multi-level intake structure makes use of the stratification within the reservoir by permitting water
with desirable thermal attributes (in this case winter condition, therefore upper layer) to be withdrawn from defined regions within the water
column. (Modified after Sherman, 2000) [Color figure can be viewed at wileyonlinelibrary.com]
756 HEGGENES ET AL.
Tvede, 2006). Similarly, Olden and Naiman (2010) refer to the Flaming
Gorge dam in USA, a somewhat more temperate location, where a
multilevel intake was installed in 1978 and increased the downstream
summer temperature by 100%, compared to using the original bottom
intake. These empirical case studies indicate the mitigation potential
with respect to downstream thermal regimes, but more studies are
needed to quantify mitigation potential and to consider different
hydropower systems (Table 5). Whereas empirical studies appear to
be few, a number of modeling studies indicate the feasibility of flexi-
ble and/or additional water reservoir intakes for obtaining desired
downstream water temperature regimes (Bartholow et al., 2001;
Halleraker et al., 2007; Hanna et al., 1999).
The few empirical studies of actual ecological effects on fish and
macroinvertebrates of changed temperature releases that exist are
mostly from Australia (Miles & West, 2011; Pardo, Campbell, &
Brittain, 1998), where amplitudes may be extreme and not compara-
ble to northern temperate regions. In an exceptional time series,
100 years of hydrological and 50 years of aquatic macroinvertebrate
assemblage data from the Flaming Gorge dam, Vinson (2001) docu-
mented substantial positive results (up to 68% increase in insect taxon
richness) after an 100% increase of downstream summer tempera-
tures. Improvements were also observed for the frequency and dura-
tion of thermal events, and the timing of extreme temperatures
(Olden & Naiman, 2010), although rate and timing of warming
remained different from before dam construction (Vinson, 2001). No
increase, however, was found in the reach immediately downstream
of the dam (0.8 km; Greendale gage). Vinson (2001) suggested this
may be explained by the combined effect of three factors: (a) the
competitive dominance of insect taxa by amphipods on this reach,
(b) low rates of immigration and colonization (from the above reser-
voir), and (c) low reproductive success of insects due to a few degrees
difference in the water temperatures between the regulated river and
natural streams in the area. An important general corollary is that not
only “traditional” habitat attributes need to be considered, monitored
and evaluated in regulated river restoration projects, but biological
interactions as well.
Here we contrast two before–after hydropower regulation tem-
perature studies to emphasize that such regulation may generate very
different biological impacts, and ecological resilience may depend cru-
cially on timing, and not only on temperature amplitude changes. In
the Alta River, North-Norway, hydropower regulation mainly
increased fall–winter temperatures (about 0.5–2
C, Figure 3a),
increasing degree-days during this egg-incubation period, resulting in
earlier hatching in spring. Ecological resilience may be affected by
later spawning in fall, and spawning time is known to vary locally
(above). Fish growth and production may remain largely unaffected,
typically not taking place in this temperature range and season. How-
ever, if ice conditions are changed, this may affect fish behavior and
survival, which has been a concern (Ugedal et al., 2008). Re-
establishing surface ice cover was a main objective of the mitigation
efforts (Figure 3a) and could be effected by a modest temperature
reduction. In contrast, a similar temperature reduction of 0.5–2
C in
Lærdalselva River, West-Norway, occurred in spring-early summer
(Figure 3b). Hydropower regulation will predictably have a major neg-
ative effect on fish growth (above) and probably also survival. Bio-
energetic modeling (Elliott et al., 1995; Hayes, 2013; Hayes, Stark, &
Shearer, 2000) suggests this apparently modest temperature change
may generate an annual weight growth reduction of about 25% for a
1 g brown trout (from 3.7 before to 2.8 g after). Unfortunately,
because there is little adaptation in fish growth to local temperature
regimes, there is little potential for ecological resilience. Thus,
unchecked hydropower regulation in northern rivers may typically
precipitate a fish production loss via downstream water temperature
reduction during the growth season, clearly justifying more focus on
thermal regimes and the mitigating potential of flexible water release
(Table 5).
4.3 | Water mixing in reservoirs
Water may also be mixed in reservoirs (destratification;
Sherman, 2000) to raise water temperature in the hypolimnion in res-
ervoirs with only bottom intakes. One alternative is a bubble plume
that creates artificial circulation within the reservoir. Other alterna-
tives include using surface mounted pumps to pump surface water
down to the bottom intake, submerged curtains to force surface water
to the intake, and curtains that prevent cold, deep water from enter-
ing the hydropower intakes (Sherman, 2000; Sherman et al., 2007).
Gray (2016) examined the effects of thermal curtains that
directed warmer surface water to the hypolimnial off-take at
Burrendong Dam, Australia. The study demonstrated an increase in
water temperature of approximately 2
C (mean daily and mean
monthly temperature, and diel temperature range) with positive envi-
ronmental effects (Lugg & Copeland, 2014), as minimum thermal
reproductive requirements for local species were met more fre-
quently. Vermeyen (2000) studied the performance of flexible curtains
installed in two reservoirs that deliver water to the Sacramento River,
USA, to reduce the warm water releases that were exceeding critical
levels for Chinook salmon (Oncorhynchus tshawytscha) egg incubation
and juvenile fish survival. The curtains in Lewiston reservoir contrib-
uted to a temperature reduction of 1–2
C, depending on baseload
power releases. However, we are not aware of similar studies in
northern hydropower regulation systems (Table 5).
5 | CONCLUSIONS AND FUTURE
RESEARCH
Water temperature is a controlling factor in ecosystem functioning.
For aquatic organisms, seasonal temperature patterns in northern
temperate river systems largely define summer as a season for devel-
opment and growth and winter as the season for tolerance, quies-
cence, starvation, and survival. Hydropower regulation typically affect
thermal regimes primarily in high head systems comprising high eleva-
tion reservoirs and transfer tunnels down to lower elevation power
plants. High head systems with storage capacity may lead to major
HEGGENES ET AL. 757
shifts from natural water temperatures. In downstream reaches, water
temperatures will typically decrease in summer but increase in winter
compared to natural conditions. Hydropeaking systems may generate
thermo-peaking exceeding natural daily variations in temperatures
and challenge aquatic organisms in terms of life cycle development
and fitness. Thus, hydropower-related water temperature changes
directly affect aquatic biota. Effects are organism-specific in a variety
of ways, in particular for the diverse benthic macroinvertebrate fauna,
whereas effects on the species poor northern fish fauna are relatively
homogeneous.
A brief overview of considerations reviewed (temperature
impacts, responses/consequences, potential mitigating measures),
with focus on the life stages of brown trout and Atlantic salmon, is
provided in Table 4. Suggested future water temperature and hydro-
power regulation research and knowledge needs are given in Table 5.
In general, effects of good practice of environmental design and
targeted mitigation measures are, unfortunately, not well docu-
mented. Much tentative knowledge is based on modelling exercises
that need to be empirically validated. Thus, there is a crucial need for
more empirical data, especially from long-term studies. The con-
founding effects of climate change must also be eliminated by the
inclusion of non-regulated systems.
ACKNOWLEDGEMENT
This work was funded by Statkraft AS (https://www.statkraft.com/)
and the University of South-Eastern Norway.
DATA AVAILABILITY STATEMENT
Raw data were generated at Norwegian Water Resources and Energy
Directorate—NVE (https://www.nve.no/english/). Derived data
supporting the findings of this study are available from the
corresponding author Jan Heggenes on request.
ORCID
Jan Heggenes https://orcid.org/0000-0003-4080-6896
Knut Alfredsen https://orcid.org/0000-0002-4076-8351
Ana Adeva-Bustos https://orcid.org/0000-0001-7655-784X
REFERENCES
Adu-Manu, K. S., Tapparello, C., Heinzelman, W., Katsriku, F. A., &
Abdulai, J.-D. (2017). Water quality monitoring using wireless sensor
networks: Current trends and future research directions. ACM Transac-
tions on Sensor Networks (TOSN), 13, 1–41.
Alvarez, D., Cano, J. M., & Nicieza, A. G. (2006). Microgeographic variation
in metabolic rate and energy storage of brown trout: Countergradient
selection or thermal sensitivity? Evolutionary Ecology, 20, 345–363.
Anderson, K., Swanson, P., Pankhurst, N., King, H., & Elizur, A. (2012).
Effect of thermal challenge on plasma gonadotropin levels and ovarian
steroidogenesis in female maiden and repeat spawning Tasmanian
Atlantic salmon (Salmo salar). Aquaculture, 334, 205–212.
Angilletta, M. J., Jr., Steury, T. D., & Sears, M. W. (2004). Temperature,
growth rate, and body size in Ectotherms: Fitting pieces of a life-
history Puzzle1. Integrative and Comparative Biology, 44, 498–509.
Angilletta, M. J., Niewiarowski, P. H., & Navas, C. A. (2002). The evolution
of thermal physiology in ectotherms. Journal of Thermal Biology, 27,
249–268.
Anttila, K., Dhillon, R. S., Boulding, E. G., Farrell, A. P., Glebe, B. D.,
Elliott, J. A. K., … Schulte, P. M. (2013). Variation in temperature toler-
ance among families of Atlantic salmon (Salmo salar) is associated with
hypoxia tolerance, ventricle size and myoglobin level. Journal of Experi-
mental Biology, 216, 1183–1190.
Archer, D. (2008). The influence of river regulation at Kielder water on the
thermal regime of the river North Tyne. BHS 10th National Hydrology
Symposium, pp. 35–41. Retrieved from https://www.researchgate.
net/publication/273137677_The_influence_of_river_regulation_at_Ki
elder_Water_on_the_thermal_regime_of_the_River_North_Tyne
Asvall, R., & Kvambekk, Å. (2001). Ny strategi for tapping av Altamagasinet
om vinteren. Endring av vanntemperaturog isregimet fra utløpet av
kraftstasjonen i Savco ved utvidet bruk av øvre inntak. NVE
Oppdragsrapport. Report number 10-2001. Retrieved from http://
publikasjoner.nve.no/oppdragsrapport/2001/oppdragsrapport2001_
10.pdf
Asvall, R. P. (1977). Vanntemperatur og isforhold i våre vassdrag. Generelt om
virkningen av vassdragsreguleringer i berørte vassdrag og tilstøtende
fjorder. Rapport Vassdragsdiektoratet. Vassdragsdirektoratet, Oslo,
Norway.
Asvall, R. P. (2008). Altautbyggingen-Vanntemperatur og isforhold om vin-
teren (2007-2008). NVE Oppdragrapport A. Norges Vassdrags og
Energidirektorat, Norges Vassdrags og Energidirektorat Oslo. p. 28.
Retrieved from http://publikasjoner.nve.no/oppdragsrapportA/2008/
oppdragsrapportA2008_13.pdf
Austin, H., Bradley, D., Steward-Rousson, I., & Milner, N. (2015). Literature
review of the influence of large impoundments on downstream tempera-
ture, water quality and ecology, with reference to the water framework
directive. APEM scientific report. Scottish Environmental Protection
Agency, Stirling, Scotland. p. 84.
Ayllon, D., Nicola, G. G., Elvira, B., Parra, I., & Almodovar, A. (2013). Ther-
mal carrying capacity for a thermally-sensitive species at the warmest
edge of its range. PLoS One, 8, 11.
Baerum, K. M., Vollestad, L. A., Kiffney, P., Remy, A., & Haugen, T. O.
(2016). Population-level variation in juvenile brown trout growth from
different climatic regions of Norway to an experimental thermal gradi-
ent. Environmental Biology of Fishes, 99, 1009–1018.
Bakken, T. H., King, T., & Alfredsen, K. (2016). Simulation of river water
temperatures during various hydro-peaking regimes. Journal of Applied
Water Engineering and Research, 4, 31–43.
Bale, J. S., & Hayward, S. A. L. (2010). Insect overwintering in a changing
climate. Journal of Experimental Biology, 213, 980–994.
Barlaup, B. T., Gabrielsen, S. E., Skoglund, H., & Wiers, T. (2008). Addition
of spawning gravel - a means to restore spawning habitat of Atlantic
salmon (Salmo salar L.), and anadromous and resident brown trout
(Salmo trutta L.) in regulated rivers. River Research and Applications, 24,
543–550.
Barneche, D. R., & Allen, A. P. (2018). The energetics of fish growth and
how it constrains food-web trophic structure. Ecology Letters, 21,
836–844.
Bartholow, J., Hanna, R., Saito, L., Lieberman, D., & Horn, M. (2001). Simu-
lated Limnological effects of the Shasta Lake temperature control
device. Environmental Management, 27, 609–626.
Bash, J. S., & Ryan, C. M. (2002). Stream restoration and enhancement pro-
jects: Is anyone monitoring? Environmental Management, 29, 877–885.
Beach, M. H. (1984). Fish pass design-criteria for the design and approval
of fish passes and other structures to facilitate the passage of migra-
tory fish in rivers Fisheries Reserach Technical Report. Ministry of
Agriculture, Fisheries and Food, Lowestoft, England. p. 48. Retrieved
from http://aquaticcommons.org/8065/
Beamish, F. W. (1979). 2. Swimming capacity. In W. S. Hoar & D. J. Randall
(Eds.), Locomotion (pp. 101–187). New York: Academic Press Inc.
Beitinger, T. L., Bennett, W. A., & McCauley, R. W. (2000). Temperature
tolerances of north American freshwater fishes exposed to dynamic
changes in temperature. Environmental Biology of Fishes, 58, 237–275.
758 HEGGENES ET AL.
Benjankar, R., Tonina, D., McKean, J. A., Sohrabi, M. M., Chen, Q., &
Vidergar, D. (2018). Dam operations may improve aquatic habitat and
offset negative effects of climate change. Journal of Environmental
Management, 213, 126–134.
Bennet, W., Rybel, V., Jenkins, M., Donohue, K., Riker, R., & Sticka, D.
(2011). Retrofitting a deep water plant intake to improve fish passage.
HydroReview, 30, 38–46.
Benson, N. G. (1955). Observations on anchor ice in a Michigan trout
stream. Ecology, 36, 529–530.
Berg, O. K., Finstad, A. G., Solem, O., Ugedal, O., Forseth, T., Niemela, E., …
Naesje, T. F. (2009). Pre-winter lipid stores in young-of-year Atlantic
salmon along a north-south gradient. Journal of Fish Biology, 74,
1383–1393.
Berg, O. K., Rod, G., Solem, O., & Finstad, A. G. (2011). Pre-winter lipid
stores in brown trout Salmo trutta along altitudinal and latitudinal gra-
dients. Journal of Fish Biology, 79, 1156–1166.
Bradshaw, W. E., Zani, P. A., & Holzapfel, C. M. (2004). Adaptation to tem-
perate climates. Evolution, 58, 1748–1762.
Breau, C., Cunjak, R. A., & Peake, S. J. (2011). Behaviour during elevated
water temperatures: Can physiology explain movement of juvenile
Atlantic salmon to cool water? Journal of Animal Ecology, 80, 844–853.
Bret, V., Bergerot, B., Capra, H., Gouraud, V., & Lamouroux, N. (2016).
Influence of discharge, hydraulics, water temperature, and dispersal on
density synchrony in brown trout populations (Salmo trutta). Canadian
Journal of Fisheries and Aquatic Sciences, 73, 319–329.
Brinck, P. (1949). Studies on swedish stoneflies (Plecoptera). Opuscula
Entomologica, 11, 1–250.
Brittain, J. E. (1982). Biology of mayflies. Annual Review of Entomology, 27,
119–147.
Brittain, J. E. (1983). The influence of temperature on nymphal growth-
rates in mountain stoneflies (Plecoptera). Ecology, 64, 440–446.
Brittain, J. E. (1990). Life history strategies in Ephemeroptera and
Plecoptera. In I. C. Campbell (Ed.), Mayflies and stoneflies (pp. 1–12).
Doderecht: Kluwer Press.
Brittain, J. E. (1991). Life history charcateristics as a determinant of the
response of mayflies and stoneflies to man-made environmental dis-
turbance. In J. Alba-Tercedor (Ed.), Overview and strategies of
Ephemeroptera anmd Plecoptera (pp. 539–545). Gainesville, Florida:
Sandhill Crane Press.
Brittain, J. E., & Eikeland, T. J. (1988). Invertebrate drift-a review.
Hydrobiologia, 166, 77–93.
Brittain, J. E., & Saltveit, S. J. (1989). A review of the effect of river regula-
tion on mayflies (Ephemeroptera). Regulated Rivers: Research & Man-
agement, 3, 191–204.
Brown, C. J. D., Clothier, W. D., & Alvord, W. (1953). Observations on ice
conditions and bottom organisms in the West Gallatin River. Montana.
Montana Academy of Sciences. 13, 21–27.
Brown, L. E., Hannah, D. M., & Milner, A. M. (2006). Thermal variability
and stream flow permanency in an alpine river system. River Research
and Applications, 22, 493–501.
Bruno, M. C., Maiolini, B., Carolli, M., & Silveri, L. (2009). Impact of hydro-
peaking on hyporheic invertebrates in an alpine stream (Trentino,
Italy). Annales De Limnologie-International Journal of Limnology, 45,
157–170.
Bruno, M. C., Siviglia, A., Carolli, M., & Maiolini, B. (2013). Multiple drift
responses of benthic invertebrates to interacting hydropeaking and
thermopeaking waves. Ecohydrology, 6, 511–522.
Budy, P., Thiede, G. P., Lobon-Cervia, J., Fernandez, G. G., McHugh, P.,
McIntosh, A., … Jellyman, P. (2013). Limitation and facilitation of one
of the world's most invasive fish: An intercontinental comparison. Ecol-
ogy, 94, 356–367.
Butler, R. L., & Hawthorne, V. M. (1979). Anchor ice, its formation and
effects on aquatic life. Science in Agriculture, 26(2), 2.
Byrne, C. J., Poole, R., Dillane, M., Rogan, G., & Whelan, K. F. (2004). Tem-
poral and environmental influences on the variation in sea trout (Salmo
trutta L.) smolt migration in the Burrishoole system in the west of Ire-
land from 1971 to 2000. Fisheries Research, 66, 85–94.
Cadbury, S., Hannah, D., Milner, A., Pearson, C., & Brown, L. (2008).
Stream temperature dynamics within a New Zealand glacierized river
basin. River Research and Applications, 24, 68–89.
Caissie, D. (2006). The thermal regime of rivers: A review. Freshwater Biol-
ogy, 51, 1389–1406.
Casas-Mulet, R., Alfredsen, K., Brabrand, A., & Saltveit, S. J. (2015). Sur-
vival of eggs of Atlantic salmon (Salmo salar) in a drawdown zone of a
regulated river influenced by groundwater. Hydrobiologia, 743,
269–284.
Casas-Mulet, R., Alfredsen, K., Brabrand, Å., & Saltveit, S. J. (2016). Hydro-
power operations in groundwater-influenced rivers: Implications for
Atlantic salmon, Salmo salar, early life stage development and survival.
Fisheries Management and Ecology, 23, 144–151.
Casas-Mulet, R., Alfredsen, K., Hamududu, B., & Timalsina, N. P. (2015).
The effects of hydropeaking on hyporheic interactions based on field
experiments. Hydrological Processes, 29, 1370–1384.
Casas-Mulet, R., Saltveit, S. J., & Alfredsen, K. (2015). The survival of
Atlantic salmon (Salmo salar) eggs during dewatering in a river sub-
jected to hydropeaking. River Research and Applications, 31,
433–446.
Casas-Mulet, R., Saltveit, S. J., & Alfredsen, K. T. (2016). Hydrological and
thermal effects of hydropeaking on early life stages of salmonids: A
modelling approach for implementing mitigation strategies. Science of
the Total Environment, 573, 1660–1672.
Catrinu-Renström, M., & Knudsen, J. (2011). Perspectives on hydropower's
role to balance non-regulated renewable power production in northern
Europe. SINTEF Report. SINTEF, Trondheim. p. 126. Retrieved from
http://hdl.handle.net/11250/2448210
Cereghino, R., Cugny, P., & Lavandier, P. (2002). Influence of intermittent
hydropeaking on the longitudinal zonation patterns of benthic inverte-
brates in a mountain stream. International Review of Hydrobiology, 87,
47–60.
Cheng, Y., Voisin, N., Yearsley, J. R., & Nijssen, B. (2020). Reservoirs Mod-
ify River thermal regime sensitivity to climate change: A case study in
the southeastern United States. Water Resources Research, 56,
e2019WR025784.
Chezik, K. A., Lester, N. P., & Venturelli, P. A. (2014). Fish growth and
degree-days I: Selecting a base temperature for a within-population
study. Canadian Journal of Fisheries and Aquatic Sciences, 71, 47–55.
Clarke, A., & Portner, H. O. (2010). Temperature, metabolic power and the
evolution of endothermy. Biological Reviews, 85, 703–727.
Clifford, H. F. (1972). A years' study of the drifting organisms in a brown-
water stream of Alberta, Canada. Canadian Journal of Zoology, 50,
975–983.
Colbo, M. (1979). Distribution of winter-developing Simuliidae (Diptera),
in eastern Newfoundland. Canadian Journal of Zoology, 57,
2143–2152.
Constantz, J. (1998). Interaction between stream temperature, streamflow,
and groundwater exchanges in alpine streams. Water Resources
Research, 34, 1609–1615.
Coulson, S. J., Convey, P., Aakra, K., Aarvik, L., Avila-Jimenez, M. L.,
Babenko, A., … Zmudczynska-Skarbek, K. (2014). The terrestrial and
freshwater invertebrate biodiversity of the archipelagoes of the
Barents Sea, Svalbard, Franz Josef Land and Novaya Zemlya. Soil Biol-
ogy & Biochemistry, 68, 440–470.
Crespel, A., Bernatchez, L., Garant, D., & Audet, C. (2013). Genetically
based population divergence in overwintering energy mobilization in
brook charr (Salvelinus fontinalis). Genetica, 141, 51–64.
Crozier, L. G., & Hutchings, J. A. (2014). Plastic and evolutionary responses
to climate change in fish. Evolutionary Applications, 7, 68–87.
Cunjak, R. A., Linnansaari, T., & Caissie, D. (2013). The complex interaction
of ecology and hydrology in a small catchment: A salmon's perspec-
tive. Hydrological Processes, 27, 741–749.
HEGGENES ET AL. 759
Dallas, H., & Ross-Gillespie, V. (2015). Sublethal effects of temperature on
freshwater organisms, with special reference to aquatic insects. Water
SA, 41, 712–726.
Danks, H. V. (2008). Aquatic insets adaptations to winter cold and ice. In J.
Lancaster & R. A. Briers (Eds.), Aquatic insects: Challenges to
populations: Proceedings of the Royal Entomological Society's 24th Sym-
posium (pp. 1–19). Wallingford, Oxfordshire, England: CAB
International.
Dekar, M. P., Magoulick, D. D., & Huxel, G. R. (2009). Shifts in the trophic
base of intermittent stream food webs. Hydrobiologia, 635, 263–277.
Dell, A. I., Pawar, S., & Savage, V. M. (2011). Systematic variation in the
temperature dependence of physiological and ecological traits. Pro-
ceedings of the National Academy of Sciences of the United States of
America, 108, 10591–10596.
Dickson, N. E., Carrivick, J. L., & Brown, L. E. (2012). Flow regulation alters
alpine river thermal regimes. Journal of Hydrology, 464, 505–516.
Dugdale, S. J., Malcolm, I. A., Kantola, K., & Hannah, D. M. (2018). Stream
temperature under contrasting riparian forest cover: Understanding
thermal dynamics and heat exchange processes. Science of the Total
Environment, 610, 1375–1389.
Einum, S., Finstad, A. G., Robertsen, G., Nislow, K. H., McKelvey, S., &
Armstrong, J. D. (2012). Natal movement in juvenile Atlantic salmon: A
body size-dependent strategy? Population Ecology, 54, 285–294.
Einum, S., & Fleming, I. A. (2000). Selection against late emergence and
small offspring in Atlantic salmon (Salmo salar). Evolution, 54, 628–639.
Einum, S., Robertsen, G., Nislow, K. H., McKelvey, S., & Armstrong, J. D.
(2011). The spatial scale of density-dependent growth and implications
for dispersal from nests in juvenile Atlantic salmon. Oecologia, 165,
959–969.
Elliott, J. A. (1981). Some aspects of thermal stress on freshwater teleosts.
In A. D. Pickering (Ed.), Stress and fish (pp. 209–246). London: Aca-
demic Press.
Elliott, J. M. (1985). Population regulation for different life-stages of migra-
tory trout salmo-trutta in a lake district stream, 1966-83. Journal of
Animal Ecology, 54, 617–638.
Elliott, J. M. (1989). The natural regulation of numbers and growth in con-
trasting populations of brown trout, salmo-trutta, in 2 lake district
streams. Freshwater Biology, 21, 7–19.
Elliott, J. M. (1994). Quantitative ecology and the Brown trout. Oxford:
Oxford University Press.
Elliott, J. M. (2009). Validation and implications of a growth model for
brown trout, Salmo trutta, using long-term data from a small stream in
north-West England. Freshwater Biology, 54, 2263–2275.
Elliott, J. M. (2011). A comparative study of the relationship between
light intensity and feeding ability in brown trout (Salmo trutta) and
Arctic charr (Salvelinus alpinus). Freshwater Biology, 56,
1962–1972.
Elliott, J. M. (2015). Density-dependent and density-independent growth
in a population of juvenile sea-trout, Salmo trutta, assessed using long-
term data from a small stream in Northwest England. Freshwater Biol-
ogy, 60, 336–346.
Elliott, J. M., & Elliott, J. A. (2010). Temperature requirements of Atlantic
salmon Salmo salar, brown trout Salmo trutta and Arctic charr
Salvelinus alpinus: Predicting the effects of climate change. Journal of
Fish Biology, 77, 1793–1817.
Elliott, J. M., & Hurley, M. A. (1997). A functional model for maximum
growth of Atlantic Salmon parr, Salmo salar, from two populations in
Northwest England. Functional Ecology, 11, 592–603.
Elliott, J. M., & Hurley, M. A. (1998a). Population regulation in adult, but
not juvenile, resident trout (Salmo trutta) in a Lake District stream.
Journal of Animal Ecology, 67, 280–286.
Elliott, J. M., & Hurley, M. A. (1998b). Predicting fluctuations in the size of
newly emerged sea-trout fry in a Lake District stream. Journal of Fish
Biology, 53, 1120–1133.
Elliott, J. M., Hurley, M. A., & Fryer, R. J. (1995). A new, improved
growth-model for brown trout. Salmo trutta. Functional Ecology, 9,
290–298.
Ellis, L. E., & Jones, N. E. (2013). Longitudinal trends in regulated rivers: A
review and synthesis within the context of the serial discontinuity
concept. Environmental Reviews, 21, 136–148.
Embody, G. C. (1934). Relation of temperature to the incubation periods
of eggs of four species of trout. Transactions of the American Fisheries
Society, 64, 281–292.
Ettema, R., Kirkil, G., & Daly, S. (2009). Frazil ice concerns for channels,
pump-lines, penstocks, siphons, and tunnels in mountainous regions.
Cold Regions Science and Technology, 55, 202–211.
Ferreira, V., & Canhoto, C. (2014). Effect of experimental and seasonal
warming on litter decomposition in a temperate stream. Aquatic Sci-
ences, 76, 155–163.
Finni, G. R., & Chandler, L. (1979). The microdistribution of Allocapnia
naiads Plecoptera: Capniidae. Journal of the Kansas Entomologcal
Socoety, 52, 93–102.
Finstad, A., Berg, O. K., Forseth, T., Ugedal, O., & Naesje, T. F. (2010).
Adaptive winter survival strategies: Defended energy levels in juvenile
Atlantic salmon along a latitudinal gradient. Proceedings of the Royal
Society B-Biological Sciences, 277, 1113–1120.
Finstad, A. G., & Forseth, T. (2006). Adaptation to ice-cover conditions in
Atlantic salmon, Salmo salar L. Evolutionary Ecology Research, 8,
1249–1262.
Finstad, A. G., Forseth, T., Faenstad, T. F., & Ugedal, O. (2004). The impor-
tance of ice cover for energy turnover in juvenile Atlantic salmon. Jour-
nal of Animal Ecology, 73, 959–966.
Finstad, A. G., & Jonsson, B. (2012). Effect of incubation temperature on
growth performance in Atlantic salmon. Marine Ecology Progress Series,
454, 75–82.
Finstad, A. G., Naesje, T. F., & Forseth, T. (2004). Seasonal variation in the
thermal performance of juvenile Atlantic salmon (Salmo salar). Freshwa-
ter Biology, 49, 1459–1467.
Forseth, T., Hurley, M. A., Jensen, A. J., & Elliott, J. M. (2001). Functional
models for growth and food consumption of Atlantic salmon parr,
Salmo salar, from a Norwegian river. Freshwater Biology, 46, 173–186.
Forseth, T., & Jonsson, B. (1994). The growth and food ration of piscivo-
rous brown trout (Salmo trutta). Functional Ecology, 8, 171–177.
Forseth, T., Larsson, S., Jensen, A. J., Jonsson, B., Naslund, I., & Berglund, I.
(2009). Thermal growth performance of juvenile brown trout Salmo
trutta: No support for thermal adaptation hypotheses. Journal of Fish
Biology, 74, 133–149.
Forseth, T., Letcher, B. H., & Johansen, M. (2010). The Behavioural flexibil-
ity of Salmon growth. In Atlantic salmon ecology (pp. 145–169). Chich-
ester: Wiley-Blackwell. https://doi.org/10.1002/9781444327755.
Frisbie, M. P., & Lee, R. E. (1997). Inoculative freezing and the problem of
winter survival for freshwater macroinvertebrates. Journal of the North
American Benthological Society, 16, 635–650.
Fry, F. E. J. (1971). The effect of environmental factors on the physiology
of fish. In W. S. Hoar & D. J. Randall (Eds.), Fish physiology (pp. 1–98).
New York, NY: Academic Press.
Gebre, S., Alfredsen, K., Lia, L., Stickler, M., & Tesaker, E. (2013). Ice effects
on hydropower systems-a review. Journal of Cold Regions Engineering,
27, 196–222.
Geris, J., Tetzlaff, D., Seibert, J., Vis, M., & Soulsby, C. (2015). Conceptual
modelling to assess hydrological impacts and evaluate environmental
flow scenarios in montane river systems regulated for hydropower.
River Research and Applications, 31, 1066–1081.
Gibbins, C., Shellberg, J., Moir, H. J., & Soulsby, C. (2008). Hydrological
influences on adult salmonid migration, spawning and embryo survival.
In D. Sear & P. DeVries (Eds.), Salmon psawning habitat in rivers: Physi-
cal controls, biological responses and approaches (pp. 195–224).
Bethesda, Maryland: American Fisheries Society.
760 HEGGENES ET AL.
Gibson, R. J., & Myers, R. A. (1988). Influence of seasonal river discharge
on survival of juvenile Atlantic salmon, salmo-Salar. Canadian Journal
of Fisheries and Aquatic Sciences, 45, 344–348.
Gillespie, B. R., Desmet, S., Kay, P., Tillotson, M. R., & Brown, L. E.
(2015). A critical analysis of regulated river ecosystem responses to
managed environmental flows from reservoirs. Freshwater Biology,
60, 410–425.
Graham, C. T., & Harrod, C. (2009). Implications of climate change for the
fishes of the British Isles. Journal of Fish Biology, 74, 1143–1205.
Gray, R. L. (2016). Effectiveness of cold water pollution mitigation at
Burrendong Dam using an innovative thermal curtain. UTS Digital Theses
Collection. Retrieved from http://hdl.handle.net/10453/44199
Greig, S. M., Sear, D. A., & Carling, P. A. (2007). A review of factors
influencing the availability of dissolved oxygen to incubating salmonid
embryos. Hydrological Processes, 21, 323–334.
Gunderson, A. R., & Stillman, J. H. (2015). Plasticity in thermal tolerance
has limited potential to buffer ectotherms from global warming. Pro-
ceedings of the Royal Society B: Biological Sciences, 282, 20150401.
Gunnes, K. (1979). Survival and development of Atlantic salmon eggs and
fry at 3 different temperatures. Aquaculture, 16, 211–218.
Haapala, A., & Muotka, T. (1998). Seasonal dynamics of detritus and asso-
ciated macroinvertebrates in a channelized boreal stream. Archiv Fur
Hydrobiologie, 142, 171–189.
Haapala, A., Muotka, T., & Markkola, A. (2001). Breakdown and
macroinvertebrate and fungal colonization of alder, birch, and willow
leaves in a boreal forest stream. Journal of the North American
Benthological Society, 20, 395–407.
Hågvar, S., & Østbye, E. (1973). Notes on some Winter-Active
Chironomidae. Norsk entomologisk tidsskrift, 20, 253–257.
Halleraker, J. H., Sundt, H., Alfredsen, K. T., & Dangelmaier, G. (2007).
Application of multiscale environmental flow methodologies as tools
for optimized management of a Norwegian regulated national salmon
watercourse. River Research and Applications, 23, 493–510.
Hamblin, P., & McAdam, S. (2003). Impoundment effects on the thermal
regimes of Kootenay Lake, the Arrow Lakes reservoir and upper
Columbia River. Hydrobiologia, 504, 3–19.
Hanna, R., Saito, L., Bartholow, J., & Sandelin, J. (1999). Results of simu-
lated temperature control device operations on in-reservoir and dis-
charge water temperatures using CE-QUAL-W2. Lake and Reservoir
Management, 15, 87–102.
Hansen, L. P., & Jonsson, B. (1985). Downstream migration of hatchery-
reared smolts of Atlantic salmon (Salmo salar L) in the river Imsa, Nor-
way. Aquaculture, 45, 237–248.
Harper, P., & Hynes, H. (1970). Diapause in the nymphs of Canadian win-
ter stoneflies. Ecology, 51, 925–927.
Hartman, K. J., & Porto, M. A. (2014). Thermal performance of three rain-
bow trout strains at above-optimal temperatures. Transactions of the
American Fisheries Society, 143, 1445–1454.
Haxton, T. J., & Findlay, C. S. (2008). Meta-analysis of the impacts of water
management on aquatic communities. Canadian Journal of Fisheries and
Aquatic Sciences, 65, 437–447.
Hayes, J. W. (2013). Brown trout growth models: User guide version 2.1.
Cawthron Report No. 571A. Cawthron Institute, Cawthron Institute,
Nelson, New Zealand. p. 46.
Hayes, J. W., Stark, J. D., & Shearer, K. A. (2000). Development and test of
a whole-lifetime foraging and bioenergetics growth model for drift-
feeding brown trout. Transactions of the American Fisheries Society,
129, 315–332.
Hedger, R. D., Sundt-Hansen, L. E., Forseth, T., Ugedal, O., Diserud, O. H.,
Kvambekk, A. S., & Finstad, A. G. (2013). Predicting climate change
effects on subarctic-Arctic populations of Atlantic salmon (Salmo salar).
Canadian Journal of Fisheries and Aquatic Sciences, 70, 159–168.
Heggberget, T. G., & Wallace, J. C. (1984). Incubation of the eggs of Atlan-
tic salmon, Salmo salar, at low-temperatures. Canadian Journal of Fish-
eries and Aquatic Sciences, 41, 389–391.
Heggenes, J., Alfredsen, K., Brittain, J. E., Bustos, A. A., Huusko, A., &
Stickler, M. (2017). Stay cool: Temperature changes and biological
responses in hydropower-regulated northern stream systems: HSN Skrift
(p. 95). Bø i Telemark: University College of Southeast Norway. http://
hdl.handle.net/11250/2477901.
Heggenes, J., Alfredsen, K., Bustos, A. A., Huusko, A., & Stickler, M. (2017).
Be cool: A review of hydro-physical changes and fish responses in win-
ter in hydropower-regulated northern streams. Environmental Biology
of Fishes, 101, 1–21.
Heino, J., Erkinaro, J., Huusko, A., & Luoto, M. (2016). Climate change
effects on freshwater fishes, conservation and management. In G. P.
Closs, M. Krkosek, & J. D. Olden (Eds.), Conservation of freshater fishes
(pp. 76–106). Cambridge: Cambridge University Press, UK.
Hildebrand, S. G. (1974). Relation of drift to benthos density and food level
in an artificial stream. Limnology and Oceanography, 19, 951–957.
Hoffsten, P. O. (2003). Effects of an extraordinarily harsh winter on
macroinvertebrates and fish in boreal streams. Archiv Fur
Hydrobiologie, 157, 505–523.
Huryn, A. D., & Wallace, J. B. (2000). Life history and production of stream
insects. Annual Review of Entomology, 45, 83–110.
Hutchison, V. H., & Maness, J. D. (1979). The role of behavior in tempera-
ture acclimation and tolerance in ectotherms. American Zoologist, 19,
367–384.
Huusko, A., Maki-Petays, A., Stickler, M., & Mykra, H. (2011). Fish can
shrink under harsh living conditions. Functional Ecology, 25,
628–633.
Hynes, H. B. N. (1970). The ecology of running waters. Toronto, Canada: Uni-
versity of Toronto Press. https://doi.org/10.4319/lo.1971.16.3.0593.
Irons, J. G., Miller, L. K., & Oswood, M. W. (1993). Ecological adaptations
of aquatic macroinvertebrates to overwintering in interior Alaska
(USA) subarctic streams. Canadian Journal of Zoology, 71, 98–108.
Jackson, H., Gibbins, C., & Soulsby, C. (2007). Role of discharge and tem-
perature variation in determining invertebrate community structure in
a regulated river. River Research and Applications, 23, 651–669.
Jensen, A. J. (1990). Growth of young migratory brown trout Salmo trutta
correlated with water temperature in Norwegian Rivers. Journal of Ani-
mal Ecology, 59, 603–614.
Jensen, A. J., Forseth, T., & Johnsen, B. O. (2000). Latitudinal variation in
growth of young brown trout Salmo trutta. Journal of Animal Ecology,
69, 1010–1020.
Jensen, A. J., & Johnsen, B. O. (1999). The functional relationship between
peak spring floods and survival and growth of juvenile Atlantic Salmon
(Salmo salar) and Brown trout (Salmo trutta). Functional Ecology, 13,
778–785.
Jensen, A. J., Johnsen, B. O., & Heggberget, T. G. (1991). Initial feeding
time of Atlantic Salmon, Salmo salar, alevins compared to river flow
and water temperature in norwegian streams. Environmental Biology of
Fishes, 30, 379–385.
Jensen, A. J., Johnsen, B. O., & Saksgard, L. (1989). Temperature require-
ments in Atlantic salmon (Salmo salar), brown trout (Salmo trutta), and
Arctic char (Salvelinus alpinus) from hatching to initial feeding com-
pared with geographic-distribution. Canadian Journal of Fisheries and
Aquatic Sciences, 46, 786–789.
Jensen, L. F., Hansen, M. M., Pertoldi, C., Holdensgaard, G.,
Mensberg, K. L. D., & Loeschcke, V. (2008). Local adaptation in brown
trout early life-history traits: Implications for climate change adaptabil-
ity. Proceedings of the Royal Society B-Biological Sciences, 275,
2859–2868.
Jonsson, B., Forseth, T., Jensen, A. J., & Naesje, T. F. (2001). Thermal per-
formance of juvenile Atlantic Salmon, Salmo salar L. Functional Ecology,
15, 701–711.
Jonsson, B., & Jonsson, N. (2009). A review of the likely effects of climate
change on anadromous Atlantic salmon Salmo salar and brown trout
Salmo trutta, with particular reference to water temperature and flow.
Journal of Fish Biology, 75, 2381–2447.
HEGGENES ET AL. 761
Jonsson, B., & Labée-Lund, J. H. (1993). Latitudinal clines in life-history
variables of anadromous brown trout in Europe. Journal of Fish Biology,
43, 1–16.
Jonsson, B., & Ruud-Hansen, J. (1985). Water temperature as the primary
influence on timing of seaward migrations of Atlantic salmon (Salar
salar) smolts. Canadian Journal of Fisheries and Aquatic Sciences, 42,
593–595.
Jonsson, N., Jonsson, B., & Hansen, L. P. (1998). Long-term study of the
ecology of wild Atlantic salmon smolts in a small Norwegian river.
Journal of Fish Biology, 52, 638–650.
Juanes, F., Gephard, S., & Beland, K. (2004). Long-term changes in migra-
tion timing of adult Atlantic salmon (Salmo salar) at the southern edge
of the species distribution. Canadian Journal of Fisheries and Aquatic
Sciences, 61, 2392–2400.
Jungwirth, M., & Winkler, H. (1984). The temperature-dependence of
embryonic-development of grayling (Thymallus thymallus), Danube
salmon (Hucho hucho), Arctic char (Salvelinus alpinus) and brown trout
(Salmo trutta fario). Aquaculture, 38, 315–327.
Kane, T. R. (1988). Relationship of temperature and time of initial feeding
of Atlantic salmon. Progressive Fish-Culturist, 50, 93–97.
King, H. R., Pankhurst, N. W., Watts, M., & Pankhurst, P. M. (2003). Effect
of elevated summer temperatures on gonadal steroid production, vitel-
logenesis and egg quality in female Atlantic salmon. Journal of Fish Biol-
ogy, 63, 153–167.
Kvambekk, A. S. (2004). Vanntemperaturer i Suldalslågen. Simulering av ure-
gulert tilstand i 1931-2002 og ulike skisseforslag til nytt van-
nføringsregime. NVE Miljørapport, Norges Vassdrags og
Energidirektorat, Norges Vassdrags og Energidirektorat Oslo. p. 50.
Retrieved from https://www.statkraft.com/globalassets/old-contains-
the-old-folder-structure/documents/no/31—vanntemperaturer-i-
suldalslagen.-simulering-av-uregulert-tilstand-i-1931-2002-og-ulike-
skisseforslag-til-nytt-vannforingsregime_tcm10-4190.pdfhttps://
www.statkraft.com/globalassets/old-contains-the-old-folder-
structure/documents/no/31—vanntemperaturer-i-suldalslagen.-
simulering-av-uregulert-tilstand-i-1931-2002-og-ulike-skisseforslag-
til-nytt-vannforingsregime_tcm10-4190.pd
Kvambekk, Å. S. (2012). Vanntemperatur i kraftverksmagasiner-Hvilke tem-
peraturforskjeller kan oppnås ved bruk av flere inntaksdyp? Rapport
Miljøbasert Vannføring, Norges Vassdrags og Energidirektorat, Norges
Vassdrags og Energidirektorat Oslo. p. 33. Retrieved from https://
publikasjoner.nve.no/rapport_miljoebasert_vannfoering/2012/
miljoebasert2012_07.pdf
Larsson, S., & Berglund, I. (2006). Thermal performance of juvenile Atlantic
salmon (Salmo salar L.) of Baltic Sea origin. Journal of Thermal Biology,
31, 243–246.
Lencioni, V. (2004). Survival strategies of freshwater insects in cold envi-
ronments. Journal of Limnology, 63, 45–55.
Letcher, B. H., Dubreuil, T., O'Donnell, M. J., Obedzinski, M.,
Griswold, K., & Nislow, K. H. (2004). Long-term consequences of varia-
tion in timing and manner of fry introduction on juvenile Atlantic
salmon (Salmo salar) growth, survival, and life-history expression.
Canadian Journal of Fisheries and Aquatic Sciences, 61, 2288–2301.
Lien, L. (1978). The energy budget of the Brown trout population of Øvre
Heimdalsvatn. Holarctic Ecology, 1, 279–300.
Lillehammer, A. (1987). Diapause and quiescence in eggs of
Systellognatha stonefly species (Plecoptera) occuring in alpine
areas of Norway. Annales de Limnologie - International Journal of
Limnology, 23, 179–184.
Lillehammer, A., Brittain, J. E., Saltveit, S. J., & Nielsen, P. S. (1989). Egg
development, nymphal growth and life-cycle strategies in Plecoptera.
Holarctic Ecology, 12, 173–186.
Linnansaari, T., Alfredsen, K., Stickler, M., Arnekleiv, J. V., Harby, A., &
Cunjak, R. A. (2009). Does ice matter? Site fidelity and movements by
Atlantic salmon (Salmo salar l.) parr during winter in a substrate
enhanced river reach. River Research and Applications, 25, 773–787.
Lobon-Cervia, J. (2014). Recruitment and survival rate variability in fish
populations: Density-dependent regulation or further evidence of
environmental determinants? Canadian Journal of Fisheries and Aquatic
Sciences, 71, 290–300.
Lobon-Cervia, J., & Mortensen, E. (2005). Population size in stream-living
juveniles of lake-migratory brown trout Salmo trutta L.: The impor-
tance of stream discharge and temperature. Ecology of Freshwater Fish,
14, 394–401.
Lobon-Cervia, J., & Rincon, P. A. (1998). Field assessment of the influence
of temperature on growth rate in a brown trout population. Transac-
tions of the American Fisheries Society, 127, 718–728.
Logez, M., & Pont, D. (2011). Variation of brown trout Salmo trutta young-
of-the-year growth along environmental gradients in Europe. Journal
of Fish Biology, 78, 1269–1276.
Lugg, A., & Copeland, C. (2014). Review of cold water pollution in the
Murray–Darling basin and the impacts on fish communities. Ecological
Management & Restoration, 15, 71–79.
Magnuson, J. J., Crowder, L. B., & Medvick, P. A. (1979). Temperature as
an ecological resource. American Zoologist, 19, 331–343.
Malcolm, I. A., Gibbins, C. N., Soulsby, C., Tetzlaff, D., & Moir, H. J. (2012).
The influence of hydrology and hydraulics on salmonids between
spawning and emergence: Implications for the management of flows in
regulated rivers. Fisheries Management and Ecology, 19, 464–474.
Martin, M. D., Brown, R. S., Barton, D. R., & Power, G. (2001). Abundance
of stream invertebrates in winter: Seasonal changes and effects of
river ice. Canadian Field-Naturalist, 115, 68–74.
McCormick, S. D., Hansen, L. P., Quinn, T. P., & Saunders, R. L. (1998).
Movement, migration, and smolting of Atlantic salmon (Salmo salar).
Canadian Journal of Fisheries and Aquatic Sciences, 55, 77–92.
Meier, K., Hansen, M. M., Normandeau, E., Mensberg, K. L. D.,
Frydenberg, J., Larsen, P. F., … Bernatchez, L. (2014). Local adaptation
at the Transcriptome level in Brown trout: Evidence from early life his-
tory temperature genomic reaction norms. PLoS One, 9, e85171.
Metcalfe, N. B., Fraser, N. H. C., & Burns, M. D. (1999). Food availability
and the nocturnal vs. diurnal foraging trade-off in juvenile salmon.
Journal of Animal Ecology, 68, 371–381.
Metcalfe, N. B., & Thorpe, J. E. (1990). Determinants of geographical varia-
tion in the age of seaward migrating salmon, Salmo salar. Journal of Ani-
mal Ecology, 59, 135–145.
Miles, N. G., & West, R. J. (2011). The use of an aeration system to pre-
vent thermal stratification of a freshwater impoundment and its
effect on downstream fish assemblages. Journal of Fish Biology, 78,
945–952.
Miller, S. W., & Judson, S. (2014). Responses of macroinvertebrate drift,
benthic assemblages, and trout foraging to hydropeaking. Canadian
Journal of Fisheries and Aquatic Sciences, 71, 675–687.
Milner, N. J., Elliott, J. M., Armstrong, J. D., Gardiner, R., Welton, J. S., &
Ladle, M. (2003). The natural control of salmon and trout populations
in streams. Fisheries Research, 62, 111–125.
Milner, N. J., Solomon, D. J., & Smith, G. W. (2012). The role of river flow
in the migration of adult Atlantic salmon, Salmo salar, through estuaries
and rivers. Fisheries Management and Ecology, 19, 537–547.
Montgomery, D. R., & Buffington, J. M. (1997). Channel-reach morphology
in mountain drainage basins. Geological Society of America Bulletin,
109, 596–611.
Moore, A., Bendall, B., Barry, J., Waring, C., Crooks, N., & Crooks, L.
(2012). River temperature and adult anadromous Atlantic salmon,
Salmo salar, and brown trout, Salmo trutta. Fisheries Management and
Ecology, 19, 518–526.
Morin, A., Rodriguez, M. A., & Nadon, D. (1995). Temporal and environ-
mental variation in the biomass spectrum of benthic invertebrates in
streams - an application of thin-plate splines and relative warp analy-
sis. Canadian Journal of Fisheries and Aquatic Sciences, 52, 1881–1892.
Myers, D. N. (2019). Innovations in monitoring with water-quality sensors
with case studies on floods, hurricanes, and harmful algal blooms. In
762 HEGGENES ET AL.
Ahuja Satinder Separation science and technology (pp. 219–283). Cam-
bridge, MA: Academic Press.
Nebeker, A. V. (1971). Effect of high winter water temperatures on adult
emergence of aquatic insects. Water Research, 5, 777–783.
Nicola, G. G., & Almodovar, A. (2004). Growth pattern of stream-dwelling
brown trout under contrasting thermal conditions. Transactions of the
American Fisheries Society, 133, 66–78.
Nuhfer, A. J., Zorn, T. G., & Wills, T. C. (2017). Effects of reduced summer
flows on the brook trout population and temperatures of a
groundwater-influenced stream. Ecology of Freshwater Fish, 26,
108–119.
Ojanguren, A. F., & Brana, F. (2003). Thermal dependence of embryonic
growth and development in brown trout. Journal of Fish Biology, 62,
580–590.
Ojanguren, A. F., Reyes-Gavilan, F. G., & Munoz, R. R. (1999). Effects of
temperature on growth and efficiency of yolk utilisation in eggs and
pre-feeding larval stages of Atlantic salmon. Aquaculture International,
7, 81–87.
Olden, J. D., Konrad, C. P., Melis, T. S., Kennard, M. J., Freeman, M. C.,
Mims, M. C., … Lytle, D. A. (2014). Are large-scale flow experiments
informing the science and management of freshwater ecosystems?
Frontiers in Ecology and the Environment, 12, 176–185.
Olden, J. D., & Naiman, R. J. (2010). Incorporating thermal regimes into
environmental flows assessments: Modifying dam operations to
restore freshwater ecosystem integrity. Freshwater Biology, 55,
86–107.
Olsson, T. I. (1981). Overwintering of benthic macroinvertebrates in ice
and frozen sediment in a north Swedish river. Holarctic Ecology, 4,
161–166.
Olsson, T. I. (1983). Seasonal variation in the lateral distribution of mayfly
nymphs in a boreal river. Holarctic Ecology, 6, 333–339.
Ornsrud, R., Gil, L., & Waagbo, R. (2004). Teratogenicity of elevated egg
incubation temperature and egg vitamin a status in Atlantic salmon,
Salmo salar L. Journal of Fish Diseases, 27, 213–223.
Ornsrud, R., Wargelius, A., Saele, O., Pittman, K., & Waagbo, R. (2004).
Influence of egg vitamin a status and egg incubation temperature on
subsequent development of the early vertebral column in Atlantic
salmon fry. Journal of Fish Biology, 64, 399–417.
Osterkamp, T., & Gosink, J. (1983). Frazil ice formation and ice cover
development in interior Alaska streams. Cold Regions Science and Tech-
nology, 8, 43–56.
Oswood, M. W., Miller, L. K., & Irons, J. G. (1991). Overwintering of fresh-
water benthic macroinvertebrates. In R. E. J. Lee & D. L. Denlinger
(Eds.), Insects at low temperature (pp. 360–375). New York, NY: Chap-
man and Hall.
Otero, J., L'Abée-Lund, J. H., Castro-Santos, T., Leonardsson, K.,
Storvik, G. O., Jonsson, B., … Baglinière, J. L. (2014). Basin-scale phe-
nology and effects of climate variability on global timing of initial sea-
ward migration of Atlantic salmon (Salmo salar). Global Change Biology,
20, 61–75.
Palmer, M. A., Lettenmaier, D. P., Poff, N. L., Postel, S. L., Richter, B., &
Warner, R. (2009). Climate change and river ecosystems: Protection
and adaptation options. Environmental Management, 44, 1053–1068.
Pardo, I., Campbell, I., & Brittain, J. (1998). Influence of dam operation on
mayfly assemblage structure and life histories in two south-eastern
Australian streams. Regulated Rivers: Research & Management, 14,
285–295.
Peel, M. C., Finlayson, B. L., & McMahon, T. A. (2007). Updated world map
of the Köppen-Geiger climate classification. Hydrology and Earth Sys-
tem Sciences Discussions, 4, 439–473.
Petrin, Z., Brittain, J. E., & Saltveit, S. J. (2013). Mayfly and stonefly species
traits and species composition reflect hydrological regulation: A meta-
analysis. Freshwater Science, 32, 425–437.
Poff, N. L., & Zimmerman, J. K. (2010). Ecological responses to altered flow
regimes: A literature review to inform the science and management of
environmental flows. Freshwater Biology, 55, 194–205.
Poole, G. C., & Berman, C. H. (2001). An ecological perspective on in-
stream temperature: Natural heat dynamics and mechanisms of
human-causedthermal degradation. Environmental Management, 27,
787–802.
Portner, H. O. (2006). Climate-dependent evolution of Antarctic ecto-
therms: An integrative analysis. Deep-sea research part Ii-topical studies
in oceanography, 53, 1071–1104.
Portner, H. O. (2010). Oxygen- and capacity-limitation of thermal toler-
ance: A matrix for integrating climate-related stressor effects in marine
ecosystems. Journal of Experimental Biology, 213, 881–893.
Pörtner, H.-O. (2002). Physiological basis of temperature-dependent
biogeography: Trade-offs in muscle design and performance in
polar ectotherms. Journal of Experimental Biology, 205,
2217–2230.
Prowse, T., Alfredsen, K., Beltaos, S., Bonsal, B. R., Bowden, W. B.,
Duguay, C. R., … Weyhenmeyer, G. A. (2011). Effects of changes in
Arctic Lake and river ice. Ambio, 40, 63–74.
Prowse, T. D. (2001). River-ice ecology. II: Biological aspects. Journal of
Cold Regions Engineering, 15, 17–33.
Quinn, T. P., McGinnity, P., & Reed, T. E. (2016). The paradox of "prema-
ture migration" by adult anadromous salmonid fishes: Patterns and
hypotheses. Canadian Journal of Fisheries and Aquatic Sciences, 73,
1015–1030.
Raddum, G. G. (1985). Effects of winter warm reservoir release on benthic
stream invertebrates. Hydrobiologia, 122, 105–111.
Raddum, G. G., & Fjellheim, A. (1993). Life cycle and production of Baetis
rhodani in a regulated river in Western Norway: Comparison of pre-
and post-regulation conditions. Regulated Rivers: Research & Manage-
ment, 8, 49–61.
Raddum, G. G., Fjellheim, A., & Velle, G. (2008). Increased growth and dis-
tribution of Ephemerella aurivillii (Ephemeroptera) after hydropower
regulation of the Aurland catchment in Western Norway. River
Research and Applications, 24, 688–697.
Reiser, D. W., & Wesche, T. A. (1979). In situ freezing as a cause of mortal-
ity in brown trout eggs. The Progressive Fish-Culturist, 41, 58–60.
Renofalt, B. M., Jansson, R., & Nilsson, C. (2010). Effects of hydropower
generation and opportunities for environmental flow management in
Swedish riverine ecosystems. Freshwater Biology, 55, 49–67.
Riedl, C., & Peter, A. (2013). Timing of brown trout spawning in alpine riv-
ers with special consideration of egg burial depth. Ecology of Freshwa-
ter Fish, 22, 384–397.
Rimmer, D. M., Paim, U., & Saunders, L. (1984). Changes in the selection of
microhabitat by juvenile Atlantic salmon (Salmo salar) at the summer-
autumn transition in a small river. Canadian Journal of Fisheries and
Aquatic Sciences, 41, 469–475.
Rishel, G. B., Lynch, J. A., & Corbett, E. S. (1982). Seasonal stream tempera-
ture changes following Forest harvesting. Journal of Environmental
Quality, 11, 112–116.
Rivers-Moore, N. A., Dallas, H. F., & Ross-Gillespie, V. (2013). Life history
does matter in assessing potential ecological impacts of thermal
changes on aquatic macroinvertebrates. River Research and Applica-
tions, 29, 1100–1109.
Robards, M. D., & Quinn, T. P. (2002). The migratory timing of adult
summer-run steelhead in the Columbia River over six decades of envi-
ronmental change. Transactions of the American Fisheries Society, 131,
523–536.
Roberts, J. H., Anderson, G. B., & Angermeier, P. L. (2016). A long-term
study of ecological impacts of river channelization on the population
of an endangered fish: Lessons learned for assessment and restoration.
Water, 8, 240.
HEGGENES ET AL. 763
Rolls, R. J., Leigh, C., & Sheldon, F. (2012). Mechanistic effects of low-flow
hydrology on riverine ecosystems: Ecological principles and conse-
quences of alteration. Freshwater Science, 31, 1163–1186.
Ross-Gillespie, V., Picker, M. D., Dallas, H. F., & Day, J. A. (2018). The role
of temperature in egg development of three aquatic insects Lestagella
penicillata (Ephemeroptera), Aphanicercella scutata (Plecoptera), Chi-
marra ambulans (Trichoptera) from South Africa. Journal of Thermal
Biology, 71, 158–170.
Saltveit, S. J. (1990). Effect of decreased temperature on growth and
smoltification of juvenile Atlantic salmon (Salmo salar) and brown trout
(Salmo trutta) in a norwegian regulated river. Regulated Rivers:
Research & Management, 5, 295–303.
Saltveit, S. J., & Brabrand, A. (2013). Incubation, hatching and survival of
eggs of Atlantic salmon (Salmo salar) in spawning redds influenced by
groundwater. Limnologica, 43, 325–331.
Saltveit, S. J., Bremnes, T., & Brittain, J. E. (1994). Effect of a changed tem-
perature regime on the benthos of a Norwegian regulated river. Regu-
lated Rivers-Research & Management, 9, 93–102.
Saltveit, S. J., Brittain, J. E., & Lillehammer, A. (1987). Stoneflies and river
regulation. In J. F. Craig & J. B. Kemper (Eds.), Regulated streams
(pp. 117–129). New York, NY: Plenum.
Sanchez-Hernandez, J., Gabler, H. M., Elliott, J. M., & Amundsen, P. A.
(2016). Use of a growth model to assess the suboptimal growth of
Atlantic salmon parr in a subarctic river. Ecology of Freshwater Fish, 25,
518–526.
Saraniemi, M., Huusko, A., & Tahkola, H. (2008). Spawning migration and
habitat use of adfluvial brown trout, Salmo trutta, in a strongly sea-
sonal boreal river. Boreal Environment Research, 13, 121–132.
Sear, D. A., & DeVries, P. (2008). Salmonid spawning habitat in Rivers: Physi-
cal controls, biological responses, and approaches to remediation.
Bethesda, MA: American Fisheries Society. https://doi.org/10.47886/
9781934874035.
Sertic Peric, M., & Robinson, C. T. (2015). Spatio-temporal shifts of
macroinvertebrate drift and benthos in headwaters of a retreating gla-
cier. Hydrobiologia, 751, 25–41.
Sherman, B. (2000). Scoping options for mitigating cold water discharges
from dams. CSIRO Land and Water Canberra. Consultancy Report
00/21, May 2000. https://www.researchgate.net/profile/Bradford-
Sherman/publication/253447153_Scoping_Options_for_Mitigating_
Cold_Water_Discharges_from_Dams/links/0deec5347621be003a000000/
Scoping-Options-for-Mitigating-Cold-Water-Discharges-from-Dams.pdf
Sherman, B., Todd, C. R., Koehn, J. D., & Ryan, T. (2007). Modelling the
impact and potential mitigation of cold water pollution on Murray cod
populations downstream of Hume dam, Australia. River Research and
Applications, 23, 377–389.
Shields, B. A., Stubbing, D. N., Summers, D. W., & Giles, N. (2005). Tempo-
ral and spatial segregation of spawning by wild and farm-reared brown
trout, Salmo trutta L., in the river Avon, Wiltshire, UK. Fisheries Man-
agement and Ecology, 12, 77–79.
Shuter, B. J., Finstad, A. G., Helland, I. P., Zweimuller, I., & Holker, F.
(2012). The role of winter phenology in shaping the ecology of fresh-
water fish and their sensitivities to climate change. Aquatic Sciences,
74, 637–657.
Skoglund, H., Einum, S., Forseth, T., & Barlaup, B. T. (2011). Phenotypic
plasticity in physiological status at emergence from nests as a
response to temperature in Atlantic salmon (Salmo salar). Canadian
Journal of Fisheries and Aquatic Sciences, 68, 1470–1479.
Skoglund, H., Einum, S., Forseth, T., & Barlaup, B. T. (2012). The penalty
for arriving late in emerging salmonid juveniles: Differences between
species correspond to their interspecific competitive ability. Functional
Ecology, 26, 104–111.
Solomon, D. J., & Lightfoot, G. W. (2008). The thermal biology of brown
trout and Atlantic salmon. Science Reports, Environment Agency, Rio
House, Waterside Drive, Aztec West, Almondsbury, Bristol, BS32
4UD. p. 42. Retrieved from https://assets.publishing.service.gov.uk/
government/uploads/system/uploads/attachment_data/file/291741/
scho0808bolv-e-e.pdf
Solomon, D. J., & Sambrook, H. T. (2004). Effects of hot dry summers on
the loss of Atlantic salmon, Salmo salar, from estuaries in south West
England. Fisheries Management and Ecology, 11, 353–363.
Staurnes, M., Sigholt, T., & Gulseth, O. A. (1994). Effects of seasonal-
changes in water temperature on the parr-smolt transformation of
Atlantic salmon and anadromous Arctic char. Transactions of the Ameri-
can Fisheries Society, 123, 408–415.
Stewart, D. C., Middlemas, S. J., & Youngson, A. F. (2006). Population
structuring in Atlantic salmon (Salmo salar): Evidence of genetic influ-
ence on the timing of smolt migration in sub-catchment stocks. Ecol-
ogy of Freshwater Fish, 15, 552–558.
Stickler, M., & Alfredsen, K. T. (2009). Anchor ice formation in streams: A
field study. Hydrological Processes, 23, 2307–2315.
Stonecypher, R. W., Hubert, W. A., & Gern, W. A. (1994). Effect of reduced
incubation temperatures on survival of trout embryos. The Progressive
Fish-Culturist, 56, 180–184.
Sunday, J., Bennett, J. M., Calosi, P., Clusella-Trullas, S., Gravel, S.,
Hargreaves, A. L., … Morales-Castilla, I. (2019). Thermal tolerance pat-
terns across latitude and elevation. Philosophical Transactions of the
Royal Society B, 374, 20190036.
Svensson, P.-O. (1966). Growth of nymphs of stream living stoneflies
(Plecoptera) in northern Sweden. Oikos, 17, 197–206.
Syrjanen, J., Kiljunen, M., Karjalainen, J., Eloranta, A., & Muotka, T. (2008).
Survival and growth of brown trout Salmo trutta L. embryos and the
timing of hatching and emergence in two boreal lake outlet streams.
Journal of Fish Biology, 72, 985–1000.
Takle, H., Baeverfjord, G., & Andersen, O. (2004). Identification of stress
sensitive genes in hyperthermic Atlantic salmon (Salmo salar)
embryos by RAP-PCR. Fish Physiology and Biochemistry, 30,
275–281.
Takle, H., Baeverfjord, G., Helland, S., Kjorsvik, E., & Andersen, O. (2006).
Hyperthermia induced atrial natriuretic peptide expression and deviant
heart development in Atlantic salmon Salmo salar embryos. General
and Comparative Endocrinology, 147, 118–125.
Taranger, G. L., & Hansen, T. (1993). Ovulation and egg survival following
exposure of Atlantic salmon, Salmo salar L., broodstock to different
water temperatures. Aquaculture Research, 24, 151–156.
Tauro, F., Selker, J., Van De Giesen, N., Abrate, T., Uijlenhoet, R.,
Porfiri, M., … Benveniste, J. (2018). Measurements and observations
in the XXI century (MOXXI): Innovation and multi-disciplinarity to
sense the hydrological cycle. Hydrological Sciences Journal, 63,
169–196.
Tetzlaff, D., Gibbins, C., Bacon, P. J., Youngson, A. F., & Soulsby, C. (2008).
Influence of hydrological regimes on the pre-spawning entry of Atlan-
tic salmon (Salmo salar L.) into an upland river. River Research and Appli-
cations, 24, 528–542.
Thorstad, E. B., Okland, F., Aarestrup, K., & Heggberget, T. G. (2008). Fac-
tors affecting the within-river spawning migration of Atlantic salmon,
with emphasis on human impacts. Reviews in Fish Biology and Fisheries,
18, 345–371.
Timalsina, N. P., Alfredsen, K. T., & Killingtveit, A. (2015). Impact of climate
change on ice regime in a river regulated for hydropower. Canadian
Journal of Civil Engineering, 42, 634–644.
Timalsina, N. P., Charmasson, J., & Alfredsen, K. T. (2013). Simulation of
the ice regime in a Norwegian regulated river. Cold Regions Science and
Technology, 94, 61–73.
Toffolon, M., Siviglia, A., & Zolezzi, G. (2010). Thermal wave dynamics in
rivers affected by hydropeaking. Water Resources Research, 46
(8), 1–18.
Turcotte, B., & Morse, B. (2013). A global river ice classification model.
Journal of Hydrology, 507, 134–148.
Tvede, A. (2006). Vanntemperatur og isforhold. Økologiske Forhold i
Vassdrag-Konsekvenser Av vannføringsendringer. En Sammenstilling
764 HEGGENES ET AL.
Av Dagens Kunnskap, pp. 27–34. Retrieved from http://publikasjoner.
nve.no/diverse/2006/oekologiskeforholdivassdrag2006.pdf
Tvede, A. M. (1994). Discharge, water temperature and glaciers in the
Aurland river basin. Norsk Geografisk Tidsskrift-Norwegian Journal of
Geography, 48, 23–28.
Ugedal, O., Naesje, T. F., Thorstad, E. B., Forseth, T., Saksgard, L. M., &
Heggberget, T. G. (2008). Twenty years of hydropower regulation in
the river Alta: Long-term changes in abundance of juvenile and adult
Atlantic salmon. Hydrobiologia, 609, 9–23.
Valdimarsson, S. K., & Metcalfe, N. B. (2001). Is the level of aggression and
dispersion in territorial fish dependent on light intensity? Animal
Behaviour, 61, 1143–1149.
Vannote, R. L., Minshall, G. W., Cummins, K. W., Sedell, J. R., &
Cushing, C. E. (1980). River continuum concept. Canadian Journal of
Fisheries and Aquatic Sciences, 37, 130–137.
Vannote, R. L., & Sweeney, B. W. (1980). Geographic analysis of thermal
equilibria - a conceptual-model for evaluating the effect of natural and
modified thermal regimes on aquatic insect communities. American
Naturalist, 115, 667–695.
Vanzo, D., Siviglia, A., Carolli, M., & Zolezzi, G. (2016). Characterization of
sub-daily thermal regime in alpine rivers: Quantification of alterations
induced by hydropeaking. Hydrological Processes, 30, 1052–1070.
Vanzo, D., Tancon, M., Zolezzi, G., Alfredsen, K. & Siviglia, A. (2016). A
modeling approach for the quantification of fish stranding risk: The case
of Lundesokna River (Norway). Paper presented at: Proceedings from
the 11th International Symposium on Ecohydraulics (ISE 2016). Engi-
neers Australia pp. 326–333. Retrieved from https://search.informit.
org/doi/10.3316/informit.687014727486689
Vermeyen, T. (2000). Application of flexible curtains to control mixing and
enable selective withdrawal in reservoirs. IAHR, Proceedings from the
5th International Symposium on Stratified Flows, Vancouver, Canada.
Retrieved from http://www.project2105.org/TC/PAP-0847.pdf
Vikingstad, E., Andersson, E., Hansen, T. J., Norberg, B., Mayer, I.,
Stefansson, S. O., … Taranger, G. L. (2016). Effects of temperature on
the final stages of sexual maturation in Atlantic salmon (Salmo salar L.).
Fish Physiology and Biochemistry, 42, 895–907.
Vikingstad, E., Andersson, E., Norberg, B., Mayer, I., Klenke, U., Zohar, Y.,
… Taranger, G. L. (2008). The combined effects of temperature and
GnRHa treatment on the final stages of sexual maturation in Atlantic
salmon (Salmo salar L.) females. Fish Physiology and Biochemistry, 34,
289–298.
Vinson, M. R. (2001). Long-term dynamics of an invertebrate assemblage
downstream from a large dam. Ecological Applications, 11, 711–730.
Wallace, J. C., & Heggberget, T. G. (1988). Incubation of eggs of Atlantic
salmon (Salmo salar) from different Norwegian streams at tempera-
tures below 1-degrees-c. Canadian Journal of Fisheries and Aquatic Sci-
ences, 45, 193–196.
Ward, J. (1994). Ecology of alpine streams. Freshwater Biology, 32, 277–294.
Ward, J. V., & Stanford, J. (1982). Thermal responses in the evolutionary
ecology of aquatic insects. Annual Review of Entomology, 27, 97–117.
Ward, J. V., & Stanford, J. A. (1979). Ecological factors controlling stream
zoobenthos with emphasis on thermal modification of regulated streams.
In The ecology of regulated streams (pp. 35–55). Boston: Springer.
Waringer, J. A. (1992). The drifting of invertebrates and particulate
organic-matter in an Austrian mountain brook. Freshwater Biology, 27,
367–378.
Webb, B., & Walling, D. (1988). Modification of temperature behaviour
through regulation of a British river system. River Research and Applica-
tions, 2, 103–116.
Webb, B., & Walling, D. (1993). Temporal variability in the impact of river
regulation on thermal regime and some biological implications. Fresh-
water Biology, 29, 167–182.
Webb, B., & Walling, D. (1997). Complex summer water temperature
behaviour below a UKregulating reservoir. River Research and Applica-
tions, 13, 463–477.
Webb, B. W., Hannah, D. M., Moore, R. D., Brown, L. E., & Nobilis, F.
(2008). Recent advances in stream and river temperature research.
Hydrological Processes, 22, 902–918.
Webb, J. H., & McLay, H. A. (1996). Variation in the time of spawning of
Atlantic salmon (Salmo salar) and its relationship to temperature in the
Aberdeenshire Dee, Scotland. Canadian Journal of Fisheries and Aquatic
Sciences, 53, 2739–2744.
Weber, C., Nilsson, C., Lind, L., Alfredsen, K. T., & Polvi, L. E. (2013). Win-
ter disturbances and riverine fish in temperate and cold regions. Biosci-
ence, 63, 199–210.
White, J. C., Hannah, D. M., House, A., Beatson, S. J. V., Martin, A., &
Wood, P. J. (2017). Macroinvertebrate responses to flow and stream
temperature variability across regulated and non-regulated rivers.
Ecohydrology, 10, e1773.
Woodward, G., Perkins, D. M., & Brown, L. E. (2010). Climate change and
freshwater ecosystems: Impacts across multiple levels of organization.
Philosophical Transactions of the Royal Society B: Biological Sciences,
365, 2093–2106.
How to cite this article: Heggenes J, Stickler M, Alfredsen K,
Brittain JE, Adeva-Bustos A, Huusko A. Hydropower-driven
thermal changes, biological responses and mitigating measures
in northern river systems. River Res Applic. 2021;37:743–765.
https://doi.org/10.1002/rra.3788
HEGGENES ET AL. 765