R I I S T A -  J A  K A L A T A L O U S  —  T U T K I M U K S I A
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Net loading system for fi sh farming 
Trash fi sh reduction and internal loading
1 / 2 0 0 8
Zeynep Pekcan-Hekim and Jukka Horppila
Net loading system for fish farming 
Trash fish reduction and internal loading
Zeynep Pekcan-Hekim and Jukka Horppila
T U T K I M U K S I A
1 / 2 0 0 8
RIISTA- JA KALATALOUS
Finnish Game and Fisheries Research Institute, Helsinki
Publisher:
Finnish Game and Fisheries Research Institute
Helsinki 2008
Cover photos: 
Mika Remes
Orders:
www.rktl.fi/julkaisut
www.juvenes.fi/verkkokauppa
Pdf publication:
http://www.rktl.fi/julkaisut/
ISBN 978-951-776-606-7 (printed)
ISBN 978-951-776-607-4 (pdf)
ISSN 1796-8860 (printed)
ISSN 1796-8879 (pdf)
Place of printing:  Tampereen Yliopistopaino Oy – Juvenes Print
Contents
Abstract ...................................................................................................................................5
Tiivistelmä ...............................................................................................................................6
Sammandrag ...........................................................................................................................7
1.  Introduction ..................................................................................................................8
2.  Internal nutrient loading by fish .................................................................................9
2.1.  Nutrient release by fish ................................................................................................... 9
2.2.  Factors affecting nutrient release rate by fish ............................................................... 9 
2.2.1. Individual body mass ......................................................................................... 9
 2.2.2. Nutrient composition of fish and their food ................................................. 10
 2.2.3. Temperature ..................................................................................................... 10
 2.2.4. Feeding habits of fish ...................................................................................... 10
2.3.  Bioturbation by fish ....................................................................................................... 11
3.  Methods for estimation of nutrient release rates by fish .......................................11
4.  Quantitative estimation of the effects of fish biomass reduction ..........................12
4.1. Estimates on the effects of biomass reduction ............................................................ 12
4.2. Caution needed in interpreting the results .................................................................. 14
5. Possible other effects of fish stock manipulations ..................................................15
References .............................................................................................................................16
Net loading system for fish farming -  
trash fish reduction and internal loading
5
Abstract 
Trash fish removal as an environmental measure to establish a new type of environmental net 
load and permission system for Baltic Sea fish farming was evaluated in a preliminary study 
in 2007.  This report is part of the project and presents the results from a literature survey. The 
aim of the survey was to determine whether any other positive consequences in addition to 
the effect of nutrient removal existed e.g., if internal loading or bioturbation by the fish would 
diminish.
Although numeric estimation is very difficult and caution is needed in interpreting the re-
sult, a very preliminary quantitative assessment of the effects is given.
It was estimated that a reduction of 300 metric tons of fish biomass in the Archipelago Sea 
(this is not the amount of fish caught) would reduce the internal nitrogen load by an amount 
that equals 10% of the fish farming load in the area and 9% of the phosphorus load.
Keywords: Fish farming, environment, nutrient load, trash fish removal
Pekcan-Hekim, Z. & Horppila, J. 2008.  Net loading system for fish farming: trash fish reduc-
tion and internal loading. Riista- ja kalatalous – Tutkimuksia 1/2008. 19 p.
R I I S T A -  J A  K A L A T A L O U S  –  T U T K I M U K S I A6
Tiivistelmä
Tässä työssä arvioidaan julkaistujen tutkimusten perusteella, missä määrin tehokkaan ka-
lastuksen avulla voisi olla mahdollista kompensoida kalankasvatuksen ravinnekuormitusta 
Saaristomerellä sen lisäksi, että kalastus poistaa ravinteita kalabiomassan mukana. Lähinnä 
tällä tarkoitetaan tehokkaan kalastuksen vaikutusta kalaston aiheuttamaan sisäiseen ravinne-
kuormitukseen.
Kalastuksen sisäisessä kuormituksessa aiheuttaman pienentymisen kvantitatiivinen arvi-
ointi voidaan tehdä vain hyvin karkealla tasolla, sillä asiaan vaikuttavat monet seikat. Pohjara-
vintoa käyttävät kalat aiheuttavat sisäistä ravinnekuormitusta, kun taas planktonia syövät kalat 
vain kierrättävät vedessä jo olevia ravinteita. 
Kalankasvatuksen ulkoinen kuormitus Saaristomerellä on n. 25 000 kg fosforia ja 
n. 200 000 kg typpeä vuodessa. Kalojen fosforin ja typen eritysnopeuksien perusteella voidaan 
laskea, että jos keskimäärin 50 g:n painoisten kalojen biomassa alueella vähenee 300 000 kg:lla 
(esim. 3 kg jokaiselta hehtaarilta 1 000 km2 alueella), vuotuinen kalojen aiheuttama sisäinen 
fosforikuormitus pienentyy n. 2 100 kg:lla. Tämä vastaisi n. 9:ää % kalankasvatuksen aiheut-
tamasta ulkoisesta fosforikuormituksesta. Typen osalta vastaava biomassan pienentäminen vä-
hentäisi vuotuista kalojen aiheuttamaa sisäistä kuormitusta n. 21 000 kg, mikä vastaisi n. 10 % 
kalankasvatuksen ulkoisesta kuormituksesta.
Laskelmat sisältävät lukuisia olettamuksia ja epävarmuustekijöitä. Arviot perustuvat ka-
lojen biomassan pienentymiseen tietyllä alueella, eivät kalansaaliin määrään. Edellä olevan 
esimerkin mukaiseen biomassan vähentymiseen pääsemiseksi voidaan tarvita moninkertainen 
saaliin määrä.
Itämeren ravintoverkot ja vaikutusketjut ovat erilaisia kuin sisävesillä, joissa suurin osa 
tässä työssä referoiduista tutkimuksista on tehty. Niiden perusteella ei voi suoraan tehdä Itä-
merta koskevia johtopäätöksiä. 
Asiasanat: hoitokalastus, kalankasvatus,  kuormitus, ravinteet, tehokalastus, ympäristö,   
 ympäristöluvat  
Pekcan-Hekim, Z. & Horppila, J. 2008.  Kalankasvatuksen nettokuormitusjärjestelmä: sisäi-
nen kuormitus ja vähäarvoisen kalan poisto. Riista- ja kalatalous – Tutkimuksia 1/2008. 19 s.
Net loading system for fish farming -  
trash fish reduction and internal loading
7
Sammandrag
På basen av publicerade undersökningar bedöms, i vilken utsträckning det med ett effektivt 
fiske skulle var möjligt att kompensera fiskodlingens näringsbelastning i Skärgårdshavet, bort-
sett från att fisket avlägsnar näringsämnen med fiskbiomassan.   Med det avses närmast den 
inverkan, som ett effektivt fiske har på fiskbeståndets inre belastning. 
Endast en grov bedömning kan göras av det kvantitativa värdet för den belastningsminsk-
ning, som åstadkoms genom fisket, eftersom förhållandet påverkas av ett flertal faktorer. De 
fiskar, som utnyttjar föda från botten förorsakar en inre belastning, medan planktonätande fis-
kar enbart cirkulerar de näringsämnen som redan finns i vattnet. 
Fiskodlingens yttre belastning i Skärgårdshavet är ca 25 000 kg fosfor och ca 200 000 
kg kväve per år. På basen av den hastighet med vilken fiskarna utsöndrar fosfor och kväve 
kan man beräkna, att om biomassan för fiskar med medelvikten 50 g minskar i området med 
300 000 kg (t.ex. 3 kg per hektar på ett 1 000 km2 stort område), minskar den inre fosforbe-
lastning, som fiskarna förorsakar, med ca 2 100 kg.  Detta skulle motsvara ca 9 % av den yttre 
belastning, som förorsakas av fiskodlingen. För kvävets del skulle en motsvarande minskning 
av biomassan reducera den årliga inre belastningen från fisken med ca 21 000 kg, vilket skulle 
motsvara ca 10 %  av fiskodlingarnas yttre belastning.
Kalkylerna innehåller ett stort antal antaganden och osäkra faktorer. Beräkningarna grun-
dar sig på en minskning av biomassan inom ett visst område och inte på den totala fiskfång-
sten. För att uppnå en minskning av biomassan i föregående exempel, kan man behöva en 
mångdubbling av fångstmängden. 
Näringsväv och orsakskedjor i Östersjön är andra än i insjöarna, där största delen av de 
refererade undersökningarna är gjorda. På basen av undersökningarna kan man inte dra slut-
satser, som direkt gäller för Östersjön.
Nyckelord: fiskodling, miljö, belastnings, näringsämnen, miljötillstånd, intensivfiske, fiskevårdande  
 fiske   
Pekcan-Hekim, Z. & Horppila, J. 2008.  Net loading system for fish farming:  trash fish reduction 
and internal loading. Riista- ja kalatalous – Tutkimuksia 1/2008. 19 s.
R I I S T A -  J A  K A L A T A L O U S  –  T U T K I M U K S I A8
1. Introduction
Fish populations can have a substantial effect on water quality. Firstly, planktivorous fish can 
consume large amounts of zooplankton, thereby decreasing the consumption of phytoplank-
ton and consequently increasing the biomass of phytoplankton (top-down control, Carpenter 
et al. 1985, Gulati et al. 1990). Thus, reduction of planktivorous fish stocks (biomanipulation) 
has been used as a restoration tool to improve the quality of water in lakes (Lammens et al. 
1990, Meijer et al. 1990, Carpenter and Kitchell 1993, Horppila et al. 1998). Biomanipulation 
by removal of planktivorous fish aims to enhance water quality by increasing the grazing of 
phytoplankton by zooplankton, especially by cladocerans (Shapiro et al. 1975, Mehner et al. 
2002). 
Secondly, benthivorous and omnivorous fish can affect nutrient availability for phyto-
plankton growth (bottom-up control, McQueen et al. 1986), because these fish translocate nut-
rients from the sediment to the water column, thus causing internal nutrient loading. Internal 
loading is an important mechanism that affects water quality in freshwater lakes and in the 
Baltic Sea (Søndergaard et al. 1999, Pitkänen et al. 2001). Despite the significant reduction 
in external loading, internal loading of nutrients from sediment can support the production of 
phytoplankton. For instance, despite the 30% decrease in external loading in the 1990s, the 
state of the Gulf of Finland did not improve. The primary reason for this was internal phospho-
rus loading, which was triggered by poor oxygen conditions at the sediment-water interface of 
the eastern Gulf (Pitkänen et al.. 2001).  In addition to oxygen conditions, the exchange of nut-
rients between water and sediment is influenced by a large number of physical and chemical 
factors such as temperature, pH and wave disturbance (Mortimer 1941, Hamilton and Mitchell 
1988, Zhu et al. 2005) and also by nutrient excretion and bioturbation by animals  (Vanni 2002, 
with references). Several studies have shown that nutrient excretion and bioturbation by ben-
thivorous fish can affect water quality without altering the zooplankton community (Drenner 
et al. 1984, Tatrai and Istvanovics 1986, Threlkeld and Drenner 1987, Horppila and Kairesalo 
1990, Tatrai et al. 1990, van Donk et al. 1990, McQueen et al. 1992). Therefore, reduction of 
benthivorous fish may be used as a tool for water quality improvement. 
Opinions on the importance of fish-mediated internal nutrient loading are variable. Inter-
nal nutrient loading caused by fish has been compared with zooplankton excretion, external 
nutrient loading, total internal loading and also nutrient demand by phytoplankton (Brabrand 
et al. 1990, Persson 1997, Horppila 1998, Mehner et al. 1998). In this report, based on the exis-
ting literature, the main aim was to estimate, to what extent external nutrient loading by fish 
farming in the Archipelago Sea, Southwest Finland, could be compensated by mass removal of 
fish through reduction of fish-mediated internal loading. The present annual phosphorus load-
ing from fish farms to the Archipelago Sea area is c. 25 000 kg and the annual nitrogen load-
ing c. 200 000 kg (2002) (http://www.ymparisto.fi/default.asp?contentid= %2083544&lan=fi, 
31.10.2007).
Net loading system for fish farming -  
trash fish reduction and internal loading
9
2. Internal nutrient loading by fish
2.1. Nutrient release by fish
Fish need nutrients for their growth, reproduction and metabolism (Kitchell et al. 1977). The 
nutrients that are not assimilated are released in faeces (egestion) while the assimilated nutri-
ents are excreted through the kidneys or similar organs. Egested nutrients are available to pri-
mary producers only after decomposition and mineralization by microbes. Nutrients such as 
phosphorus (P) and nitrogen (N) are excreted in inorganic form (phosphate and ammonia) and 
can be directly available for phytoplankton and primary producers (Brabrand et al. 1990). 
2.2. Factors affecting nutrient release rate by fish
2.2.1.  Individual body mass
The amount of nutrients excreted per unit body mass per unit time decreases with increasing 
body mass. Adult fish have lower mass-specific excretion rates than underyearling fish as a re-
sult of inverse allometric relationship between body size and respiration rate (Schindler et al. 
1993). To obtain correct estimates of phosphorus release, it is thus important to consider the 
size structure of the fish population (Tarvainen et al. 2002). The relationship between nutrient 
release rate (μg g-1 h-1; NH
4
-N and PO
4
-P) and  fish wet weight (g) from the literature with 95% 
confidence intervals can be found in Figure 1 (Tarvainen et al. 2005).
Figure. 1. Relationship between nutrient release rate of dissolved PO
4
-P (μg g -1 h -1) and NH
4
-N (μg g-1 
h-1) with fish wet weight (g) from the literature with 95% confidence intervals (Tarvainen et al. 2005). 
PO
4
-P: A: Mather et al. (1995), B: Lamarra (1975), C: Brabrand et al. (1990), D: Heino et al. (unpublis-
hed). NH
4
-N: A: Forsberg and Summerfelt (1992), B: Zakes et al. (2001), C: Zakes and Demska-Zakes 
(2002), D: Mather et al. (1995), E: Yager and Summerfelt (1993).
Wet weight (g)
PO4–P
R2 =0.66, t = –2.80, P = 0.049
10
5
1
0,1
1    5       10                50    100       500   1000
µg
 g
-1
 h
-1
Wet weight (g)
µg
 g
-1
 h
-1
NH4–N
R2 =0.63, t = –3.46, P = 0.011
10
5
1
0,1
1    5       10                50     100              500   1000
R I I S T A -  J A  K A L A T A L O U S  –  T U T K I M U K S I A10
2.2.2.  Nutrient composition of fish and their food
Nutrient excretion rates are functions of the nutrient composition of animals and their food. 
Animal species maintain relatively constant body nutrient contents per unit body mass (Vanni 
2002). Thus, during growth an animal will incorporate nutrients at rates needed to maintain 
this nutrient composition and will excrete nutrients that are assimilated but not needed for 
growth. The amount of nutrients excreted will thus be dependent on the nutrient content of the 
food the animal is consuming. Fish feeding on nutrient-rich food resources will excrete more 
nutrients than fish feeding on nutrient-poor diets (Vanni 2002). On the other hand, an animal 
with a relatively low nutrient content in its body will allocate fewer nutrients for growth and 
will excrete more nutrients than an animal with high body nutrient composition. For instance, 
among vertebrate taxa from tropical streams, the mass-specific P excretion rate varied 10-fold 
and was negatively related to animal body P content (Vanni et al. 2002).
2.2.3.  Temperature
Nutrient excretion rates of fish increase with temperature (Lin et al. 2005), due to the increase 
in metabolic rates with increasing temperature (Vanni 2002). For instance, in Lake Balaton in 
Hungary excretion of total nitrogen by bream (Abramis brama) increased significantly with 
increasing water temperature,  from 1.3 g N ind-1 at 6.2 °C to 7.4 g N ind-1 at 22.7 °C for age 
groups 0+ to 7+ (Tatrai 1987). 
2.2.4.  Feeding habit of fish 
The contribution of fish to internal loading is also dependent on the feeding habits of the fish. 
Fish can physically move nutrients between habitats or ecosystems (Vanni 1996). This pro-
cess is usually accompanied by transformation of nutrients from one chemical form to another. 
For example, when benthivorous fish feed on benthic prey and excrete nutrients into the water 
column, they translocate nutrients from benthic to pelagic habitats and convert nutrients from 
particulate to dissolved forms. In this case, new P is added to the water column and internal 
loading occurs (Lamarra 1975, Andersson et al. 1978, Brabrand et al. 1990). 
Planktivorous fish recycle nutrients but do not add new nutrients to the water column. 
They feed on zooplankton in the water column and excrete nutrients back again into the wa-
ter column, recycling nutrients within that habitat. Hence, planktivorous fish do not contribute 
to internal nutrient loading (e.g. Nakashima and Leggett 1980, Havens 1993). For instance in 
Lake Köyliönjärvi, P release of the dense roach (Rutilus rutilus) stock had only minor signifi-
cance for internal phosphorus loading because roach fed mainly on zooplankton (Tarvainen et 
al. 2002). Similar results were obtained in experimental studies, in which access to the bottom 
was prevented in some experimental units and allowed in others. For instance, Havens (1993) 
showed that the total phosphorus concentration in the water did not increase in mesocosms 
where fish were not allowed to feed from the bottom, but remained at a levels similar to those 
in mesocosms with no fish (Figure 2). In mesocosms, where feeding from the bottom was pos-
sible, phosphorus concentration was substantially elevated. 
Net loading system for fish farming -  
trash fish reduction and internal loading
11
Figure 2. Total phosphorus concentrations in the mesocosms and in the surrounding lake during the ex-
periments by Havens (1993). The fish species used in the experiments were brown bullhead (Ictalurus 
nebulosus) shiner (Notropis sp.) and smallmouth bass (Micropterus dolomieui). In mesocosms with net, 
fish were not able to feed from the bottom. Redrawn from Havens (1993).
2.3.  Bioturbation by fish
Fish physically disturb the sediment by feeding and other activities. Bottom-feeding fish such 
as roach and bream can release nutrients by disturbing the surface sediments while searching 
for food (Persson and Hamrin 1994). Breukelaar et al. (1994) showed a positive relationship 
between bream and carp (Cyprinus carpio) biomass with both suspended solids and sediment 
found in traps in the experiments. These fish also contributed significantly to turbidity in the 
experimental set ups (Breukelaar et al. 1994). It is very difficult to distinguish internal nutrient 
loading caused by bioturbation from nutrient excretion by benthivorous fish (Lamarra 1975). 
3. Methods for estimation of nutrient release 
 rates by fish
Excretion rates can be estimated using bioenergetics models or by direct measurement (Vanni 
2002). Using bioenergetics, the excretion rate is estimated as nutrients ingested minus nut-
rients allocated for egestion and growth (Kraft 1992, Horppila et al. 1998, Tarvainen et al. 
2002). Direct measurements can be done by capturing animals in the field and placing them 
in containers in which the accumulation of nutrients is quantified (Persson 1997, Hood et al. 
2005, Tarvainen et al. 2005). Both methods have advantages and disadvantages, but there are 
limited data suggesting that they yield similar excretion rates and ratios for fish (Vanni 2002). 
0 5 10 15  20   25   30
150
120
90
60
30
0
Day
To
ta
l P
 (µ
g 
l -
1 )
Lake
Fish
Fish+net
No fish
R I I S T A -  J A  K A L A T A L O U S  –  T U T K I M U K S I A12
4. Quantitative estimation of the effects of fish   
 biomass reduction
4.1. Estimates on the effects of biomass reduction 
As presented above, numerous factors affect nutrient release rate by fish. It is thus possible to 
make only very rough estimates of the effects of fish biomass reduction on internal nutrient 
loading. Therefore, the effects of biomass reduction were estimated with the different nutrient 
release rates presented in the literature. 
The literature review by Tarvainen et al. (2005) showed that the PO
4
-P release rate by fish 
having individual weights from 1 to 1000 g varies approximately from 10 to 1 μg g-1 h-1 (Figure 
1). For NH
4
-N, the release rate varied from 5 to 50 μg g-1 h-1 for fish weights of 1-500 g (Fi- 
gure 1). These ranges of nutrient release rates form the x-axis of Figures 3 and 4. The estimated 
reduction of fish-induced internal nutrient loading was calculated by multiplying the mass-spe-
cific nutrient release rates of fish by the biomass reduction of fish (y-axis). From the figures it 
is thus possible to read the estimated annual reduction in fish-induced internal nutrient loading 
with each combination of nutrient release rate by individual fish and biomass reduction caused 
by intensive fishing. The calculations were based on the assumption that nutrient release by 
fish occurs for 24 h per day during 150 days of the year (Tarvainen 2007). 
Figure 3. Estimated reduction of  fish-induced internal P loading (kg)  with different combinations of 
fish biomass reduction (kg x 104) and mass-specific PO
4
-P release rates of fish obtained in previous stu-
dies, covering fish weights approximately from 1 g to 1000 g (Figure 1). 
1 2 3 4 5 6 7 8 9 10
1
2
3
4
5
6
7
8
9
10
F
h
ed
u
on
K
10
00
00
30 000
18 000
18 000
22 000
22 000
24 000
26 000
28 000
20 000
20 000
8 000
6 000
10 000
12 000
14 000
16 000
8 000
10 000
12 000
14 000
4 000
16 000
6 0004 000
16 000
12 000
8 000
8 000
2 000
4 000
6 000
10 000
14 000
4 000
6 000
1  2  3  4  5  6  7  8  9  1 0
1 0
9
8
7
6
5
4
3
2
1
Phosphorus excretion_g g  –1 h –1
Fi
sh
 re
du
ct
io
n 
Kg
 x
 1
00
 0
00
Large fish ------------------>  Small fish
Net loading system for fish farming -  
trash fish reduction and internal loading
13
For instance, a 300 000 kg biomass reduction of fish releasing 8 μg P g-1 h-1 would result ap-
proximately in an 8 000 kg annual reduction in fish-induced internal P loading (Figure 3). It is 
necessary to consider, however, that the calculation is based on the assumption that fish bio-
mass reduction is comprised exclusively of benthivorous fish. Since most fish species, inclu-
ding cyprinids, are zooplanktivorous as juveniles and switch to benthic food when they grow 
in size (Persson 1983, Persson and Greenberg 1990), the highest values of individual nutrient 
release in the x-axis are probably not realistic. For an average size of 50 g for a benthivorous 
fish, a nutrient release rate of 2 μg P g-1 h-1 should be used (Figures 1 and 3). Using this value, 
 a 300 000 kg biomass reduction would cause c. 2 100 kg annual reduction in fish-induced in-
ternal P loading, corresponding to 9% of the external loading by fish farming. For nitrogen, 
similar calculations were made based on the published NH
4
-N release rates by fish (Figure 4).
Figure 4. Estimated reduction in  fish-induced internal N loading (kg)  with different combinations of 
fish biomass reduction (kg x 104) and mass-specific NH
4
-N release rates of fish obtained in previous 
studies, covering fish weights approximately from 1 g to 500 g (Figure 1). 
Nitrogen excretion_g g  –1 h –1
Fi
sh
 re
du
ct
io
n 
Kg
 x
 1
0 
00
0
1 0  2 0  3 0  4 0  5 0
1 0 0
9 0
8 0
7 0
6 0
5 0
4 0
3 0
2 0
1 0
10 20 30 40 50
0
0
0
0
0
0
0
0
0
0
140 000
70 000
100 000
80 000
110 000
90 000
120 000
130 000
60 000
50 000
60 000
70 000
30 000
50 000
40 000
30 000
80 000
90 000
40 000
10 000 20 000
Large fish ------------------>  Small fish
R I I S T A -  J A  K A L A T A L O U S  –  T U T K I M U K S I A14
Assuming an average fish size of 50 g for a benthivorous fish, a nutrient release rate of 
20 μg N g-1 h-1 should be used (Figure 1). Using this value, for instance, a 300 000 kg biomass 
reduction of benthivorous fish would cause c. 21 000 kg annual reduction in fish-induced in-
ternal N loading, corresponding to 10% of the external loading by fish farming.
4.2. Caution needed in interpreting the results
In reading the figures, it is important to bear in mind that the calculations were based on the 
reduction in benthivorous fish biomass in a given area and not on the fish catch. 
The possible catch amounts of different fish species in the Archipelago Sea area are eva-
luated to be about 250 tonnes (Mäkinen 2007). Additionally, use of some excretion period 
other than 24 h x 150 days would of course change the calculation results. However, outside 
the growing season the food intake and nutrient excretion rates are low, due to low water tem-
peratures. 
The behaviour of fish may be affected by mass removal. A change in the abundance of 
benthivorous fish may, for instance, affect the biomass of the zoobenthos (Svensson et al. 
1999, Leppä et al. 2003), with consequences for the feeding habits of the remaining fish and 
thus for their effect on nutrient dynamics. Several studies have shown that the effect of fish on 
internal nutrient loading may depend on the availability of benthic food (Horppila and Kaire-
salo 1992, Zambrano et al. 2001, Tarvainen et al. 2005).
It is also very important to consider that the studies cited have been mostly conducted in 
freshwater environments. In most aquatic ecosystems, attention has focused on the cycling of 
nitrogen (N) and phosphorus (P) because they are the nutrients most likely to limit primary 
producers. For internal loading, phosphorus is often taken into account because P usually li-
mits the growth rates or maximum yields of phytoplankton during summer in temperate fresh-
water lakes (Carpenter et al. 1985). However, this is not the case for the Baltic Sea since N is 
the limiting factor for phytoplankton growth in certain areas (Pitkänen and Tamminen 1995). 
In the Baltic Sea, the low N:P ratio of internal loading also partly explains the limiting role of 
N in primary production, despite the high N:P ratio of external inputs (Pitkänen and Tamminen 
1995). The capacity of saline water sediments to permanently retain P is generally poorer than 
that of freshwater sediments because of the formation of insoluble Fe(II)S in anoxic saline wa-
ter systems (Pitkänen and Tamminen 1995). This process reduces the number of potentially 
available adsorption sites for P (Caraco et al. 1990)
Most top-down control studies have been conducted in lakes and those that have been 
conducted in the Baltic Sea have focused on the lower trophic levels (zooplankton and algae) 
(Rudstam et al. 1992, Kivi et al. 1993). A strong coupling between fish and lower trophic le-
vels was suggested to be due to the higher fish yield per unit of primary production in coas-
tal areas, including the Baltic Sea (Rudstam et al. 1994). However, in comparing freshwater 
lakes with brackish lakes, Jeppesen et al. (1994) concluded that in brackish lakes the effects 
of planktivorous fish stock manipulations on water quality are weak compared with the effects 
in freshwater lakes. 
Net loading system for fish farming -  
trash fish reduction and internal loading
15
5. Possible other effects of fish stock manipulations
Planktivorous fish can alter the species composition and size structure of zooplankton com-
munities (Hrbácek et al. 1961, Drenner et al. 1984, Attayde and Hansson 2001). The change 
in size structure of zooplankton can lead to a change in consumption rate of phytoplankton, 
since large-sized zooplankton can consume higher biomasses of phytoplankton. Consequently 
the nutrient excretion rates of zooplankton may increase and lead to enhanced phytoplankton 
production (Vanni 1987). Possible changes in the N:P ratio can also affect phytoplankton com-
munities (Carpenter et al. 1992). Benthivorous fish can affect the biomass and density of zoo-
benthos. Changes in the biomass of zoobenthos may have consequences for nutrient dynamics, 
because benthic invertebrates can play an important role in nutrient cycling by their burrowing 
activity that causes resuspension of the sediment and increases or decreases nutrient release 
from the bottom (Tátrai 1987, Philips et al. 1994). 
The presence of benthivorous fish may prevent sediment consolidation (Scheffer et al. 
2003). When benthivorous fish feed from the sediment, they leave small pits in the sediment 
surface (Lammens and Hoogenboezem 1991). These pits increase the effects of wind-induced 
waves and disturbance of the bottom sediment. After removal of benthivorous fish, the sedi-
ment may become more stable and firm, leading to reduction in sediment resuspension due to 
wind (Scheffer et al. 2003).
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