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): Teppo Vehanen, Ari Huusko, Eva Bergman, Åsa Enefalk, Pauliina Louhi and Tapio Sutela Title: American mink (Neovison vison) preying on hatchery and wild brown trout (Salmo trutta) juveniles in semi-natural streams Year: 2022 Version: Published version Copyright: The Author(s) 2022 Rights: CC BY 4.0 Rights url: http://creativecommons.org/licenses/by/4.0/ Please cite the original version: Vehanen, T., Huusko, A., Bergman, E., Enefalk, Å., Louhi, P., & Sutela, T. (2022). American mink (Neovison vison) preying on hatchery and wild brown trout (Salmo trutta) juveniles in semi-natural streams. Freshwater Biology, 67, 433– 444. https://doi.org/10.1111/fwb.13852 Freshwater Biology. 2022;67:433–444. �  |  433wileyonlinelibrary.com/journal/fwb Received: 10 March 2021  |  Revised: 2 November 2021  |  Accepted: 8 November 2021 DOI: 10.1111/fwb.13852 O R I G I N A L A R T I C L E American mink (Neovison vison) preying on hatchery and wild brown trout (Salmo trutta) juveniles in semi-natural streams Teppo Vehanen1  | Ari Huusko1 | Eva Bergman2 | Åsa Enefalk2 | Pauliina Louhi1 | Tapio Sutela1 This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2021 The Authors. Freshwater Biology published by John Wiley & Sons Ltd. 1Natural Resources Institute Finland, Helsinki, Finland 2River Ecology and Management Research Group, Department of Environmental and Life Sciences, Karlstad University, Karlstad, Sweden Correspondence Teppo Vehanen, Natural Resources Institute Finland, Helsinki, Finland. Email: teppo.vehanen@luke.fi Present address Åsa Enefalk, County Administrative Board Värmland, Karlstad, Sweden Abstract 1. Predator–prey interactions are one of the main ecological factors influencing the structure of fish communities. The impact of wading and diving semi-aquatic predators on riverine fish populations is poorly known. We studied the effect of feral American mink predation on brown trout juveniles during winter in two ex- periments conducted in semi-natural streams (length 26 m, width 1.5 m). 2. In the first experiment, we compared the vulnerability of age-1+ hatchery (length 142 ± 16 mm, average ± SD) and wild (112 ± 8 mm) brown trout of similar genetic origin in sympatry and allopatry. In the second experiment we used age-0+ brown trout (79 ± 5 mm), increased habitat heterogeneity by addition of fine wood, and compared those to treatments without fine wood addition. 3. Hatchery fish were more vulnerable to mink predation than their wild counterpart, and the predation rate increased with increasing body size among the hatchery trout. Predation by mink on wild trout was higher in sympatric than in allopatric treatments suggesting that stocking of hatchery fish may increase predation on wild conspecifics. Increased habitat heterogeneity resulted in reduced predation rate. 4. The results show that a large size of hatchery fish in small streams was a negative trait, which was opposite to the mainstream observations of salmonid stockings made directly of feeding areas in lakes and oceans. Adding habitat heterogene- ity was found important for habitat enhancements in streams with mammalian predators. 5. We highlight the importance of taking all the habitats during the life cycle of migratory fish into account in management decisions and carefully considering whether using hatchery fish to support wild populations in streams. K E Y W O R D S fish stocking, habitat heterogeneity, mesocarnivore, mink predation, semi-aquatic predator www.wileyonlinelibrary.com/journal/fwb mailto: https://orcid.org/0000-0003-3441-6787 http://creativecommons.org/licenses/by/4.0/ mailto:teppo.vehanen@luke.fi http://crossmark.crossref.org/dialog/?doi=10.1111%2Ffwb.13852&domain=pdf&date_stamp=2021-11-23 434  |     VEHANEN et al. 1  | INTRODUC TION Predation has both direct and indirect effects on the structure of fish communities (Allouche & Gaudin, 2001; Giam & Olden, 2016; Jackson et al., 2001). The direct effect of predation can be defined as mortality of prey species, which can be variably high and selective for many traits, such as prey size, other morphological characteristics and prey availability (He & Kitchell, 1990; Hoey & McCormick, 2004; Pinnegar et al., 2003). Responses to predation can lead to indirect in- teractions among prey species. These can be behavioural (Metcalfe et al., 1999; Vehanen, 2003) and physiological (Archard et al., 2012) adaptations or morphological changes (Svanbäck & Eklöv, 2006; Vinterstare et al., 2020). Indirect predation effects include also changes in habitat selec- tion, and it has been suggested that shallow areas are preferred by small fish, but avoided by large fish (Power, 1987; Schlosser, 1987; but see Sheaves, 2001). Small-sized fish are expected to take refuge in shallow areas to reduce predation risk from predatory fish, which are considered gape-limited predators (Mihalitsis & Bellwood, 2017; Nilsson & Brönmark,  2000). Thus, a larger body size of prey fish provides per se a shelter from predatory fishes (Hyvärinen & Vehanen,  2004). However, even if shallow areas create refuges against fish predators, fish can increase encounters with terrestrial piscivores, as wading and diving predators forage effectively in shal- low water (Crowder et  al.,  1997; Power,  1987). Prey vulnerability to predation generally decreases as the environmental complexity increases (Beukers & Jones, 1997; Livernois et al., 2019; Nelson & Bonsdorff, 1990). Whereas the effects of fish predation on salmonid fish popula- tions are relatively well known, the effect of terrestrial mammalian predators is less clear. Pacific salmon (Oncorhynchus spp.) have been found to be an important resource for terrestrial wildlife through the allocation of nutrients to terrestrial predators (Hilderbrand et  al.,  2004; Levi et  al.,  2012, 2020). The most widely mentioned terrestrial predators on salmon are bears, but the range of verte- brate consumers includes various species, and also several avian foragers (Levi et al., 2015; Shardlow & Hyatt, 2013). Thus the mi- grating salmonids role to the ecosystem function can be high (Gende et  al.,  2002), even when iteroparous migratory species such as Atlantic salmon (Salmo salar) or brown trout (Salmo trutta) are consid- ered (Enbom, 2015; Jonsson & Jonsson, 2003; also see Cairns, 2006). When large carnivores have been lost or are rare, smaller sized meso- carnivores play a larger role in the prey community structure (Roemer et al., 2009). In many European freshwater systems, the otter (Lutra lutra) and American mink (Neovison vison) play a key role as semi- aquatic predators (Holland et al., 2018). Both predators feed on fish, including salmonids, and otters are known to prey also on large sal- monids (Carss et  al.,  1990), whereas mink, due their smaller body size, typically forage on smaller sized fish (Erlinge, 1969). The American mink was introduced to Europe from North America for fur farming, and escaped minks have established naturally reproducing populations around Europe (Bonesi & Palazon,  2007). We have relatively limited knowledge about the impacts of this invasive species on its prey species, although sev- eral studies have indicated that it is a potential predator for a wide range of prey fauna (Bonesi & Palazon, 2007). Seasonal changes in the mink's diet originate from changes in the availability of specific prey categories, e.g. poikilothermic fish are an important part of the diet of the homeothermic mink especially in winter (Chibowski et al., 2019; Filip’echev et al., 2016; Gerell, 1967, 1969) due to lower water temperatures and the limited escape ability of fish at low tem- peratures (Huusko et al., 2007). Lindstrom and Hubert (2004) concluded that mink predation might have a substantial effect on the winter mortality of salmo- nids in Wyoming mountain streams. Correspondingly, Heggenes and Borgstrøm (1988) suggested that mink predation may be a major cause of mortality in small streams, but the predation efficiency is likely to vary with stream characteristics. For example, in a structur- ally enhanced stream, where trout juveniles have better possibilities to find cover from predation compared to a stream with simplified habitat, mink predation was not an important source of mortal- ity for brook trout (Salvelinus fontinalis) during summer (Burgess & Bider, 1980). Erlinge (1969) reported that mink largely fed on fish, but the majority of the fish diet consisted of non-salmonid fish, suggesting that brown trout could find hiding places from mink pre- dation in a naturally complex river environment. Where mink are sympatric with otters, mink is a more generalised carnivore, and therefore switches diet opportunistically (Wise et al., 1981). Stream characteristics can affect mink predation efficiency, which has been observed, for example, when bears prey on salmonids (Andersson & Reynolds, 2018). Despite, the concerns about the harmful effects to native fishes (Aas et  al.,  2018; Pister,  2001)), fish stocking is one of the most common methods to mitigate adverse effects on fish stocks (Cowx, 1994). Predation by feral predators, such as mink, is partic- ularly concerning where fish stocking is used as management tool. Salmonids are a group of fishes with high socio and ecological value where fish stocking is used heavily for management and conser- vation (Aas et  al.,  2018; Armstrong,  2005; Krueger & May,  1991). Hatchery fish are typically raised to large sizes as larger fish have higher survival rates in the wild (the bigger is better hypothesis; Sogard,  1997). However, hatchery reared salmonids have lower survival rates in the wild compared to their wild counterparts or to fish raised in enriched environments (Einum & Fleming,  2001; Hyvärinen & Rodewald, 2013; Larocque et al., 2020). One major rea- son for lower survival in the wild is that stocked individuals have not been raised in environments that stimulate predator avoidance behaviours (Mes et  al.,  2019; Olla et  al.,  1998), and they are thus heavily preyed upon (Alioravainen et  al.,  2018; Berejikian,  1995; Einum & Fleming, 2001). Surprisingly, there are no published studies comparing whether hatchery fish are more vulnerable than wild fish to semi-aquatic mammalian predators, such as the American mink. Habitat restoration is another mitigation tool to enhance the survival and reproduction of depleted fish stocks. The restoration of natural salmonid reproduction areas has recently gained more interest in fish management than more traditional hatchery fish      |  435VEHANEN et al. releases. Perhaps the most used technique is in-stream habitat restoration (Krall et al., 2019; Roni et al., 2008), which aims to re- establish channel complexity through the placement of structures (Nilsson et al., 2015; Vehanen et al., 2010). More complex habitat di- versity is expected to increase the suitability of the habitat for juve- nile salmonids, and provides more refuge from predators (Beukers & Jones, 1997; Höjesjö et al., 2014). Adding large or fine wood (FW) into the stream channel results in positive responses in the densities of juvenile salmonids (Louhi et al., 2016; Nagayama & Nakamura, 2010; Roni et al., 2015), and wood can also function as cover from pred- ators. However, there is an ongoing debate about how success- ful the in-stream restoration efforts have been, and what are the most influential measures behind successful in-stream restorations (Krall et al., 2019; Marttila et al., 2019; Nilsson et al., 2015; Stewart et al., 2009; Taylor et al., 2019). In this study, we present the results of two experiments run in semi-natural streams to test the hypotheses that: (1) hatchery fish are more vulnerable to predation from the semi-aquatic predator, American mink, than wild fish; (2) a larger body size protects fish from mink predation; and (3) habitat complexity in the form of FW protects fish from mink predation. Even if the original aims for the experiments were different (see below), the design of the experi- ments allowed us to study the effects of the unexpected mink pre- dation on juvenile brown trout. 2  | METHODS This study assessed the mortality of brown trout juveniles from mink predation in stream channels at the Kainuu Fisheries Research Station (Natural Resources Institute Finland, http://www.kfrs.fi/en/ front​page/, 64°30′N, 27°10′E). In the experiments, several months after the experimental start, we noticed tracks of feral minks in the snow in the experimental stream area, and it became obvious that the minks had preyed on the experimental fish in both experiments. Thus mink predation took place unexpectedly, and we were able to study its effect on brown trout juveniles in these experimental setups. The first experiment was designed to study the long-term patterns in the strength of competition between hatchery and wild brown trout (Huusko & Vehanen, 2011), and the second experiment was designed to examine growth effects on brown trout with and without wood addition (Enefalk et al., 2019). The detailed descrip- tions of study designs are given for Experiment 1 by Huusko and Vehanen (2011) and for Experiment 2 by Enefalk et al. (2019) and are only summarised here. 2.1 | Semi-natural streams We conducted the experiments in six 26  m long and 1.5  m wide outdoor artificial stream channels. In the first experiment, we used three of the six stream channels, and in the second experiment, we used all six channels. The bottom of the streambed consisted of natural material, a 10–15 cm layer of gravel and pebbles (15–40 mm in diameter). Each stream was divided into three 8.5-m long sections (upstream–middle–downstream) with wire mesh panels (mesh size 10 mm [Experiment 1] or 6 mm [Experiment 2]). All channels shared the same water source drained from the nearby Lake Kivesjärvi, thus having the same temperature regime. Experiment 1 took place from 30th August to 8th November 2006, and experiment 2 from 17th August 2013 to 24th February 2014. During Experiment 1, water temperature decreased from 17.0 to 2.1°C, while during experi- ment 2, it decreased from 17.1 to 1.1°C. The stream channels sup- ported benthic invertebrate communities similar to those present in a nearby stream in terms of both species composition and densities (Korsu et  al.,  2009; Vehanen,  2006). Thus, trout were feeding on natural food during the experiments. 2.2 | Brown trout Brown trout juveniles from the same adfluvial brown trout stock were used in both experiments, and originated from the Kuusinkijoki River, north-eastern Finland. The river has its own wild genetically divergent, lake-migrating brown trout stock (Huusko et  al.,  1990; Lemopoulos et  al.,  2018). This brown trout stock has been main- tained for stocking purposes in the state fish hatcheries in Finland. The hatchery brown trout juveniles used in Experiment 1 were hatched and reared using normal procedures on the Kuusamo Fish Farm, a state hatchery about 300 km from the experimental site. The brown trout were the first-generation progeny of wild parents from the Kuusinkijoki River. The average rearing density in 4-m2 indoor ponds was 3 kg/m2. During rearing, the fish were fed with artificial food pellets. The wild counterparts for the hatchery fish, also aged 1+, were caught using electrofishing methods from the Raatekoski Rapid (66°23′88.8″N, 29°67′41.1″E) in the Kuusinkijoki River. Directly after capture, we transported both the hatchery-reared and wild fish to the study site in separate oxygenated containers. For recovery, we placed the fish in separate holding tanks with a low water flow (velocity 0.02–0.10 m/s) for 48 hr, and during this time, we provided no food. Thereafter the fish were anaesthetised with clove oil, measured (total length, mm and mass, g), and tagged with passive integrated transponder (PIT) tags (HDX Oregon RFID, Portland, USA, tag size 23 mm × 3.65 mm, weight 0.6 g) in their body cavities for individual recognition. After the fish had recovered from tagging and behaved normally, they were randomly placed into the different stream sections according to the study design. The brown trout used in the Experiment 2 were produced in the same hatchery, Kuusamo Fish Farm, and originated from wild par- ents from the Kuusinkijoki River. We transferred the brown trout to the experimental site already in their late yolk-sac phase in early June 2014. Altogether, 175 age-0+ individuals were placed into each of the six channels used in the experiment. In the channels we did not feed the fish, instead they started to feed on natural food. After two months, we collected all fish by multi-pass sampling with a DC electro shocker. The flow in the channels was reduced to a low level http://www.kfrs.fi/en/frontpage/ http://www.kfrs.fi/en/frontpage/ 436  |     VEHANEN et al. and carefully searched to make sure no fish were left in the channels. The collected brown trout were kept in a holding tank (3.5 m2, water volume 1.4 m3, and flow 1.5 L/s). From the tank, 360 fish were ran- domly selected, anaesthetised with clove oil, measured (fork length, mm and weight, g), and tagged with a PIT tag (HDX Oregon RFID, tag size 12 mm × 2.15 mm, weight 0.1 g), and then placed into the stream sections according to the study design. 2.3 | Study design In Experiment 1, all channels had a similar discharge regime, 43.7 ± 7.1 L/s (average ± 1 SD). Each study section comprised an upstream riffle and a downstream pool section. The water veloc- ity in the riffle part of each section ranged between 20–60  cm/s (depth 15–25 cm, mean substrate diameter 15 cm) and in the pool 0–20  cm/s (25–35  cm, 4  cm, respectively). We used a substitu- tive experimental design as such a design is useful for drawing conclusions for wider contexts (Vehanen et  al.,  2009; Weber & Fausch, 2003, 2005; Yamamoto et  al.,  2008). The design included three treatments, with three replicates, in randomly selected sec- tions: (1) 10 wild brown trout (WBT); (2) five WBT and five hatchery- reared brown trout (HBT); and (3) 10 HBT, resulting in trout densities of 0.78 ind. m−2 in each stream section (Figure 1). The fish densities used in the experiment were comparable to average field densities of age-1+ brown trout in natural trout streams in northern Finland (Korsu et  al.,  2007). The length of the WBT at the beginning of the experiment was 112 ± 8 mm (average ± SD, n = 45) and their mass was 13.9 ± 3.0 g, and for HBT the corresponding values were 142  ±  16  mm (n  =  45) and 33.7  ±  12.1  g, respectively. Thus, the hatchery trout were larger than the wild trout, with the result that the fish biomasses in the treatment areas were different. However, the aim was to follow the standard stocking procedure in which hatchery brown trout are added to support wild stocks. This meant that no changes in the rearing procedures, for example, by restrict- ing feeding or selecting small-sized hatchery fish to result in match- ing sizes, were applied for the hatchery fish. During the study period, we located the fish seven times with intervals of 7–14 days over the course of the study at noon by slowly moving a customised portable PIT antennae (Texas Instruments TIRIS S-2000; Linnansaari et al., 2007) about 20 cm above the water surface in the upstream direction. When a fish was located, we marked its individual code on a map. In Experiment 2, the discharge regime in all channels was 59.0 ± 5 L/s (mean ± 1 SD). Three gravel deflectors (triangle shaped, with a side length of 0.5 m) were placed into each section protrud- ing from the water surface. These deflectors shaped the water flow into meandering patterns. Two bricks with an arch underside (height 4 cm) were placed in each section to provide additional sheltering sites for fish. The water depth was 16.1 ± 1.2 cm in treatment areas with additions of FW, and 16.3  ±  1.2 in control sections without wood additions. The water velocities were 24.4  ±  2.6  cm/s and 24.9  ±  2.1  cm/s, respectively. The study design was similar to a stratified random design (Figure 2). There were three channel sec- tions in each of the six channels, for a total of 18 sections. According to the study design, equal numbers of sections were assigned to treatment areas with FW added, and control with no wood added (Figure 2). The FW load used in the study equalled 50 m3 of wood/ha of the stream bottom surface (Enefalk et al., 2019). About 1 m long (Ø = 1 cm) willow sticks (Salix sp.) were collected in early June. The sticks were tied in bundles of 25–26 sticks and were waterlogged for 2 months before use in the experiment. In Experiment 2, eight waterlogged willow stick bundles were placed in the upstream area of the FW treatment sections and the rest of the sections (length 6.5  m) remained free-flowing. Each channel section was stocked with 20 brown trout. The fork length of the brown trout individuals in the FW treatment section was 78 ± 5 mm (mean ± 1 SD) and mass 5.5 ± 1.1 g, and correspondingly 79 ± 5 mm and 5.5 ± 1.1 g in the controls. Again, the fish density in the Experiment 2 was selected to be within the range of natural densities of juvenile brown trout in streams in Scandinavia (Korsu et al., 2009). Similarly to Experiment 1, the daytime distribution of brown trout in the channel sections was determined by slowly moving a customised portable PIT antenna (Texas Instruments TIRIS S-2000; Linnansaari et al., 2007), approximately 20 cm above the water sur- face. During the study period, in September, October, and December, brown trout were captured by electrofishing, counted, and mea- sured for length and mass for growth and survival responses, and returned back to the channels. 2.4 | The mink and mink predation After 10 weeks from the start of Experiment 1, at the onset of winter in November 2006, we noticed tracks of feral minks in the F I G U R E 1   Schematic presentation of the study design in three semi-natural streams in Experiment 1. Treatment positions for the substitute design in the channels are indicated by the letters. 10W = 10 wild brown trout, 10H = 10 hatchery brown trout, and 5W + 5H = five wild brown trout and five hatchery brown trout in stream sections separated by wire mesh panels Inflow Outflow 0 5 10 15 20 25 10W 10W 10W 5W + 5H 5W + 5H 5W + 5H 10H 10H 10H VALVES LE N G TH      |  437VEHANEN et al. experimental stream area. After checking the channels, it became obvious that the minks had preyed on the brown trout in all the streams and sections. A wire mesh fence surrounds the area of the experimental streams and it is unclear which route the minks used to access the stream channel area. The snow cover in the area was about 20 cm deep, and the stream sections had border ice of 20–40 cm in width. From the mink tracks, we could estimate that two minks had visited the experimental area several times in early November. The mink had collected the brown trout they had killed into piles under the border ice of the streams in sites with a very shallow water depth, probably as food storage. Many of them were not eaten at all or were only partly eaten. We collected the dead fish and electrofished the ones remaining alive, measured their total length and mass, and recorded the individual PIT tag codes. PIT tags from the channels and the surrounding area were also located with the customised portable reader. Altogether, we found 17 tags (nine HBT and eight WBT tags) loose and signalling from the channel bot- tom. Before the mink invasion, only two out of 90 fish had lost their tags (during the positioning of the brown trout at the end of October, fish found alive but tags were recovered from the bottom), thus the tags in the bottom could be classified as eaten by mink. In Experiment 2, we observed mink tracks for the first time in the experimental stream area in late January 2014. The snow cover was about 10 cm deep, and from the mink tracks on the snow, it was judged that three individuals had visited the area. During the next 2  weeks, we checked mink tracks and blood patches in the snow all weekdays, and estimated that the minks were preying on brown trout in the experimental channels. To protect the fisheries station's experimental activities, a local hunter trapped and removed one mink. Following the detection of mink predation, we electrofished the flumes for brown trout on 24 February and 6 May 2014. On both occasions, we recorded their PIT tag codes, fork length and mass. Fish that were present in the flumes up until the last electrofishing in December but were not recorded during these two electrofishing occasions were classified as preyed upon by mink. This is an over- estimation of the predation rate as it also includes other natural mortality. The natural mortality outside mink predation (see Results, Experiment 2 for exact numbers) was, however, relatively low. For example, out of the fish that were detected in the October elec- trofishing sampling (319 trout), 8.5% (27 trout) were not detected alive in later samplings, and thus had died by natural causes between October and December. There was no statistical difference in num- bers of dead fish between treatments (FW/no FW, 12/15 trout, chi square test, χ2 = 0.333, df = 1, p = 0.564). Thus, the natural mortality outside the mink predation was low, and there was no difference between treatments. Because we could not individually identify out of PIT-tagged trout the fish that were preyed upon by mink from the fish that had died from other sources of natural mortality, we used total mortality as a measure of mink predation mortality in our analyses. 2.5 | Data analysis Substitutive comparisons (see Weber & Fausch, 2003) in Experiment 1 were made between allopatric treatments (i.e. either wild or hatch- ery fish separately in each section) and sympatric treatments (i.e. both wild and hatchery fish in each section) using a t-test. The sig- nificance level used in the analyses was 0.05. The response variables were survival (%), the size of fish (length in mm) killed by mink and surviving, respectively. The survival was also compared between al- lopatric treatments. In the Experiment 2, the survival (%) of brown trout in channels with and without FW was analysed using a mixed ANOVA model. The treatments (FW addition, control with no wood addition) were used as fixed factors and the experimental channels (1–6) as random factors. The possible size selectivity of mink preda- tion was examined using a mixed ANOVA approach using the size of the fish (killed by mink or those that survived) as response variables and experimental channels as random factors. Statistical analyses were done using Systat 13 and SPSS 25 statistical software programs. 3  | RESULTS 3.1 | Experiment 1 Out of 87 brown trout, 58.6% or 51 fish (38 HBT and 13 WBT) were killed by mink. Three fish (two HBT and one WBT) were lost: their fate could not be identified as they or their tags were not found in the flumes or in their surroundings inside the wire mesh fence bor- dering the area. There was a significant difference in survival of allopatric WBT and HBT (t3 = 6.918, p < 0.002, Figure 3) such that the HBT had a survival of 10.4%, which was clearly lower than the 83.3% for the WBT. The survival of the sympatric WBT (46.7%) was significantly lower than the survival of the allopatric WBT (t3 = 3.317, p = 0.029). F I G U R E 2   Schematic presentation of the study design in three semi-natural streams in Experiment 2. Treatment positions for the design in the channels are indicated by the letters. W, fine wood addition; C, control, no wood added. 20 brown trout were in each stream section separated by wire mesh panels Inflow Outflow 0 5 10 15 20 25 VALVES LE N G TH w w c w w c w c c c c w c w w c w c 438  |     VEHANEN et al. For HBT, there was no treatment effect on the survival (t3 = 0.336, p = 0.754), Both in the sympatric and allopatric treatments the average length of HBT fish (measured in October before mink invasion) that had been preyed on were significantly greater than that of the sur- viving fish (Figure  4, allopatry: t3  =  −7.816, p  =  0.004; sympatry: t3 = −2.567, p = 0.0083). No similar effects were found for the WBT in allopatry (t3 = 0.386, p = 0.719) and in sympatry mink seem to prey on larger WBT individuals, but the difference was insignificant (t3 = −1.822, p = 0.142, Figure 4a). 3.2 | Experiment 2 After the mink visits in February 2014, in total 56.6% of the trout in the FW sections, and 35.7% in control sections were found alive, compared to the trout numbers present in early December. The survival of the brown trout was significantly higher in the treatment areas with FW addition compared to control treatment areas with no wood addition (F = 10.570, df = 1, p = 0.023, Figure 5a), but there was no effect of channel (F = 1.830, df = 5, p = 0.262) or treatment- channel interaction (F = 1.603, df = 5, p = 0.290). Neither did the minks select brown trout of any particular size (fish lengths based on December 2013 measurements of pit-tagged fish, F  =  2.402, df = 1, p = 0.182). In treatment areas where the habitat complexity was increased by adding FW smaller trout seemed to survive better, but this was insignificant (Figure 5b). The channel factor (F = 0.358, df = 5, p = 0.858) and the treatment-channel interaction (F = 1.563, df = 5, p = 0.171) were insignificant. 4  | DISCUSSION Our study showed that the predation by mink can cause high mortality among juvenile salmonids in small streams. In addition, we found that the mortality of wild brown trout was significantly higher when in sympatry with hatchery trout than it was in al- lopatric wild trout treatments. Thus, presence of hatchery fish can increase the predation effect from mink on wild brown trout juveniles. High predation by feral mink should be considered in management actions when restoring and conserving salmonid populations in small streams. Increased habitat complexity by FW in our small streams decreased the mortality of brown trout juveniles from predation by mink, as our hypothesis was, and in- creasing habitat complexity can be used to decrease mink pre- dation. We found considerably lower survival rates in hatchery brown trout than in wild trout of the same genetic origin, which is in accordance with earlier findings that hatchery fish are more vulnerable to predation than wild fish (Einum & Fleming, 2001). It also confirms our hypothesis that hatchery brown trout are more vulnerable to mink predation than wild brown trout. Among the hatchery brown trout under mink predation pressure, smaller trout individuals survived better than larger individuals, which suggests that stocking of large-sized juveniles in small streams might not be beneficial. The result of large trout surviving poorer was the opposite to our hypothesis that large body size protects F I G U R E 3   Average survival (±1 SE) of brown trout from mink predation in the outdoor semi-natural streams in Experiment 1. Black symbols indicate hatchery-reared brown trout (HBT), the open symbols indicate wild brown trout (WBT). N = 3 for each treatment, error bars represent standard error of the mean F I G U R E 4   The average total length (±1 SE) of brown trout that survived (open symbols) and that were preyed on by mink (black symbols) in the Experiment 1 in semi-natural streams. (a) Wild brown trout, (b) hatchery-reared brown trout. Allopatry: fish of one origin present, sympatry: both fish origins present. N = 3 for each treatment, error bars represent standard error of the mean      |  439VEHANEN et al. from mink predation, but in this study we cannot rule out a differ- ent outcome in a different habitat. 4.1 | Mortality of hatchery and wild brown trout High mortality from mink predation was found in the experimental channels, built to simulate small natural streams. Similarly to our re- sults, minks have been observed to prey effectively on brown trout juveniles in corresponding environments in small natural streams (Heggenes & Borgstrøm, 1988; Lindstrom & Hubert, 2004). When together with hatchery trout, wild trout were preyed on more than in treatments when only wild trout were present. This indi- cates that the occurrence of hatchery fish might increase predation pressure, thereby exposing wild conspecifics to a higher preda- tion risk. Earlier research has shown that large releases can attract predators and thereby reducing the production of wild populations (Nickelson, 2003; Van Alen, 2000), or that the larger size of hatch- ery salmonids increase predation because they are in the predators’ preferred size range (Nelson et  al.,  2019). Both minks and otters are known to use hatchery fish as a food resource by visiting fish farms for prey (Manikowska-Ślepowrońska et al., 2016), and otters also use salmonid-rich streams of stocked salmonids during the win- ter (Jacobsen,  2005; Ludwig et  al.,  2002). We suggest that hatch- ery brown trout, when stocked among the wild trout, can increase predation from semi-aquatic predators, thus increasing the preda- tion mortality also among wild fish, as we observed in semi-natural streams. Higher predation on hatchery fish may be the result of behavioural differences. Predator-naïve hatchery fish have not developed the same predator avoidance behaviours as wild fish (Mes et al., 2019; Olla et al., 1998). Although predator-naïve brown trout are able to develop antipredator behaviour, it commonly occurs when they sense faeces from minks that have been feeding on their conspe- cifics (Rosell et al., 2013). During the winter, brown trout juveniles shift to deeper water or use more cover to avoid predation (Huusko et al., 2007). We did not observe any significant differences in hab- itat use; both the wild and hatchery trout used mostly the flowing stream habitat (see Enefalk et al., 2019; Huusko & Vehanen, 2011). Our results support the earlier results that hatchery fish are more vulnerable to predation than wild fish (Einum & Fleming, 2001), also in the case with a semi-aquatic predator. 4.2 | The effect of body size We hypothesised that larger brown trout would be less preyed upon by mink, which was contradicted by our results. Instead, we found higher predation rates on larger brown trout. Generally, larger fish individuals both in marine (Sogard, 1997; Vehanen et al., 1993) and freshwater lake environments (Hesthagen & Johnsen,  1992; O'Grady,  1984; Vehanen,  1998) avoid predation better compared to smaller individuals when the main predation pressure stems from piscivorous, gape-limited, fish predators. The bigger is better hy- pothesis, i.e. that fish with a larger body size have higher survival than smaller individuals, has led managers to use of large hatchery ju- veniles for stocking in streams and rivers (Harvey et al., 2016; Nelson et al., 2019). These stocked hatchery juveniles are typically consid- erably larger than their wild conspecifics at the same age. Although a large body size is beneficial against gape-limited predators in the sea or lake environments, our results from small streams show that a smaller body size could be beneficial when coping with wading and diving non–gape-limited predators. In streams, small body size has been found to be an advantage over larger individuals in terms of survival in brown trout (Carlson et al., 2008) and cutthroat trout (Oncorhynchus clarkii) (Uthe et al.,2016) juveniles. Also, for bird pre- dation, enhanced survival of smaller individuals has been observed in rare cases among salmonids (Sogard, 1997). Therefore, the gener- ality of the bigger is better hypothesis could be questioned. Size-selective mortality may also have prolonged consequences for salmonids (Russell et al., 2012; Sogard, 1997). Specifically, mor- tality during different life stages of salmonids are not independent of each other, i.e. characteristics carried over from their juvenile phase in rivers may be important determinants of their survival during their growth phase also in marine or freshwater environments (Russell et al., 2012). To maximise adult returns of declining Atlantic salmon F I G U R E 5   (a) Survival (%) of brown trout in two treatment areas with the addition of small wood and a control (no wood addition), in Experiment 2 in semi-natural streams. N = 9 for both treatments. (b) Size (fork length in mm) of brown trout that were preyed on and those that survived in both treatments. Box plots: box length shows the range within which the central 50% of the values fall, vertical line marks the median, asterisks are outliers 20 30 40 50 60 70 80 90 Small wood addition Control Treatment AliveDead 80 90 100 110 120 130 Small wood addition Control Treatment (a) (b) Su rv iv al % Le ng th m m 440  |     VEHANEN et al. stocks, restoration efforts should focus on the freshwater life-stages to maximise the number and the size of emigrating smolts (Gregory et al., 2019). In shallow water in coastal marine environments, how- ever, otters select larger fish for prey from the prevailing fish com- munity (Cote et al., 2008). It is obvious that the effect of body size on the vulnerability to predation is dependent on the environment and the predator or mix of predators (Livernois et al., 2019), which should be taken into account in management. In shallow water with wading and diving non–gape-limited predators, a large body size of juvenile brown trout was a disadvantage. 4.3 | Habitat heterogeneity and predation mortality As hypothesised, we observed that the mortality from mink pre- dation was lower in treatment areas where FW was added to the streams. FW adds heterogeneity to the habitat by providing shelter for juvenile fish. Before the mink entered our semi-natural streams, Enefalk et al. (2019) had found that brown trout aggregated among wood bundles, whereas fish in the control sections the fish were more evenly distributed, and that the growth rate was significantly lower in treatment areas with wood addition than without wood ad- dition. Here, we found (Experiment 2) brown trout juveniles were willing to use the cover from the wood addition, possibly to increase survival from predation, even at the cost of slower growth. Similar results have been observed for chub (Squalius cephalus), that was ob- served to grow significantly less under bird predation threat, and the authors argued that the increased use of cover led to costs in terms of lost feeding opportunities but to benefits in terms of predator avoidance (Allouche & Gaudin, 2001). The smaller body can also be viewed as a benefit in terms of lowered predation rates, as diving non–gape-limited predators seem to prefer larger prey. Understanding the relationships among habitat preferences and how these affect predation can be complex and challenging. Habitat preferences differ between size classes of brown trout juveniles. For example, larger juveniles prefer deeper water and coarser sub- strate during winter compared to smaller size class (Mäki-Petäys et  al.,  1997). Also, larger salmonids prefer to shelter among larger wood (Langford et al., 2012). It may be that the habitat in our re- search units in semi-natural streams, consisting of FW material, up- stream riffle, and a downstream pool section, were more suited for small-sized hatchery trout. We acknowledge that our inferences may not extend to larger rivers with larger wood and more diverse hab- itat. Habitat complexity strongly affects predation rates due to the availability of preferred refuges (Beukers & Jones, 1997; Nelson & Bonsdorff, 1990). In successful restoration projects, it is important to have knowledge about the habitat preferences of native species as to create useful refuge habitats (Billman et al., 2013). Restoration is specifically important for migrating salmonids, that have com- plex life cycles involving different habitats, and are thus especially vulnerable for habitat changes (Vagg & Hepworth,  2006). In an example of habitat improvement, both crayfish (Cambarus bartoni) and brook trout (Salvelinus fontinalis) production increased, but the mink population only exploited the crayfish population (Burgess & Bider, 1980). This implied that crayfish were easier catch for mink, whereas trout were capable of taking advantage of the increased complexity to avoid mink predation also during summer period. In another example otters were found to prey on hatchery brown trout in one stream, but not in another, due to different fish community composition in the two streams (Jacobsen,  2005). This supports the idea that wading/diving predators adapt their foraging to local conditions. As mink predation efficiency seems to vary with habi- tat characteristics (Heggenes & Borgstrøm, 1988), prey populations might benefit from habitat restoration. Overall, these results high- light the complexity of predator–prey interactions and pinpoint the need to follow up fish populations in newly restored habitats, to en- sure that the target species benefits from the restoration. Additionally, habitat preferences vary seasonally as fish typically shift to low velocity areas in the winter (Huusko et al., 2007). In ju- venile salmonids, these changes in habitat use typically are observed at micro- to mesohabitat scale as they prefer lower water velocity areas within riffles, such as streambank areas or larger cover sub- strate, or emigrate relatively short distances to pools (Cunjak, 1996). These changes in preferred habitats have been related to the need to conserve energy at low water temperatures, but also to obtain shel- ter from endothermic predators and piscivorous fish (Valdimarson & Metcalfe, 1998). Thus, it is possible that overwinter survival of sal- monids increases if fish has a possibility to move between habitats or in search of preferred pool areas (Elso & Greenberg, 2001). If suit- able winter habitats are not present, brown trout can demonstrate greater movements (Huusko et al., 2007), presumably to seek more suitable habitat. Larger scale movements between summer habi- tat in lakes and winter habitat in rivers to avoid predation are also common in fishes (Skov et al., 2013). While our semi-natural streams consisted of pool and riffle habitat, juvenile brown trout could not emigrate outside this area. Therefore, the movements of fish were more limited than in a natural stream, and this could have intensified the predation intensity from the mink. However, the physical habitat (water depth and velocity) in our semi-natural streams was within the preferred range of brown trout juveniles during winter in the region (Mäki-Petäys et al., 1997). 4.4 | Fish stocking to support wild populations in streams Stocking of hatchery fish is a globally used method to support weakening fish stocks (Cowx, 1994). Fish stocking can increase the densities of fish, but there is strong evidence of negative ecological effects on wild populations (Aas et al., 2018; Huusko et al., 2018; Uusi-Heikkilä et al., 2018). For managers, an important decision is whether or not to use hatchery fish to support wild populations. Our results suggested that stocking hatchery fish among wild trout could increase the predation pressure from terrestrial predators, such as mink. The brown trout used in our study, both hatchery and wild juveniles, originated from one of the last remaining viable      |  441VEHANEN et al. naturally reproducing adfluvial brown trout stocks in Finland, and the status of this stock is considered endangered due to anthropo- genic impacts (Huusko et al., 1990, 2018). As this is a worldwide trend where migrating salmonids, both anadromous and adfluvial, are considered vulnerable (Freyhof, 2014; Hyvärinen et al., 2019; Ricciardi & Rasmussen, 1999), it is essential to understand the con- sequences of hatchery releases in all habitats and all life cycles. 5  | CONCLUSIONS Our study has management implications specifically in small streams, which are important for brown trout (Jonsson et  al.,  2001; Sutela et al., 2020; Vehanen et al., 2020). Survival during the juvenile stages is highly important for the growth of the brown trout populations (Elliott, 1994), and predation is one of the main drivers of fish com- munity structure (Gebrekiros,  2016). Our results illustrate the im- portance of considering how the needs differ between life stages of migrating salmonids in terms of suitable habitat conditions for feed- ing and predator avoidance. Second, before supporting wild salmo- nid stocks with hatchery juveniles, the characteristics of hatchery fish, such as the hatchery background, should be considered. For ex- ample, occurrence of hatchery fish may increase predation on wild fish and a large individual size is not necessarily beneficial in small streams. Third, we encourage increasing the habitat heterogeneity, for example by adding wood to streams, in restoration and habitat enhancement efforts, as this is likely to reduce predation from wad- ing and diving predators, such as feral American mink. In addition to restoration of habitat complexity, other management actions should be considered to mitigate the predation effect of feral mink. These could include eradication (removing all feral mink from a limited area), control (keeping mink numbers low) by trapping or hunting or prevention of further escapes of feral mink (Bonesi & Palazon, 2007). ACKNOWLEDG MENTS All authors contributed to the writing of the article. The staff of the Kainuu Fisheries Research Station, Natural Resources Institute Finland, offered logistical help during the experiment. Experiment 1 was approved by the Finnish ethics committee on animal experiments (licence 8/05), and Experiment 2 was con- ducted according to animal experiment legislation in Finland, license EVISA-2458-04.10-03.2011. DATA AVAIL ABILIT Y S TATEMENT Data are available from the authors upon reasonable request. ORCID Teppo Vehanen  https://orcid.org/0000-0003-3441-6787 R E FE R E N C E S Aas, Ø., Cucherousset, J., Fleming, I. A., Wolter, C., Höjesjö, J., Buoro, M., … Arlinghaus, R. (2018). Salmonid stocking in five North Atlantic ju- risdictions: Identifying drivers and barriers to policy change. Aquatic Conservation: Marine and Freshwater Ecosystems, 28(6), 1451–1464. https://doi.org/10.1002/aqc.2984 Alioravainen, N., Hyvärinen, P., Kortet, R., Härkönen, L., & Vainikka, A. (2018). Survival of crossbred brown trout under experimental pike predation and stocking in the wild. Boreal Environment Research, 23, 267–281. Allouche, S., & Gaudin, P. (2001). Effects of avian predation threat, water flow and cover on growth and habitat use by chub, Leuciscus ceph- alus, in an experimental stream. Oikos, 94(3), 481–492. https://doi. org/10.1034/j.1600-0706.2001.940310.x Andersson, L. C., & Reynolds, J. D. (2018). Habitat features mediate selec- tive consumption of salmon by bears. Canadian Journal of Fisheries and Aquatic Sciences, 75(6), 955–963. https://doi.org/10.1139/cjfas​ -2017-0055 Archard, G. A., Earley, R. L., Hanninen, A. F., & Braithwaite, V. A. (2012). Correlated behaviour and stress physiology in fish exposed to dif- ferent levels of predation pressure. Functional Ecology, 26(3), 637– 645. https://doi.org/10.1111/j.1365-2435.2012.01968.x Armstrong, J. D. (2005). Spatial variation in population dynamics of juvenile Atlantic salmon: Implications for conservation and man- agement. Journal of Fish Biology, 67(Suppl B), 35–52. https://doi. org/10.1111/j.0022-1112.2005.00929.x Berejikian, B. A. (1995). The effects of hatchery and wild ancestry and experience on the ability of steelhead trout fry (Oncorhynchus mykiss) to avoid a benthic predator. Canadian Journal of Fisheries and Aquatic Sciences, 52(11), 2476–2482. https://doi.org/10.1139/ f95-838 Beukers, J. S., & Jones, G. P. (1997). Habitat complexity modifies the im- pact of piscivores on a coral reef fish population. Oecologia, 114(1), 50–59. https://doi.org/10.1007/s0044​20050419 Billman, E. J., Kreitzer, J. D., Creighton, J. C., Habit, E., McMillan, B., & Belk, M. C. (2013). Habitat enhancement and native fish con- servation: Can enhancement of channel complexity promote the coexistence of native and introduced fishes? Environmental Biology of Fishes, 96(4), 555–566. https://doi.org/10.1007/s1064​ 1-012-0041-2 Bonesi, L., & Palazon, S. (2007). The American mink in Europe: Status, impacts, and control. Biological Conservation, 134(4), 470–483. https://doi.org/10.1016/j.biocon.2006.09.006 Burgess, S. A., & Bider, J. R. (1980). Effects of stream habitat improve- ments on invertebrates, trout populations, and mink activity. The Journal of Wildlife Management, 44(4), 871–880. https://doi. org/10.2307/3808315 Cairns, D. K. (2006). A review of predator-prey and competitive inter-specific interactions in Atlantic salmon (Salmo salar) (pp. 17). Canadian Science Advisory Secretariat, Research Document 2006/019. http://www.dfo-mpo.gc.ca/CSAS/ Carlson, S., Olsen, E., & Vollestad, A. (2008). Seasonal mortality and the effect of body size: A review and an empirical test using individual data on brown trout. Functional Ecology, 22(4), 663–673. https:// doi.org/10.1111/j.1365-2435.2008.01416.x Carss, D. N., Kruuk, H., & Conroy, J. W. H. (1990). Predation on adult Atlantic salmon, Salmo salar L., by otters, Lutra lutra (L.), within the River Dee system, Aberdeenshire, Scotland. Journal of Fish Biology, 37(6), 935–944. https://doi.org/10.1111/j.1095-8649.1990.tb035​ 97.x Chibowski, P., Zalewski, A., Suska-Malawska, M., & Brzeziński, M. (2019). Study on geographical differences in American mink diets reveals variations in isotopic composition of potential mink prey. Mammal Research, 64(3), 343–351. https://doi.org/10.1007/s1336​4-019- 00419​-4 Cote, D., Gregory, R. S., & Stewart, H. M. J. (2008). Size-selective pre- dation by river otter (Lontra canadensis) improves refuge proper- ties of shallow coastal marine nursery habitats. Canadian Journal of Zoology, 86(11), 1324–1328. https://doi.org/10.1139/Z08-120 https://orcid.org/0000-0003-3441-6787 https://orcid.org/0000-0003-3441-6787 https://doi.org/10.1002/aqc.2984 https://doi.org/10.1034/j.1600-0706.2001.940310.x https://doi.org/10.1034/j.1600-0706.2001.940310.x https://doi.org/10.1139/cjfas-2017-0055 https://doi.org/10.1139/cjfas-2017-0055 https://doi.org/10.1111/j.1365-2435.2012.01968.x https://doi.org/10.1111/j.0022-1112.2005.00929.x https://doi.org/10.1111/j.0022-1112.2005.00929.x https://doi.org/10.1139/f95-838 https://doi.org/10.1139/f95-838 https://doi.org/10.1007/s004420050419 https://doi.org/10.1007/s10641-012-0041-2 https://doi.org/10.1007/s10641-012-0041-2 https://doi.org/10.1016/j.biocon.2006.09.006 https://doi.org/10.2307/3808315 https://doi.org/10.2307/3808315 http://www.dfo-mpo.gc.ca/CSAS/ https://doi.org/10.1111/j.1365-2435.2008.01416.x https://doi.org/10.1111/j.1365-2435.2008.01416.x https://doi.org/10.1111/j.1095-8649.1990.tb03597.x https://doi.org/10.1111/j.1095-8649.1990.tb03597.x https://doi.org/10.1007/s13364-019-00419-4 https://doi.org/10.1007/s13364-019-00419-4 https://doi.org/10.1139/Z08-120 442  |     VEHANEN et al. Cowx, I. G. (1994). Stocking strategies. Fisheries Management and Ecology, 1(1), 15–30. https://doi.org/10.1111/j.1365-2400.1970.tb000​03.x Crowder, L. B., Squires, D. D., & Rice, J. A. (1997). Nonadditive effects of terrestrial and aquatic predators on juvenile estuarine fish. Ecology, 78(6), 1796–1804. Cunjak, R. A. (1996). Winter habitat of selected stream fishes and poten- tial impacts from land-use activity. Canadian Journal of Fisheries and Aquatic Sciences, 53(Suppl. 1), 267–282. https://doi.org/10.1139/ f95-275 Einum, S., & Fleming, I. G. (2001). Implications of stocking: Ecological interactions between wild and released salmonids. Nordic Journal of Freshwater Research, 75(1), 56–70. Elliott, J. M. (1994). Quantitative ecology and the brown trout (p. 286). Oxford University Press. Elso, J. I., & Greenberg, L. A. (2001). Habitat use, movements and survival of individual 0+ brown trout (Salmo trutta) during winter. Archiv Fur Hydrobiologie, 152(2), 279–295. Enbom, M. (2015). The effect of migratory fish on freshwater ecosystem nu- trient dynamics. Examensarbete i ämnet biologi 2015:15. Swedish University of Agricultural Sciences, Faculty of Forest Science, p. 29. http://stud.epsil​on.slu.se Enefalk, Å., Huusko, A., Louhi, P., & Bergman, E. (2019). Fine stream wood decreases growth of juvenile brown trout (Salmo trutta L.). Environmental Biology of Fishes, 102(5), 759–770. https://doi. org/10.1007/s1064​1-019-00869​-4 Erlinge, S. (1969). Food habits of the otter Lutra lutra L. and the mink Mustela vison Schreber in a trout water in southern Sweden. Oikos, 20(1), 1–7. Filip’echev, A. O., Belyachenko, A. V., & Savonin, A. A. (2016). The American mink Neovison vison Schreber, 1777 (Carnivora, Mustelidae) in the floodplain and right-shore ecosystems of the Volgograd water reservoir: Seasonal changes in its spatial struc- ture, feeding, and temporal activity. Biology Bulletin, 43(10), 1407– 1415. https://doi.org/10.1134/S1062​35901​6100046 Freyhof, J. (2014). Salmo salar. The IUCN Red List of Threatened Species 2014: e.T19855A2532398. https://www.iucnr​edlist.org Gebrekiros, S. T. (2016). Factors affecting stream fish community compo- sition and habitat suitability. Journal of Aquaculture & Marine Biology, 4(2), 00076. https://doi.org/10.15406/​jamb.2016.04.00076 Gende, S. M., Edwards, R. T., Willson, M. F., & Wipfli, M. S. (2002). Pacific salmon in aquatic and terrestrial ecosystems. BioScience, 52(10), 917–928. Gerell, R. (1967). Food selection in relation to habitat in mink (Mustela vison Schreber) in Sweden. Oikos, 18(2), 233–246. https://doi. org/10.2307/3565101 Gerell, R. (1969). Activity patterns of the mink Mustela vison Schreber in Southern Sweden. Oikos, 20(2), 451–460. https://doi. org/10.2307/3543208 Giam, X., & Olden, J. D. (2016). Environment and predation govern fish community assembly in temperate streams. Global Ecology and Biogeography, 25(10), 1194–1205. https://doi.org/10.1111/ geb.12475 Gregory, S. D., Ibbotson, A. T., Riley, W. D., Nevoux, M., Lauridsen, R. B., Russell, I. C., … Rivot, E. (2019). Atlantic salmon return rate increases with smolt length. ICES Journal of Marine Science, 76(6), 1702–1712. https://doi.org/10.1093/icesj​ms/fsz066 Harvey, A. C., Solberg, M. F., Troianou, E., Carvalho, G. R., Taylor, M. I., Creer, S., … Glover, G. A. (2016). Plasticity in growth of farmed and wild Atlantic salmon: Is the increased growth rate of farmed salmon caused by evolutionary adaptations to the commercial diet? BMC Evolutionary Biology, 16, 264. https://doi.org/10.1186/s1286​ 2-016-0841-7 He, X., & Kitchell, J. F. (1990). Direct and indirect effects of predation on a fish community: A whole-lake experiment. Transactions of the American Fisheries Society, 119(5), 825–835. Heggenes, J., & Borgstrøm, R. (1988). Effect of mink, Mustela vison Schreber, predation on cohorts of juvenile Atlantic salmon, Salmo salar L., and brown trout, S. trutta L., in three small streams. Journal of Fish Biology, 33(6), 885–894. https://doi.org/10.1111/ j.1095-8649.1988.tb055​36.x Hesthagen, T., & Johnsen, B. O. (1992). Effects of fish density and size on survival, growth and production of hatchery-reared brown trout (Salmo trutta L.) in lakes. Fisheries Research, 15(1–2), 147–156. https://doi.org/10.1016/0165-7836(92)90010​-Q Hilderbrand, G. V., Farley, S. D., Schwartz, C. C., & Robbins, C. T. (2004). Importance of salmon to wildlife: Implications for integrated man- agement. Ursus, 15(1), 1–9. Hoey, A. S., & McCormick, M. I. (2004). Selective predation for low body condition at the larval-juvenile transition of a coral reef fish. Oecologia, 139(1), 23–29. https://doi.org/10.1007/s0044​ 2-004-1489-3 Höjesjö, J., Gunve, E., Bohlin, T., & Johnsson, J. I. (2014). Addition of structural complexity–contrasting effect on juvenile brown trout in a natural stream. Ecology of Freshwater Fish, 24(4), 608–615. https:// doi.org/10.1111/eff.12174 Holland, A. M., Schauber, E. M., Nielsen, C. K., & Hellgren, E. C. (2018). Stream community richness predicts apex predator occupancy dy- namics in riparian systems. Oikos, 127(10), 1422–1436. https://doi. org/10.1111/oik.05085 Huusko, A., Greenberg, L., Stickler, M., Linnansaari, T., Nykänen, M., Vehanen, T., … Alfredsen, K. (2007). Life in the ice lane: The winter ecology of stream salmonids. River Research and Applications, 23(5), 469–491. https://doi.org/10.1002/rra.999 Huusko, A., Vainikka, A., Syrjänen, J. T., Orell, P., Louhi, P., & Vehanen, T. (2018). Life-history of the adfluvial brown trout (Salmo trutta L.) in eastern Fennoscandia. In J. Lobon-Cervia, & N. Sanz (Eds.), Brown trout. Biology ecology and management. John Wiley & Sons Ltd. Huusko, A., van der Meer, O., & Koljonen, M.-L. (1990). Life history pat- terns and genetic differences in brown trout (Salmo trutta L.) in the Koutajoki river system. Polskie Archiwum Hydrobiologii, 36(1), 63–77. Huusko, A., & Vehanen, T. (2011). Do hatchery-reared brown trout affect the growth and habitat use of wild congeners? Fisheries Management and Ecology, 18(3), 258–261. https://doi. org/10.1111/j.1365-2400.2010.00770.x Hyvärinen, E., Juslén, A., Kemppainen, E., Uddström, A., & Liukko, U.-M. (Eds.) (2019). The 2019 Red List of Finnish species. Ympäristöministeriö & Suomen ympäristökeskus. Helsinki. http://hdl.handle.net/10138/​ 299501 Hyvärinen, P., & Rodewald, P. (2013). Enriched rearing improves survival of hatchery-reared Atlantic salmon smolts during migration in the River Tornionjoki. Canadian Journal of Fisheries and Aquatic Sciences, 70(9), 1386–1395. https://doi.org/10.1139/cjfas​-2013-0147 Hyvärinen, P., & Vehanen, T. (2004). Effect of brown trout body size on post-stocking survival and pike predation. Ecology of Freshwater Fish, 13(2), 77–84. https://doi.org/10.1111/j.1600-0633.2004.00050.x Jackson, D. A., Peres-Neto, P. R., & Olden, J. D. (2001). What controls who is where in freshwater fish communities – The roles of biotic, abiotic, and spatial factors. Canadian Journal of Fisheries and Aquatic Sciences, 58(1), 157–170. https://doi.org/10.1139/cjfas​-58-1-157 Jacobsen, L. (2005). Otter (Lutra lutra) predation on stocked brown trout (Salmo trutta) in two Danish lowland rivers. Ecology of Freshwater Fish, 14(1), 59–68. https://doi.org/10.1111/j.1600-0633.2004.00076.x Jonsson, B., & Jonsson, N. (2003). Migratory Atlantic salmon as vectors for the transfer of energy and nutrients between freshwater and marine environments. Freshwater Biology, 48(1), 21–27. https://doi. org/10.1046/j.1365-2427.2003.00964.x Jonsson, B., Jonsson, N., Brodtkorb, E., & Ingebrigtsen, P.-J. (2001). Life-history traits of brown trout vary with the size of small streams. Functional Ecology, 15(3), 310–317. https://doi. org/10.1046/j.1365-2435.2001.00528.x https://doi.org/10.1111/j.1365-2400.1970.tb00003.x https://doi.org/10.1139/f95-275 https://doi.org/10.1139/f95-275 http://stud.epsilon.slu.se https://doi.org/10.1007/s10641-019-00869-4 https://doi.org/10.1007/s10641-019-00869-4 https://doi.org/10.1134/S1062359016100046 https://www.iucnredlist.org https://doi.org/10.15406/jamb.2016.04.00076 https://doi.org/10.2307/3565101 https://doi.org/10.2307/3565101 https://doi.org/10.2307/3543208 https://doi.org/10.2307/3543208 https://doi.org/10.1111/geb.12475 https://doi.org/10.1111/geb.12475 https://doi.org/10.1093/icesjms/fsz066 https://doi.org/10.1186/s12862-016-0841-7 https://doi.org/10.1186/s12862-016-0841-7 https://doi.org/10.1111/j.1095-8649.1988.tb05536.x https://doi.org/10.1111/j.1095-8649.1988.tb05536.x https://doi.org/10.1016/0165-7836(92)90010-Q https://doi.org/10.1007/s00442-004-1489-3 https://doi.org/10.1007/s00442-004-1489-3 https://doi.org/10.1111/eff.12174 https://doi.org/10.1111/eff.12174 https://doi.org/10.1111/oik.05085 https://doi.org/10.1111/oik.05085 https://doi.org/10.1002/rra.999 https://doi.org/10.1111/j.1365-2400.2010.00770.x https://doi.org/10.1111/j.1365-2400.2010.00770.x http://hdl.handle.net/10138/299501 http://hdl.handle.net/10138/299501 https://doi.org/10.1139/cjfas-2013-0147 https://doi.org/10.1111/j.1600-0633.2004.00050.x https://doi.org/10.1139/cjfas-58-1-157 https://doi.org/10.1111/j.1600-0633.2004.00076.x https://doi.org/10.1046/j.1365-2427.2003.00964.x https://doi.org/10.1046/j.1365-2427.2003.00964.x https://doi.org/10.1046/j.1365-2435.2001.00528.x https://doi.org/10.1046/j.1365-2435.2001.00528.x      |  443VEHANEN et al. Korsu, K., Huusko, A., & Muotka, T. (2007). Niche characteristics ex- plain the reciprocal invasion success of stream salmonids in dif- ferent continents. Proceedings of the National Academy of Sciences of the United States of America, 104(23), 9725–9729. https://doi. org/10.1073/pnas.06107​19104 Korsu, K., Huusko, A., & Muotka, T. (2009). Does the introduced brook trout (Salvelinus fontinalis) affect growth of the native brown trout (Salmo trutta)? Naturwissenschaften, 96(3), 347–353. https://doi. org/10.1007/s0011​4-008-0482-9 Krall, M., Clark, C., Roni, P., & Ross, K. (2019). Lessons learned from long-term effectiveness monitoring of instream habitat projects. North American Journal of Fisheries Management, 39(6), 1395–1411. https://doi.org/10.1002/nafm.10381 Krueger, C. C., & May, B. (1991). Ecological and genetic-effects of sal- monid introductions in North-America. Canadian Journal of Fisheries and Aquatic Sciences, 48(1), 66–77. https://doi.org/10.1139/f91-305 Langford, T. E. L., Langford, J., & Hawkins, S. J. (2012). Conflicting ef- fects of woody debris on stream fish populations:Implications for management. Freshwater Biology, 57(5), 1096–1111. https://doi. org/10.1111/j.1365-2427.2012.02766.x Larocque, S. M., Johnson, T. B., & Fisk, A. T. (2020). Survival and migration patterns of naturally and hatchery-reared Atlantic salmon (Salmo salar) smolts in a Lake Ontario tributary using acoustic teleme- try. Freshwater Biology, 65(5), 835–848. https://doi.org/10.1111/ fwb.13467 Lemopoulos, A., Uusi-Heikkilä, A., Vasemägi, A., Huusko, A., Kokko, H., & Vainikka, A. (2018). Genome-wide divergence patterns support fine-scaled genetic structuring associated with migration tendency in brown trout. Canadian Journal of Fisheries and Aquatic Sciences, 75(10), 1680–1692. https://doi.org/10.1139/cjfas​-2017-0014 Levi, T., Darimont, C. T., MacDuffee, M., Mangel, M., Paquet, P., & Wilmers, C. C. (2012). Using grizzly bears to assess harvest- ecosystem tradeoffs in salmon fisheries. PLoS Biology, 10(4), e1001303. https://doi.org/10.1371/journ​al.pbio.1001303 Levi, T., Hilderbrand, G. V., Hocking, M. D., Quinn, T. P., White, K. S., Adams, M. S., … Wilmers, C. C. (2020). Community ecology and conservation of bear-salmon ecosystems. Frontiers in Ecology and Evolution, 8, 513304. https://doi.org/10.3389/fevo.2020.513304 Levi, T., Wheat, R. E., Allen, J. M., & Wilmers, C. C. (2015). Differential use of salmon by vertebrate consumers: Implications for conserva- tion. PeerJ, 3, e1157. https://doi.org/10.7717/peerj.1157 Lindstrom, J. W., & Hubert, W. A. (2004). Mink predation on radio-tagged trout during winter in a low-gradient reach of a mountain stream, Wyoming. Western North American Naturalist, 64(4), 551–553. Linnansaari, T., Roussel, J.-M., Cunjak, R. A., & Halleraker, J. V. (2007). Efficacy and accuracy of portable PIT-antennae when locating fish in ice-covered streams. Hydrobiologia, 582(9), 281–287. https://doi. org/10.1139/F10-093 Livernois, M. C., Fodrie, F. J., Heck, K. L., & Powers, S. P. (2019). Emergent intraspecifc multiple predator effects shape estuarine trophic dynamics across a gradient of habitat complexity. Journal of Experimental Marine Biology and Ecology, 511, 120–128. Louhi, P., Vehanen, T., Huusko, A., Mäki-Petäys, A., & Muotka, T. (2016). Long-term monitoring reveals the success of salmonid habitat res- toration. Canadian Journal of Fisheries and Aquatic Sciences, 73(12), 1733–1741. https://doi.org/10.1139/cjfas​-2015-0546 Ludwig, G. X., Hokka, V., Sulkava, R., & Ylönen, H. (2002). Otter Lutra lutra predation on farmed and free-living salmonids in boreal freshwater habitats. Wildlife Biology, 8(1), 193–199. https://doi.org/10.2981/ wlb.2002.033 Mäki-Petäys, A., Muotka, T., Huusko, A., Tikkanen, P., & Kreivi, P. (1997). Seasonal changes in habitat use and preference by juvenile brown trout, Salmo trutta, in a northern boreal river. Canadian Journal of Fisheries and Aquatic Sciences, 54(3), 520–530. Manikowska-Ślepowrońska, B., Szydzik, B., & Jakubas, D. (2016). Determinants of the presence of conflict bird and mammal species at pond fisheries in western Poland. Aquatic Ecology, 50(1), 87–95. https://doi.org/10.1007/s1045​2-015-9554-z Marttila, M., Louhi, P., Huusko, A., Vehanen, T., Mäki-Petäys, A., Erkinaro, J., … Muotka, T. (2019). Synthesis of habitat restoration impacts on young-of-the-year salmonids in boreal rivers. Reviews in Fish Biology and Fisheries, 29(3), 513–527. https://doi.org/10.1007/s1116​0-019- 09557​-z Mes, D., van Os, R., Gorissen, M., Ebbesson, L. O. E., Finstad, B., Mayer, I., & Vindas, M. A. (2019). Effects of environmental enrichment on forebrain neural plasticity and survival success of stocked Atlantic salmon. Journal of Experimental Biology, 222, jeb212258. https://doi. org/10.1242/jeb.212258 Metcalfe, N. B., Fraser, N. H. C., & Burns, M. D. (1999). Food availability and the nocturnal vs. diurnal foraging trade-off in juvenile salmon. The Journal of Animal Ecology, 68(2), 371–381. Mihalitsis, M., & Bellwood, D. R. (2017). A morphological and functional basis for maximum prey size in piscivorous fishes. PLoS One, 12(9), e0184679. https://doi.org/10.1371/journ​al.pone.0184679 Nagayama, S., & Nakamura, F. (2010). Fish habitat rehabilitation using wood in the world. Landscape Ecology and Engineering, 6(2), 289– 305. https://doi.org/10.1007/s1135​5-009-0092-5 Nelson, B. W., Shelton, A. O., Anderson, J. H., Ford, M. J., & Ward, E. J. (2019). Ecological implications of changing hatchery practices for Chinook salmon in the Salish Sea. Ecosphere, 10(11), e02922. https://doi.org/10.1002/ecs2.2922 Nelson, W. G., & Bonsdorff, E. (1990). Fish predation and habitat com- plexity: Are complexity thresholds real? Journal of Experimental Marine Biology and Ecology, 141(2–3), 183–194. https://doi. org/10.1016/0022-0981(90)90223​-Y Nickelson, T. (2003). The influence of hatchery coho salmon (Oncorhynchus kisutch) on the productivity of wild coho salmon populations in Oregon coastal basins. Canadian Journal of Fisheries and Aquatic Sciences, 60(9), 1050–1056. https://doi.org/10.1139/ f03-091 Nilsson, A., & Brönmark, C. (2000). Prey vulnerability to a gape- size limited predator: Behavioural and morphological impacts on northern pike piscivory. Oikos, 88(3), 539–546. https://doi. org/10.1034/j.1600-0706.2000.880310.x Nilsson, C., Polvi, L. E., Gardeström, J., Hasselquist, E. M., Lind, L., & Sarneel, J. M. (2015). Riparian and in-stream restoration of boreal streams and rivers: Success or failure? Ecohydrology, 8(5), 753–764. https://doi.org/10.1002/eco.1480 O'Grady, M. F. (1984). The importance of genotype, size on stocking and stocking date to the survival of brown trout (Salmo trutta) released in Irish lakes (pp. 178–191). EIFAC Technical Paper No. 42, Suppl 1. Olla, B. L., Davis, M. W., & Ryer, C. H. (1998). Understanding how the hatchery environment represses or promotes the development of behavioral survival skills. Bulletin of Marine Science, 62(2), 531–550. Pinnegar, J. K., Trenkel, V. M., Tidd, A. N., Dawson, W. A., & Du buit, M. H. (2003). Does diet in Celtic Sea fishes reflect prey avail- ability? Journal of Fish Biology, 63(1), 197–212. https://doi. org/10.1111/j.1095-8649.2003.00204.x Pister, E. P. (2001). Wilderness fish stocking: History and perspec- tive. Ecosystems, 4(4), 279–286. https://doi.org/10.1007/s1002​ 1-001-0010-7 Power, E. (1987). Predator avoidance by grazing fishes in temperate and tropical streams: Importance of stream depth and prey size. In W. C. Kerfoot, & A. Sih (Eds.), Predation: Direct and indirect impacts on aquatic communities (pp. 333–351). University Press of New England. Ricciardi, A., & Rasmussen, J. B. (1999). Extinction rates of North American freshwater fauna. Conservation Biology, 13(5), 1220– 1222. https://doi.org/10.1046/j.1523-1739.1999.98380.x https://doi.org/10.1073/pnas.0610719104 https://doi.org/10.1073/pnas.0610719104 https://doi.org/10.1007/s00114-008-0482-9 https://doi.org/10.1007/s00114-008-0482-9 https://doi.org/10.1002/nafm.10381 https://doi.org/10.1139/f91-305 https://doi.org/10.1111/j.1365-2427.2012.02766.x https://doi.org/10.1111/j.1365-2427.2012.02766.x https://doi.org/10.1111/fwb.13467 https://doi.org/10.1111/fwb.13467 https://doi.org/10.1139/cjfas-2017-0014 https://doi.org/10.1371/journal.pbio.1001303 https://doi.org/10.3389/fevo.2020.513304 https://doi.org/10.7717/peerj.1157 https://doi.org/10.1139/F10-093 https://doi.org/10.1139/F10-093 https://doi.org/10.1139/cjfas-2015-0546 https://doi.org/10.2981/wlb.2002.033 https://doi.org/10.2981/wlb.2002.033 https://doi.org/10.1007/s10452-015-9554-z https://doi.org/10.1007/s11160-019-09557-z https://doi.org/10.1007/s11160-019-09557-z https://doi.org/10.1242/jeb.212258 https://doi.org/10.1242/jeb.212258 https://doi.org/10.1371/journal.pone.0184679 https://doi.org/10.1007/s11355-009-0092-5 https://doi.org/10.1002/ecs2.2922 https://doi.org/10.1016/0022-0981(90)90223-Y https://doi.org/10.1016/0022-0981(90)90223-Y https://doi.org/10.1139/f03-091 https://doi.org/10.1139/f03-091 https://doi.org/10.1034/j.1600-0706.2000.880310.x https://doi.org/10.1034/j.1600-0706.2000.880310.x https://doi.org/10.1002/eco.1480 https://doi.org/10.1111/j.1095-8649.2003.00204.x https://doi.org/10.1111/j.1095-8649.2003.00204.x https://doi.org/10.1007/s10021-001-0010-7 https://doi.org/10.1007/s10021-001-0010-7 https://doi.org/10.1046/j.1523-1739.1999.98380.x 444  |     VEHANEN et al. Roemer, G. W., Gompper, M. E., & Van Valkenburgh, B. (2009). The Ecological role of the mammalian mesocarnivore. BioScience, 59(2), 165–173. https://doi.org/10.1525/bio.2009.59.2.9 Roni, P., Beechie, T., Pess, G., & Hanson, K. (2015). Wood placement in river restoration: Fact, fiction, and future direction. Canadian Journal of Fisheries and Aquatic Sciences, 72(3), 466–478. https:// doi.org/10.1139/cjfas​-2014-0344 Roni, P., Hanson, K., & Beechie, T. (2008). Global review of the physical and biological effectiveness of stream habitat rehabilitation tech- niques. North American Journal of Fisheries Management, 28(3), 856– 890. https://doi.org/10.1577/M06-169.1 Rosell, F., Holtan, L. B., Thorsen, J. G., & Heggenes, J. (2013). Predator-naive brown trout (Salmo trutta) show antipredator behaviours to scent from an introduced piscivorous mammalian predator fed conspecifics. Ethology, 119(4), 303–308. https://doi.org/10.1111/eth.12065 Russell, I. C., Aprahamian, M. W., Barry, J., Davidson, I. C., Fiske, P., Ibbotson, A. T., … Todd, C. D. (2012). The influence of the freshwater environ- ment and the biological characteristics of Atlantic salmon smolts on their subsequent marine survival. ICES Journal of Marine Science, 69(9), 1563–1573. https://doi.org/10.1093/icesj​ms/fsr208 Schlosser, I. J. (1987). The role of predation in age and size related hab- itat use by stream fishes. Ecology, 68(3), 651–659. https://doi. org/10.2307/1938470 Shardlow, T. F., & Hyatt, K. D. (2013). Quantifying associations of large vertebrates with salmon in riparian areas of British Columbia streams by means of camera-traps, bait stations, and hair samples. Ecological Indicators, 27(1), 97–107. https://doi.org/10.1016/j.ecoli​ nd.2012.11.011 Sheaves, M. (2001). Are there really few piscivorous fishes in shallow estuarine habitats? Marine Ecology Progress Series, 222, 279–290. https://doi.org/10.3354/meps2​22279 Skov, C., Chapman, B. B., Baktoft, H., Brodersen, J., Brönmark, C., Hansson, L.-A., … Nilsson, P. A. (2013). Migration confers survival benefits against avian predators for partially migratory freshwa- ter fish. Biology Letters, 9(2), 20121178. https://doi.org/10.1098/ rsbl.2012.1178 Sogard, S. M. (1997). Size-selective mortality in the juvenile stage of te- leost fishes: A review. Bulletin of Marine Science, 60(3), 1129–1157. Stewart, G. B., Bayliss, H. R., Showler, D. A., Sutherland, W. J., & Pullin, A. S. (2009). Effectiveness of engineered in-stream structure mitigation measures to in-crease salmonid abundance: A system- atic review. Ecological Applications, 19(4), 931–941. https://doi. org/10.1890/07-1311.1 Sutela, T., Vehanen, T., & Jounela, P. (2020). Longitudinal patterns of fish assemblages in European boreal streams. Hydrobiologia, 847(11), 3277–3290. https://doi.org/10.1007/s1075​0-020-04330​-x Svanbäck, R., & Eklöv, P. (2006). Genetic variation and phenotypic plas- ticity: Causes of morphological and dietaryvariation in Eurasian perch. Evolutionary Ecology Research, 8(1), 37–49. Taylor, J. J., Rytwinski, T., Bennett, J. R., Smokorowski, K. E., Lapointe, N. W. R., Janusz, R., … Cooke, S. J. (2019). The effectiveness of spawning habitat creation or enhancement for substrate-spawning temperate fish: A systematic review. Environmental Evidence, 8(19), 1–32. https://doi.org/10.1186/s1375​0-019-0162-6 Uthe, P., Al-Chokhachy, R., Zale, A. V., Shepard, B. B., McMahon, T. E., & Stephens, T. (2016). Life history characteristics and vital rates of Yellowstone Cutthroat Trout in two headwater basins. North American Journal of Fisheries Management, 36, 1240–1253. https:// doi.org/10.1080/02755​947.2016.1206643 Uusi-Heikkilä, S., Perälä, T., & Kuparinen, A. (2018). Species’ ecological functionality alters the outcome of fish stocking success predicted by a food-web model. Royal Society Open Science, 5(8), 180465. https://doi.org/10.1098/rsos.180465 Vagg, R., & Hepworth, H. (2006). Migratory species and climate change: Impacts of a changing environment on wild animals. UNEP/CMS Secretariat. Valdimarson, S. K., & Metcalfe, N. B. (1998). Shelter selection in juve- nile Atlantic salmon, or why do salmon seek shelter in winter? Journal of Fish Biology, 52(1), 42–49. https://doi.org/10.1111/ j.1095-8649.1998.tb015​51.x Van Alen, B. W. (2000). Status and stewardship of salmon stocks in Southeast Alaska. In E. E. Knudsen, C. R. Steward, D. D. MacDonald, J. E. Williams, & D. W. Reiser (Eds.), Sustainable fisheries manage- ment: Pacific salmon (pp. 161–194). CRC Press. Vehanen, T. (1998). Ecological factors affecting the success of piscivorous salmonid (Salmo) stocking. PhD Thesis. University of Oulu. Acta Universitas Ouluensis A 303. Vehanen, T. (2003). Adaptive flexibility in the behaviour of juvenile Atlantic salmon: Short-term responses to food availability and threat from predation. Journal of Fish Biology, 63(4), 1034–1045. https://doi.org/10.1046/j.1095-8649.2003.00228.x Vehanen, T. (2006). Intra- and interspecific competition in hatchery landlocked salmon and brown trout in semi-natural streams. Environmental Biology of Fishes, 76(2), 255–264. https://doi. org/10.1007/s1064​1-006-9031-6 Vehanen, T., Aspi, J., & Pasanen, P. (1993). The effect of size, fin erosion, body silvering and precocious maturation on recaptures in Carlin-tagged Baltic salmon (Salmo salar L.). Annales Zoologici Fennici, 30(4), 277–285. Vehanen, T., Huusko, A., & Hokki, R. (2009). Competition between hatchery-raised and wild brown trout Salmo trutta in enclosures – Do hatchery releases have negative effects on wild popula- tions? Ecology of Freshwater Fish, 18(2), 261–268. https://doi. org/10.1111/j.1600-0633.2008.00343.x Vehanen, T., Huusko, A., Mäki-Petäys, A., Louhi, P., Mykrä, H., & Muotka, T. (2010). Effects of habitat rehabilitation on brown trout (Salmo trutta) in boreal forest streams. Freshwater Biology, 55(10), 2200– 2214. https://doi.org/10.1111/j.1365-2427.2010.02467.x Vehanen, T., Sutela, T., & Harjunpää, A. (2020). The effects of ecoregions and local environmental characteristics on spatial patterns in boreal riverine fish assemblages. Ecology of Freshwater Fish, 29(4), 739– 751. https://doi.org/10.1111/eff.12550 Vinterstare, J., Hulthén, K., Nilsson, D. E., Nilsson, P. A., & Brönmark, C. (2020). More than meets the eye: Predator-induced pupil size plas- ticity in a teleost fish. Journal of Animal Ecology, 89(10), 2258–2267. Weber, E. D., & Fausch, K. D. (2003). Interactions between hatchery and wild salmonids in streams: Differences in biology and evidence for competition. Canadian Journal of Fisheries and Aquatic Sciences, 69(8), 1018–1036. https://doi.org/10.1139/F03-087 Weber, E. D., & Fausch, K. D. (2005). Competition between hatchery- reared and wild Juvenile Chinook Salmon in enclosures in the Sacramento River, California. Transactions of the American Fisheries Society, 134(1), 44–58. https://doi.org/10.1577/FT03-189.1 Wise, M. H., Linn, I. J., & Kennedy, C. R. (1981). A comparison of the feeding biology of mink Mustela vison and otter Lutra lutra. Journal of Zoology, 195(2), 181–213. https://doi.org/10.1111/ j.1469-7998.1981.tb034​58.x Yamamoto, S., Nakamura, H., & Koga, K. (2008). Interaction between hatchery and wild juvenile white-spotted charr Salvelinus leucomae- nis in a stream enclosure experiment. Journal of Fish Biology, 73(4), 861–869. https://doi.org/10.1111/j.1095-8649.2008.01978.x How to cite this article: Vehanen, T., Huusko, A., Bergman, E., Enefalk, Å., Louhi, P., & Sutela, T. (2022). American mink (Neovison vison) preying on hatchery and wild brown trout (Salmo trutta) juveniles in semi-natural streams. Freshwater Biology, 67, 433–444. https://doi.org/10.1111/fwb.13852 https://doi.org/10.1525/bio.2009.59.2.9 https://doi.org/10.1139/cjfas-2014-0344 https://doi.org/10.1139/cjfas-2014-0344 https://doi.org/10.1577/M06-169.1 https://doi.org/10.1111/eth.12065 https://doi.org/10.1093/icesjms/fsr208 https://doi.org/10.2307/1938470 https://doi.org/10.2307/1938470 https://doi.org/10.1016/j.ecolind.2012.11.011 https://doi.org/10.1016/j.ecolind.2012.11.011 https://doi.org/10.3354/meps222279 https://doi.org/10.1098/rsbl.2012.1178 https://doi.org/10.1098/rsbl.2012.1178 https://doi.org/10.1890/07-1311.1 https://doi.org/10.1890/07-1311.1 https://doi.org/10.1007/s10750-020-04330-x https://doi.org/10.1186/s13750-019-0162-6 https://doi.org/10.1080/02755947.2016.1206643 https://doi.org/10.1080/02755947.2016.1206643 https://doi.org/10.1098/rsos.180465 https://doi.org/10.1111/j.1095-8649.1998.tb01551.x https://doi.org/10.1111/j.1095-8649.1998.tb01551.x https://doi.org/10.1046/j.1095-8649.2003.00228.x https://doi.org/10.1007/s10641-006-9031-6 https://doi.org/10.1007/s10641-006-9031-6 https://doi.org/10.1111/j.1600-0633.2008.00343.x https://doi.org/10.1111/j.1600-0633.2008.00343.x https://doi.org/10.1111/j.1365-2427.2010.02467.x https://doi.org/10.1111/eff.12550 https://doi.org/10.1139/F03-087 https://doi.org/10.1577/FT03-189.1 https://doi.org/10.1111/j.1469-7998.1981.tb03458.x https://doi.org/10.1111/j.1469-7998.1981.tb03458.x https://doi.org/10.1111/j.1095-8649.2008.01978.x https://doi.org/10.1111/fwb.13852 Vehanen et al 2021.pdf Freshwater Biology - 2021 - Vehanen - American mink