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Renewable energy projects such as hydropower facilities contribute towards meeting the world`s growing energy demands and urgent need for mitigating climate change. However, such infrastructure has the potential to substantially alter the environment which, in turn, can induce new challenges related to for instance fish migration conditions. As a consequence, local adaptations related to pre-development migration conditions may be affected for influenced populations. To explore selection regimes operating at a river hydropower plant, we monitored Atlantic salmon smolt individuals during their seaward migration. When passing the hydropower plant, the smolts chose between a surface fish passage or a submerged turbine intake. Smolts were scored for behavioural type (basal locomotor activity, net restrain (a measure of escape responses) and willingness to leave a familiar environment) prior to their migration choice, and we found that smolts with high basal activity had higher probability of using the fish passage than the turbine intake. In addition, migration route choice was a partly consistent trait in that fish that had previously passed a hydroelectric facility by using a fish passage rather than the turbine intake were significantly more likely to use it again when faced with the same choice. Higher mortality among turbine migrants could potentially reduce or eliminate particular behaviour types within populations- and the corresponding population genetic diversity that is essential to cope with future environmental challenges.

While many studies provide microscale relationships between fish and habitat characteristics, studies covering longer river reaches are scarce. Modern remote sensing techniques may enable new and effective ways of mapping and assessing mesoscale habitat characteristics. Using green LIDAR-derived bathymetry and hydraulic modelling, we tested how mesoscale depth and velocity were related to fish counts of adult European grayling (Thymallus thymallus L.) and brown trout (Salmo trutta L.) in 500 m river sections in three separate periods during the year. Using riverbank sinuosity from aerial images and a Froude number-based index from the hydraulic model as proxies for mesoscale spatial and hydraulic heterogeneity, we tested for temporal correlations with river section fish counts of adult European grayling and brown trout. Results showed that mesoscale mean depth and velocity were correlated to period fish counts of adult European grayling. Using mixed model analysis we found that riverbank sinuosity and the Froude number-based index were significantly correlated with river section occurrence of adult European grayling during spawning. The results can be used to assess how flow-induced changes and channel adjustments at the mesoscale level can influence access to and use of relevant habitats in rivers occupied by European grayling and brown trout. Open Access. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

Anthropogenic structures in rivers are major threats for fish migration and effective mitigation is imperative given the worldwide expansion of such structures. Fish behaviour is strongly influenced by hydrodynamics, but little is known on the relation between hydraulics and fish fine scale-movement. We combined 3D Computational fluid dynamics modelling (CFD) with 2D and 3D fish positioning to investigate the relation between hydrodynamics and the downstream movement of Atlantic salmon smolts (Salmo salar). We show that fish use fine-scale flow velocity and turbulence as navigation cues of fine-scale movement behaviour. Tri-dimensional swimming speed and swimming direction can be explained by adjustments of fish to flow motion, which are linked to fish swimming mode. Fish diverge from the flow by swimming at speeds within or higher than their prolonged speeds (0.38–0.73 m s−1). Flow direction plays a pivotal role on fish swimming performance, with high upstream and downwards velocities impacting swimming the most. Turbulence is also influential, by benefiting swimming performance at low TKE (< 0.03 m2 s−2) or constraining it at higher levels. We show that fish behaviour is affected by interactions of several hydraulic variables that should be considered jointly.

Source at: http://doi.org/10.1139/cjfas-2016-0408 Résumé : L’étude fait état de preuves empiriques appuyant le paradigme de « croissance–survie » chez les phases marines de l’omble chevalier (Salvelinus alpinus) et de la truite de mer (Salmo trutta). Ce paradigme postule que les individus plus grands ou a` croissance plus rapide sont plus susceptibles de survivre que leurs conspécifiques plus petits ou a` croissance plus lente. Nous avons utilisé des données de capture de longue durée (25 ans) obtenues d’un piège dans la rivière Halselva, en Norvège, durant les migrations annuelles entre les milieux marins et d’eau douce. Des résultats semblables ont été obtenus pour les deux espèces. Il y a une corrélation positive entre la croissance durant le séjour en mer et les taux de retour, témoignant d’un lien entre un taux de survie plus élevé et la croissance. Le taux de croissance, la survie et la durée du séjour en mer des individus migrant pour la première fois sont corrélés, ce qui donne a` penser que des conditions ambiantes communes en mer influencent la productivité annuelle des poissons. Les températures de l’eau douce et de mer ont une incidence sur le moment de la migration, alors que les variations annuelles de la croissance et de la survie en mer ne sont pas corrélées a` la température. Cela porte a` croire qu’un autre facteur, tel que la variation de l’apport énergétique, constituerait la principale cause de variation de la croissance annuelle. En outre, le taux de croissance en mer des deux espèces pourrait refléter la production annuelle globale de l’écosystème du fjord, particulièrement en ce qui concerne leurs proies principales. [Traduit par la Rédaction] This study found empirical evidence supporting the “growth–survival” paradigm in the marine phase of Arctic char (Salvelinus alpinus) and brown trout (Salmo trutta). The paradigm postulates that larger or faster-growing individuals are more likely to survive than smaller or slower-growing conspecifics. The study employed long-term (25 year) capture data from a trap in the River Halselva in Norway during annual migration between marine and freshwater environments. Similar results were found for both species. Growth during the sea sojourn and return rates were positively correlated, linking increased survival with growth. Specific growth rate, survival, and duration of the sea sojourn of first-time migrants were correlated, suggesting that common environmental conditions at sea influence annual fish productivity. Freshwater and sea temperatures affected migration timing, whereas annual variation in marine growth and survival did not correlate with temperatures. This suggests that other factors such as variation in energy intake were the main source of annual growth variations. Moreover, the marine growth rate of the two species may signal annual overall fjord ecosystem production, especially related to their main prey.

An effect of hydropower and hydropeaking regulation in rivers is stranding of fish. Those fishes that survive stranding may experience stranding as a stressful situation. In four experimental stranding experiments (each with 6 individuals in 10 control and 10 treatment replicates), the energetic consequences of two forms of stranding (i.e. trapping and beaching) of juvenile Atlantic salmon (Salmo salar) were investigated in summer and winter. Restricted food access in the experimental channels ensured that effects of hydropeaking could be revealed. Mean fish length ranged between 60 mm and 110 mm among experiments. Both during the winter and summer experiments fish did not grow in length, neither in the control nor in the treatment channels and fish lost body mass as well as body fat in all experiments (body fat in summer trapping experiment not determined). The four experiments revealed similar results: stranding did not affect growth or energy content. Despite the severity of the stranding and the resulting mortality, which was especially high during summer, no stranding related effects on fish performance could be detected. Copyright © 2017 by authors and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0).http://creativecommons.org/licenses/by/4.0/

The performance and movements of juvenile Atlantic salmon Salmo salar exposed to variable water discharge (simulating hydropeaking) but with a stable water-covered area were studied in six experimental stream channels, both during the winter and summer. Thirty fish were stocked into each channel, and the growth, body fat and movements of the fish were followed for about 2.5 months in each season. During the winter, no effect of hydropeaking was documented on performance or movement. In the summer, fish experiencing hydropeaking had lower body mass, lower body fat, and higher movement rates than the control fish. In general, effect sizes were small, and the rapid and frequent changes in water discharge and water level in the present study had small effects on the performance of juvenile Atlantic salmon. The cumulative long-term effect at the population level is unknown, but a reduced growth rate of 10% and a reduction in body fat of 16% in the hydropeaking experiments in the summer might to some extent translate into increased smolt age and lower overwintering survival. Copyright © 2014 John Wiley & Sons, Ltd.

a b s t r a c t In this paper, we present a spatially explicit individual-based mechanistic model for predicting the freshwater population abundance of Atlantic salmon (Salmo salar L.). The model simulates the complete life-cycle at small spatial (∼50 m) and temporal (weekly) scales. Processes such as spawning, recruitment, mortality and movements are predicted using deterministic and stochastic functions, parameterized using relationships established from the literature and empirical observations. The model was used to simulate salmon population abundance within a river in western-central Norway (River Nausta). The sensitivity of population abundance to fry and parr carrying capacities, and parr and post-smolt densityindependent mortalities was analyzed. The optimally parameterized model predicted similar abundances and age compositions across the developmental stages of the population to those estimated from empirical observations within the river. The effect of carrying capacities and mortalities on abundance was strongest at the developmental stage at which they operated, but also affected the abundance of subsequent developmental stages or the progeny, suggesting negative (dampening) feedback mechanisms.

The spatial distribution of Atlantic salmon Salmo salar young-of-the-year (0+) and 1 year old parr (1+) from the 2006 spawning cohort in a 5125 m reach of the River Skauga in central Norway was documented. A high degree of similarity was found between the distribution of 0+ and 1+ parr based on catches at 205 transects sampled in both years. Cross-correlations and partial cross-correlations (correcting for habitat variables) confirmed significant positive association between the two distributions on a small spatial scale (within 100 m) and a clear pattern of decreasing correlation with distance.

In this study, cost effective (in terms of reducing loss of power production) measures for increasing bypass migration of Atlantic salmon Salmo salar were developed and tested by establishing statistical models for timing of smolt migration and favourable diversion of water to the bypass. Initial tracking of radio-tagged smolts showed very low bypass migration under normal hydropower operations. Bypass migration increased when bypass discharge was experimentally increased and a model was developed that described relationships between total river discharge, bypass diversion and smolt migration route. Further improvements were obtained by installing two strobe lights at the power-production tunnel entrance that increased bypass migration during the night, but not during daytime. According to the behaviour of radio-tagged fish, the implemented measures contributed to increasing the annual percentage of bypass migration from 11 to 64%, and according to model predictions to 60-74% when the hydropower facilities were operated according to the developed models. To ensure correct timing of discharge diversion a smolt migration model was developed based on environmental variables that could successfully predict the general pattern of migration timing. The concept presented for improving smolt migration past hydropower intakes should be applicable in many systems where migration past hydropower installations cannot easily be solved by screening systems.

Successful transitions from relying on yolk to exogenous feeding may be strongly influenced by temperature conditions experienced both during embryonic development, through effects on juvenile phenotype, and during initiation of feeding. Here we simultaneously assess these two effects of temperature treatments (2, 5, 8, and 12 °C) in Atlantic salmon ( Salmo salar ). Fry emerging from low incubation temperatures were smaller and had metabolized more energy prior to emergence, but had larger yolk sacs and higher mass specific energy levels, than those experiencing higher temperatures. After emergence, activity, feeding, and growth increased significantly with increasing temperature, but fry were able to initiate feeding and maintain positive growth at all four temperatures. Larger energy stores may provide an advantage when emerging at cold temperatures with a low potential for feeding activity, whereas having a large body size, which is primarily of importance in competitive interactions, may be less important owing to cryptic feeding and sheltering at low temperatures. However, the adaptive significance of the observed phenotypic response to incubation temperature remains untested.

International audience; We test whether the spatial distribution of spawning sites in a natural population is related to the density and individual growth rate of juvenile stream-dwelling salmonids. For this purpose, fine-scale data on the spatial distribution of Atlantic salmon (Salmo salar) nest sites, juvenile densities and body size, and habitat characteristics were combined. The area of nests summed over reaches of 25 m in length was positively correlated with local young-of-the-year (YOY) densities and was the strongest predictor of local densities. In turn, increased YOY densities were related to slower growth, and YOY density was the strongest predictor of YOY size. Measured habitat variables had little effect on YOY density over the spatial scale examined, which suggests that redistribution of YOY salmon according to habitat preferences was limited. The distribution of spawning sites was therefore the best predictor of local densities and subsequently YOY growth. Thus, during the YOY stage when dispersal is limited, patchy spawning distributions combined with local densitydependence may be the dominant factor responsible for spatial variation in growth. Résumé : Nous vérifions si la répartition spatiale des sites de fraie dans une population naturelle est reliée à la densité et au taux de croissance individuelle des jeunes salmonidés d'eau courante. Pour ce faire, nous avons combiné des données à échelle fine sur la répartition spatiale des sites de nidification de saumons atlantiques (Salmo salar), sur les densités et les tailles corporelles des jeunes et sur les caractéristiques de l'habitat. Il existe une corrélation positive entre la surface cumulée des nids sur des sections de 25 m de longueur et la densité locale des jeunes de l'année (YOY); c'est la meilleure variable prédictive des densités locales. Par ailleurs, les densités accrues de YOY sont reliées à une croissance ralentie et la densité des YOY est la meilleure variable prédictive de la taille des YOY. Les variables mesurées de l'habitat ont peu d'effet sur la densité des YOY à l'échelle spatiale considérée, ce qui laisse croire qu'il y a une redistribution limitée des saumons YOY d'après leurs préférences d'habitat. La répartition des sites de fraie est donc la meilleure variable prédictive des densités locales et subséquemment de la croissance des YOY. Ainsi, durant le stade YOY pendant lequel la dispersion est limitée, la répartition contagieuse de la fraie, combinée à une densité-dépendance locale, peut être le principal facteur responsable de la variation spatiale de la croissance.

Summary 1. Variations in the strength of ecological interactions between seasons have received little attention, despite an increased focus on climate alterations on ecosystems. Particularly, the winter situation is often neglected when studying competitive interactions. In northern temperate freshwaters, winter implies low temperatures and reduced food availability, but also strong reduction in ambient light because of ice and snow cover. Here, we study how brown trout [Salmo trutta (L.)] respond to variations in ice-cover duration and competition with Arctic charr [Salvelinus alpinus (L.)], by linking laboratory-derived physiological performance and field data on variation in abundance among and within natural brown trout populations. 2. Both Arctic charr and brown trout reduced resting metabolic rate under simulated ice-cover (darkness) in the laboratory, compared to no ice (6-h daylight). However, in contrast to brown trout, Arctic charr was able to obtain positive growth rate in darkness and had higher food intake in tank experiments than brown trout. Arctic charr also performed better (lower energy loss) under simulated ice-cover in a semi-natural environment with natural food supply. 3. When comparing brown trout biomass across 190 Norwegian lakes along a climate gradient, longer ice-covered duration decreased the biomass only in lakes where brown trout lived together with Arctic charr. We were not able to detect any effect of ice-cover on brown trout biomass in lakes where brown trout was the only fish species. 4. Similarly, a 25-year time series from a lake with both brown trout and Arctic charr showed that brown trout population growth rate depended on the interaction between ice breakup date and Arctic charr abundance. High charr abundance was correlated with low trout population growth rate only in combination with long winters. 5. In conclusion, the two species differed in performance under ice, and the observed outcome of competition in natural populations was strongly dependent on duration of the ice-covered period. Our study shows that changes in ice phenology may alter species interactions in Northern aquatic systems. Increased knowledge of how adaptations to winter conditions differ among coexisting species is therefore vital for our understanding of ecological impacts of climate change.

A common environment experiment was conducted to assess the magnitude of the difference in growth, con- sumption, activity rate, and spatial and temporal patterns of habitat use between morphologically different populations of Arctic char (Salvelinus alpinus) originating from two Norwegian lakes. These two lakes contrasted sharply in terms of sur- face area, depth, elevation, length of the winter period, and fish community structure (presence-absence of brown trout, Salmo trutta). The experimental framework encompassed four littoral enclosures (average volume, 146 m 3 ) stocked with char from either of the two populations with duplicated treatments. Char morphology was quantified with numerical image analysis, food consumption was estimated using caesium analysis ( 133 Cs), and activity cost and patterns were determined using video cameras. Char populations were morphologically distinct and reacted differently in growth (1.9-fold difference between populations), food consumption (3-fold difference), and spatial activity patterns (20-fold difference) to the condi- tions prevailing in the enclosures. The results highlight that functional differences between morphologically distinct char may drive important differences in their bioenergetic and behavioural responses when exposed to similar environmental conditions. Such functional differences should be incorporated when developing habitat or trophic cascade models. Resume´ : Une experience en conditions communes a eterealisee afin de quantifier la magnitude des differences de crois- sance, de consommation, d'activiteet de patrons spatiaux et temporels d'utilisation d'habitat chez des ombles chevaliers (Salvelinus alpinus) provenant de deux populations morphologiquement differentes, issues de deux lacs de Norvege. Ces deux lacs differaient de maniere marquee en terme d'aire, de profondeur, d'altitude, de la duree de leur periode d'hi- vernage et de structure de leurs communautes de poissons (presence ou absence de truite brune, Salmo trutta). Le cadre experimental utilisecomprenait quatre enclos de grandes tailles (volume moyen, 146 m 3 ), chacun ensemenceavec des om

The pre-winter lipid stores of young-of-the-year (YOY, age 0 year) Atlantic salmon Salmo salar were analysed along a north-south gradient from c. 71 to 58 degrees N, with winter conditions ranging from >200 days of ice cover to no ice. The rivers sampled in Northern Norway represent some of the most northerly S. salar rivers. There was an increase in lipid content with increasing latitude, and mean lipid content (size adjusted to common mass) for YOY in northern rivers were almost three times higher: 0.035 g compared to 0.013 g in southern rivers. The relationship was not sensitive to variation in sampling time or variation in YOY body size. The lipid stores, however, varied markedly between rivers and also between neighbouring rivers, indicating different strategies or opportunities for pre-winter lipid storage both at latitudinal and local scales.

Summary 1. A new laboratory-based growth model for brown trout (Salmo trutta) was used to explore latitudinal variation in growth among natural populations. The model included the effects of differences in ambient temperatures and fish size among populations. Annual growth rates of anadromous brown trout parr from 22 Norwegian populations at 61–70 °N were compared with predictions from the growth model. Published field data from one Spanish, 15 British and four Danish populations at 44–58 °N were included in the analysis to increase the latitudinal range. 2. Among the Norwegian populations, the ratio between observed and predicted growth rates was not significantly different from 1·00 in eight rivers, but was significantly higher in eight, and was significantly lower in six. Observed growth was highest, relative to predicted growth, in the coldest rivers. In Spanish, British and Danish rivers, observed growth did not exceed predicted growth. 3. The ratio between observed and predicted annual growth rate decreased significantly with increasing annual mean temperature. Observed annual growth was higher than predicted growth only in rivers with an annual mean temperature lower than 5·1 °C, and this indicates that some kind of thermal adaptation may occur in trout populations in the coldest rivers. 4. Regression analyses showed that besides the direct effects of temperature and body size predicted by the growth model, annual growth rates were significantly related to annual mean temperature, densities of juvenile salmonids, duration of twilight (average for May–August) and latitude. Adding these variables to the original model increased the explanatory power from 73·9 to 80·6%.

Summary 1Contrasting results of relations between standard metabolic rate, behaviour and performance suggest that the evolutionary trade-offs in energy allocation between metabolisms and other parts of the energy budget depend on environmental conditions. Here, we show how habitat complexity changes the relationship between standard metabolic rate, behaviour and performance (winter energy loss-rates) in juvenile Atlantic salmon (Salmo salar L.). 2Juvenile Salmon, individually screened for standard metabolic rate, were stocked in replicated semi-natural stream channels with and without sheltering possibilities under ledge cover. We recorded the use of ledged areas, energy-loss rates and feeding activity during a 61-day period. 3In the channels with ledge cover, individuals with relatively higher standard metabolic rates used areas outside of the cover more frequently and had higher feeding frequency than individuals with lower standard metabolic rates. No relationship was apparent between areas used, feeding and standard metabolic rate for the individuals within the no-cover treatments. In channels without cover, there was a negative relationship between standard metabolic rate and performance (energy balance), whereas standard metabolic rate did not affect performance in channels with ledge cover. 4The effect of standard metabolic rate on performance depended on habitat complexity expressed through differences in behaviour. In channels with ledge cover, individuals with higher standard metabolic rates were apparently able to compensate for higher metabolic costs by utilizing a riskier habitat outside the cover and attaining a higher feeding success. 5The present study demonstrates that the relationship between standard metabolic rate and performance may be dependent on habitat complexity and vary on small spatial scales, contributing to the persistence of large variation in standard metabolic rate within populations.

– The seasonal variation in lipid class composition (triacylglycerols, free fatty acids, cholesterols and polar lipids) was described for juvenile Atlantic salmon sampled at 12 occasions over an 18-month period in the Norwegian River Alta (70°N). Lipid class composition was determined for pooled samples of groups of fish sorted by cohort and condition factor. Mass-specific lipid content ranged from 2.3% and 8.1% of fresh mass, and mass-specific triacylglycerol content between 0.16% and 5.6% of fresh mass. Total lipid content decreased by 34–57% during the winter period, before increasing by 155–176% between May and July. A large proportion of the variation in total lipid content was due to variation in triacylglycerol content (r2 = 0.95), except for variations at low total lipids levels when triacylglycerols were absent. The temporal variation in polar lipid content was, although to a lesser degree, related to total lipid content (r2 = 0.62), indicating that the fish also utilised polar lipids as an energy source during periods of starvation. Analyses of 39 individual parr collected in May showed a large individual variation in lipid class composition in the most critical period of the year. It is concluded that information on variation in lipid class composition may give more precise and detailed information of energy status during critical periods, although total lipid content will in most cases give a fair description of the energy status of the fish.

An accidental release of non-toxic waste from decommissioned wood pulp industry in the River Numedalslagen, Southern Norway, occurred in the upper part of the accessible stretches for anadromous fish during a study of migration behaviour of radio tagged Atlantic salmon (Salmo salar, n = 32, body length 51-99 cm). The fish had completed the migration phase and initiated the resident phase characterised by little movement until spawning. When the wooden fibres and pulp were released, 16 of 32 (50%) salmon showed an immediate avoidance response. Six (19%) salmon moved upstream and 10 (31%) salmon moved downstream. Of the salmon moving downstream, eight (25%) moved all the way to sea (average 14.8 km). Four re-entered the River Numedalslagen, two entered a neighbouring river and two were not recorded later. Fish moving downstream but without moving to the sea, moved on average 5.3 km during the episode, whereas those moving upstream moved on average 6.7 km. All fish recorded after the episode (n = 30) survived until the spawning season. The study demonstrates that fish in nature may show an evident avoidance response, even to non-toxic contaminants. For Atlantic salmon, the size of the spawning population may be reduced by fish leaving the river (13% left the river and never came back). Moreover, the dispersal of salmon to other rivers may increase, and the distribution of the spawning population within the contaminated river may shift.

By comparing the population frequency distributions for specific somatic energy between samplings using quantile–quantile (QQ) plots, we tested for energy-related mortality of juvenile (2- and 3-year-old) Atlantic salmon (Salmo salar) sampled at monthly intervals throughout three consecutive winters in a Norwegian river located at 70°N. Between several of the sampling periods, changes in the distributions of specific energy were observed corresponding to removal of low-energy individuals. By using energetic modelling we demonstrated that metabolic processes or feeding could not be responsible for the shifts in the shape of the energy distributions and that negative-energy-dependent mortality was the most likely explanation for the observations. No changes in mean size of the fish or in the shape of the size distributions were observed between successive sampling periods, indicating that mortality was linked to levels of storage energy rather than to body size per se. Our study indicated a critical body energy level for survival of juvenile salmon at approximately 4400–4800 J·g–1, corresponding to a depletion of storage lipids.

In the present thesis, winter mortality was linked to levels of stored energy by a field study from a northern Atlantic salmon population where population frequency distributions for specific somatic energy were compared among sampling periods. Interspecific differences in seasonal acclimatization were studied by performing common environment experiments on Atlantic salmon and establish models for thermal performance of winter acclimatized fish. These models were then compared to previously published thermal performance models for summer acclimatized salmon. Furthermore, the effect of changes in ice-cover conditions on the energy budget of the individual fish was studied using a series of common environment experiments in both laboratory and semi-natural environment. The latter experiments aimed at testing both interspecific and intraspecific variation in the effect of ice-cover. The results from these experiments were linked to winter mortality using bioenergetic modelling, where the input parameters originated from both field studies and experiments. The purpose of this conceptual modelling was not to provide quantitative predictions, but to demonstrate how environmental induced changes in the energy budget of individual fish may affect populations through the energy dependence of winter survival.

Summary 1 Under benign laboratory tank conditions we compared food consumption and metabolism of Atlantic salmon (Salmo salar) juveniles exposed to simulated ice cover (darkness) with fish in natural short, 6 h light, day length (without ice). Three different populations along an ice-cover gradient were tested (59°N−70°N). 2 Resting metabolism was on average 30% lower under simulated ice cover (6·6 J g−1 day−1) than under natural day length (9·4 J g−1 day−1), and the response was similar for all populations. Northern salmon grew equally well in dark and light conditions, whereas the southern grew significantly poorer in the dark. Fish from all populations fed more under natural day length than in the dark and the northern population had higher consumption than the southern. The relative high growth of fish from the northern population in the dark compared to the southern populations was due partly to higher consumption and partly to higher growth efficiency. Fish from the southern populations had negative growth efficiency in the dark. 3 We also studied the importance of ice cover under more hostile conditions in stream channels using the northern population only. Juveniles held in channels with simulated ice cover lost less energy (20 J g−1 day−1) than those held in channels with transparent cover (26 J g−1 day−1). This difference in energy loss was due partly (50%) to higher food consumption under simulated ice (4·5 and 1·6 J g−1, respectively) and partly (30%) to light-induced differences in resting metabolic rate. 4 In conclusion, both experiments showed lower metabolic costs in darkness under simulated ice cover than without ice. Under benign laboratory conditions the response to light (ice cover) varied among populations and only the northern population were able to attain positive growth in the dark. Under semi-natural conditions the lack of ice cover induced strong negative effects on the energy budget. Because energetic deficiencies are assumed to be an important cause of winter mortality, our study indicates that ice break-ups or removal following climatic change may affect winter survival significantly, particularly in northern populations.

Individual daily food intake, mass-specific growth rate and growth efficiency in groups of juvenile brown trout Salmo trutta were compared in tank experiments with three water level regimes (fluctuating, stable high and low water levels) and two temperature regimes (fluctuating between 10 and 14° C and constant 14° C) to simulate events during hydropeaking in regulated rivers. Fish exposed to high stable water level showed higher food intake and growth rate, and higher or similar growth efficiency than fish exposed to fluctuating or stable low water level. Both groups of slow-growing and fast-growing individuals fed less and grew slower at stable low and fluctuating water level than at stable high water level. Furthermore, growth and growth efficiency were lower in brown trout exposed to stable low water level and fluctuating temperature, particularly for groups of fish with slow growth. Temperature did not have any effect at high water level. For groups of fast-growing fish, there was no difference in growth efficiency between treatments. It is concluded that fluctuating water level and temperature have a potentially detrimental effect on growth in juvenile brown trout and effects are more severe in slow- than fast-growing fish.

The within- and among-population variation in individual growth rate of brown trout Salmo trutta L. was studied in five small neighbouring streams (seven isolated populations) within a distance of 70 km in east Norway. Observed growth rate was only weakly correlated with predicted maximum growth rate based on laboratory models, and there was a significant interaction with site. A generalized linear model showed that growth rate was positively correlated with temperature, but also that growth rate decreased as the summer season progressed. This might indicate either a seasonal decline in food availability or appetite, or a change in energy allocation strategy. Growth rate decreased with increasing fish age, probably as an effect of sexual maturation of older fish, and differential allocation of protein and lipid among different size-groups of brown trout. After adjusting for variation in temperature, season, and fish mass, there was still significant among-site variation in growth rate. A significant part of this variation was due to variation in brown trout density and the presence or absence of Alpine bullhead Coitus poecilopus. Growth rate decreased with increasing brown trout density, and was lower in the presence than in the absence of Alpine bullhead after correcting for variation in brown trout density. This last result may indicate the presence of interspecific competition.

Summary 1 Experimental data for maximum growth and food consumption of Atlantic Salmon (Salmo salar L.) parr from five Norwegian rivers situated between 59 and 70°N were analysed and modelled. The growth and feeding models were also applied to groups of Atlantic Salmon growing and feeding at rates below the maximum. The data were fitted to the Ratkowsky model, originally developed for bacterial growth. 2 The rates of growth and food consumption varied significantly among populations but the variation appeared unrelated to thermal conditions in the river of population origins. No correlation was found between the thermal conditions and limits for growth, thermal growth optima or maximum growth, and hypotheses of population-specific thermal adaptation were not supported. Estimated optimum temperatures for growth were between 16 and 20 °C. 3 Model parameter estimates differed among growth-groups in that maximum growth and the performance breadth decreased from fast to slow growing individuals. The optimum temperature for growth did not change with growth rate. 4 The model for food consumption (expressed in energy terms) peaked at 19–21 °C, which is only slightly higher than the optimal temperature for growth. Growth appeared directly related to food consumption. Consumption was initiated ≈2 °C below the lower temperature for growth and terminated ≈1·5 °C above the upper critical temperature for growth. Model parameter estimates for consumption differed among growth-groups in a manner similar to the growth models. 5 By combining the growth and consumption models, growth efficiencies were estimated. The maximum efficiencies were high, 42–58%, and higher in rivers offering hostile than benign feeding and growth opportunities.

Temporal changes in densities of young brown trout (Salmo trutta ), mainly of age 0+, and in water quality (pH and alkalinity) were assessed by means of electrofishing in lake tributaries in three acidic, softwater watercourses in western and southwestern Norway; Gaular and Vikedal (1987–1999) and Bjerkreim (1988–1999). Approximately 74 sites were sampled each year. Most of the streams were acidic with mean annual pH levels between 5.1–5.9. Alkalinity and pH increased significantly in all three areas during the study period. Brown trout fry densities increased significantly during the period in Vikedal and Bjerkreim. Also in Gaular, the density of young brown trout has exhibited a positive trend in recent years. We suggest that the increase in the density of young brown trout is because the study areas have became less acidified during recent years due to reduction in sulphate deposition.

1. We explored the mechanisms determining age and size at juvenile migration in brown trout Salmo trutta L. A 133Cs tracer methodology was used to estimate food consumption of juvenile brown trout in a Norwegian stream, and the energy budgets of early migrants and stream residents were compared. 2. Fast-growing brown trout migrated to the lake earlier and at a smaller body size than slower-growing individuals. The 2+ migrants were significantly larger than those that remained 1 or more years longer in the stream. The 3+ migrants were significantly larger than the 2+ migrants. Some fast-growing males matured in the stream, whereas all females left the stream before maturing sexually. 3. The food consumption and the energy budgets for 2+ migrants were more than four times higher than those of the resident 2+ fish. Total energy allocated to growth was also higher among migrants, and the total metabolic costs were five times higher among migrants than among resident fish. 4. The proportional energy allocation to growth among the 2+ migrants was much lower (about half) than that of those remaining longer in the stream. The reduction in the proportion of energy available for growth from age 1+ to 2+ was larger among migrants (88%) than among resident fish (68%). Reduction in the proportion of energy available for growth is a probable explanation for why migrations are initiated at age 2. 5. Our study supports the hypothesis that fast-growing individuals shift their niche earlier and at a smaller body size than slower-growing individuals because they maintain higher metabolic rates and are energetically constrained at a younger age by limited food resources than slow growers.

We examined the relationship between young brown trout ( Salmo trutta) density in lake tributaries, and water chemistry and habitat variables. The study was carried out during the autumn in three acidic, softwater river systems in western and southwestern Norway; Gaular and Vikedal (1987–1993) and Bjerkreim (1988–1993). The streams had mean calcium concentrations of 0.35 mg L-1 (Gaular), 0.52 mg L-1 (Vikedal) and 0.84 mg L-1 (Bjerkreim). The concentration of inorganic Al was generally low, with mean values of 8.40 (Gaular), 22.22 (Vikedal) and 43.36 μg L-1 (Bjerkreim). In multiple regressions that involved different water chemistry variables, brown trout density correlated best with calcium concentration and with a combination of calcium and pH; the Ca2+:H+ ratio. In Vikedal and Gaular, calcium explained 51 and 57%, respectively, of the variability in brown trout densities. Althoug alkalinity exhibited the best correlation with brown trout density in Bjerkreim ( r2=0.33), it was similar to that of the model that included all major ions plus pH. The Ca2+:H+ ratio had a larger effect for variability in brown trout density in Gaular (r2=0.66) than calcium alone. In Vikedal and Bjerkreim, the Ca2+:H+ ratio also correlated with brown trout density, but considerably less than in Gaular. The predictive power of habitat variables was much lower than that of water chemistry; the single most important factors were altitude in Gaular (r2=0.22), mean water temperature in Vikedal (r2=0.11) and depth SD (index of heterogeneity) in Bjerkreim (r2=0.07). Models that included both habitat and water chemistry variables showed that the density of young brown trout was predicted primarily by calcium concentrations in Gaular (r2=0.75) and Vikedal (r2=0.54), as opposed to pH in Bjerkreim (r2=0.25). Habitat had low effect in all three river systems (r2=0.01–0.04). The final model explained 86, 68 and 32%, respectively, of the variability in brown trout density in the three catchments. Thus, water chemistry variables seem to be factors that limit the density of young brown trout in acidic softwater streams.

We describe a mechanistic model to explore the environmental and metabolic determinants of radiocesium kinetics in a freshwater fish, the brown trout (Salmo trutta). Our model, which incorporates submodels for fish growth, food consumption, ambient temperature, and radiocesium turnover, was validated and calibrated to observations in three year-classes of brown trout from a Norwegian lake contaminated with Chernobyl fallout. Observed growth and independent estimates of food consumption matched simulations. Whereas our model successfully described trends and seasonal dynamics of radiocesium in brown trout, it was highly sensitive to the temperature submodel with model uncertainty largely driven by uncertainties in absorption efficiency, feeding rate, and temperature. We used the model to simulate both fallout and steady-state (constant prey radiocesium) conditions. Peak radiocesium in fish after a fallout depended upon time of year, fish size, rate of food consumption and growth, and water temperature regime. Among the modeled lake types (temperate, boreal, and subalpine), peak radioactivity was highest for brown trout living in the warmest lake, feeding and growing at a maximum rate, and for fallout occurring early in spring. In these lakes, increasing growth rate from 80 to 100% of the maximum nearly doubled peak radioactivity. The model predicted that the relationship between body size and fish radioactivity changed from negative to positive within 2 yr after a fallout. At steady state, the model predicted seasonal dynamics in accumulation of radiocesium, with maximum biomagnification in autumn and minimum in early spring. Maximum biomagnification was related to fish size (weak negative slope), growth rate, and temperature regime, and a metamodel was developed for predicting biomagnification.

Habitat utilization and the life history of browntrout Salmo trutta and Arctic charr Salvelinus alpinus were investigated in fivesympatric populations and five allopatric brown troutpopulations in Hoylandet catchment, a atmosphaericlow deposition area in Mid Norway. There was asignificant inverse correlation in abundance ofepibenthic Arctic charr and brown trout in theselakes, indicating that the latter species is dominant.The largest numbers of sympatric brown trout andArctic charr were caught in epibenthic habitat. In twolakes, brown trout to some extent also occurredpelagically, while pelagic individuals of Arctic charrwere found in all five lakes. The main food items forboth epibenthic and pelagic brown trout wereterrestrial surface insects and chironomid pupae.Zooplankton was the primary food item for Arctic charrin both habitats. Although the age distribution wasvery different in the populations, neither speciesseem to suffer from recruitment failure. There was nosignificant difference in survival rates betweensympatric populations of brown trout and Arctic charr.We found a significant inverse correlation betweenepibenthic catches of brown trout and the mean weightof 4+ fish, the most abundant age group. However, ifusing weight data for three-year-old fish, no suchrelationship was found for Arctic charr. Brown troutand Arctic charr reached asymptotic lengths of197–364 mm and 259–321 mm, respectively. Both speciestypically reached sexual maturity at age 2–3, and nomaturation-induced mortality was evident. We concludethat fish populations in Hoylandet lakes areregulated throughout their lifes by inter- andintraspecific competition.

Theoretical considerations suggest that the relative abundance of age-specific limiting resources determines the ontogenetic timing of density dependence. Structural shelters may represent one such resource which can become increasingly scarce with increasing body size. Here we use a time series of juvenile Atlantic salmon (Salmo salar) densities and ask whether ontogenetic patterns of density-dependent losses in two separate reaches of a river can be predicted by considering their shelter abundances. The analyses were conducted using sampling site data (n = 30) as well as stream-reach averages. Loss rates from the egg to the young-of-the-year stage were density-dependent in both reaches. For the transition from the young-of-the-year to the yearling stage, when shelters are more likely to become limiting, the results were sensitive to the spatial scale of analysis. On the reach scale, among-year variation in loss rates was positively correlated with density in the reach with the lowest shelter abundance, whereas no such effect was found in the other reach. This demonstrates that the ontogenetic timing of density dependence can vary among areas within populations, and hence among populations, and that this variation can be explained by quantification of age-specific limiting factors. For analyses at the sample site scale this pattern was reversed, with stronger density dependence in the reach with highest shelter abundance. However, this result was clearly driven by immigration into low density sites, which masked the true reach-level effect. Thus, our study also exemplifies how population level regulation inferred from patch- or trap-based data that fails to account for animal movements can be biased. © Springer Verlag. This is the authors' accepted and refereed manuscript to the article. The final publication is available at https://link.springer.com/article/10.1007%2Fs10144-013-0387-0

This study investigated the importance of competition with brown trout Salmo trutta as a driver of the morphological and behavioural divergence of two morphs of Arctic charr Salvelinus alpinus. The morphs originated from two lakes differing in absence or presence of the competitor. The bioenergetics and behaviour of S. alpinus were quantified in replicate experimental enclosures (mean volume: 150 m(3)) stocked with 15 S. alpinus of one morph or the other and in the absence or presence of nine S. trutta. The presence of S. trutta decreased growth rate, affected food consumption and increased activity costs in S. alpinus, but provided little support for the hypothesis that competition with S. trutta is a major driver of the divergence of the two S. alpinus morphs. Both morphs responded similarly in terms of mean growth and consumption rates per enclosure, but the association between individual morphology and growth rate reversed between allopatric and sympatric enclosures. While the activity patterns of the two morphs were unaffected by the presence of S. trutta, their swimming speed and activity rate differed. Since the profound differences in the structure of the physical habitat of the source lakes provided a more likely explanation for the difference observed among these two morphs than interspecific competition, it is hypothesized that physical habitat may sometimes be a significant driving force of the phenotypic divergence.

Using thermal growth data from eight populations of anadromous and lake-feeding brown trout Salmo trutta, hypotheses of adaptation to local optima and countergradient variation in growth were tested. The adaptation to local optima hypothesis suggests that natural selection can shift optimal performance temperatures to match the prevailing temperature in a new or changed thermal niche. In contradiction, the countergradient variation hypothesis suggests that populations from hostile environments perform better than conspecifics from benign environments at all temperatures. In this study, growth capacity varied between populations but there was no significant correlation between any of the estimated thermal performance parameters (e.g. lower and upper thermal growth limits, optimal temperature for growth and maximum growth capacity) and natural climatic conditions among populations. Hence, S. trutta growth response to temperature lends no support for either of the two suggested thermal adaptation hypotheses. Instead, growth capacity among populations tended to correlate positively with female size at maturity.

The study explored the combined effects of density, physical habitat and different discharge levels on the growth of juvenile Atlantic salmon Salmo salar in artificial streams, by manipulating flow during both summer and winter conditions. Growth was high during all four summer trials and increased linearly with discharge and mean velocity. Differences in fish densities (fish m −3 ) due to differences in stream volume explained a similar proportion of the variation in mean growth among discharge treatments. Within streams, the fish aggregated in areas of larger sediment size, where shelters were probably abundant, while growth decreased with increasing densities. Fish appeared to favour the availability of shelter over maximization of growth. Mean growth was negative during all winter trials and did not vary among discharge treatments. These results suggest that increased fish densities are a major cause of reduced summer growth at low discharge, and that habitat-mediated density differences explain the majority of the growth variation across habitat conditions both during summer and winter.

A fast, simple, and reliable method for estimating food consumption using radioisotopes is presented. The method gives both sensitive short-term and robust long-term estimates of food consumption for brown trout (Salmo trutta). The estimates of daily food rations were similar to those estimated from rates of gastric evacuation and laboratory experiments. The radioisotope method can be used to detect variations in ration size due to temperature, fish size, and food availability. Information on radiocesium absorption from this study and earlier published data on radiocesium retention can be used to estimate daily ration for both piscivorous and benthivorous brown trout. The radioisotope method can be used in radiocesium-contaminated lakes, but it could also be used in other experimental situations by adding stable cesium to small water bodies. The same method is also applicable for use with other stable or radioactive isotopes.

Current knowledge suggests that patterns of energy storage and depletion in animals are governed by behavioural trade-offs between risks associated with feeding and future energy demands. However, the length of adverse periods varies over geographical or climatic gradients. To explore the potential for genotypic sources of variation in behavioural trade-offs, we compared the winter energy-depletion patterns among 13 wild populations of juvenile Atlantic salmon ( Salmo salar L.) along a latitudinal gradient (58–70°N) and performed common-environment experiments of energy-state-dependent feeding. In the wild, winter lipid-depletion rates were lower for northern than for southern populations. The variation in spring lipid levels among the population was lower than autumn variation, with storage lipid levels clustered close to critical limits for survival. In semi-natural stream channels with natural food supply, hatchery-reared fish originating from northern populations showed a positive scaling of feeding activity with decreasing energy levels, whereas southern populations did not. In conclusion, juvenile Atlantic salmon from northern populations defend their energy levels more strongly than fish from southern populations. Adaptive variation in feeding activity appears important for this difference. Thus, the present study shows a link between geographical patterns in storage energy trajectories and adaptive differences in state-dependent feeding motivation.

Extensive mortality in Atlantic salmon fry was reported in the River Aelva from 2002 to 2004. Dead fish were collected in late summer 2006, and live fish were sampled by electrofishing in September the same year. At autopsy and in histological sections, the fish kidneys were found to be pale and considerably enlarged. Proliferative lesions with characteristic PKX cells were seen in a majority of the fish. DNA from kidney samples of diseased fish was subjected to PCR and sequencing, and the amplified sequences matched those of Tetracapsuloides bryosalmonae. We concluded that this myxozoan transmitted from bryozoans was the main cause of the observed mortality in salmon fry in 2006. Results from quantitative electrofishing in 2005 and 2006, combined with the observed fry mortality from 2002 to 2004, show that the smolt production in the river is severely reduced and that T. bryosalmonae is the most likely explanation for this decline. The present study is the first to report a considerable negative population effect in wild Atlantic salmon due to proliferative kidney disease (PKD). It also represents the northernmost PKD outbreak in wild fish. The river is regulated for hydroelectric power purposes, causing reduced water flow and elevated summer temperatures, and the present PKD outbreak may serve as an example of increased disease vulnerability of northern fish populations in a warmer climate.

The annual variability in growth and life history traits of brown trout (Salmo trutta L.) and Arctic charr (Salvelinus alpinus (L.)) in Lake Atnsjoen, a Norwegian subalpine lake, was studied over a period of 13 years (1985-1997). The extent to which life-history characteristics recorded on one occasion can be regarded as representative for the population was explored. We found inter-cohort variation in growth for both species; estimates of asymptotic length (L ∞) in ten cohorts ranged between 225-305 mm (CV = 10.5%) for brown trout and 273-301 mm (CV = 4.1%) for Arctic charr. However, this variation was much lower than inter-population variation for brown trout based on single samples from 169 populations (CV = 24.6%). In Lake Atnsjoen, annual growth increment correlated highly with the number of days warmer than 7 °C (R2 = 0.60-0.89) for brown trout, and days warmer than 10 °C (R2 = 0.40-0.58) for Arctic charr. Females of Arctic chaff were younger at sexual maturity than males, while no such difference was found in brown trout. Generally speaking, early maturing individuals of both species grew faster, particularly from age-2 and onwards, than immature individuals. Early maturing individuals, however, were smaller at maturity than those maturing one year older. Age and size at maturity were significantly correlated with asymptotic lengths only in Arctic chaff females.

After the accident at the Chernobyl nuclear reactor in 1986, the concentration of radioactive caesium (134Cs and 137Cs) in fish was expected to decline rapidly. The estimated ecological half-life (the time needed to reduce the average caesium concentration by 50%) was 0.3 to 4.6 years1,2. Since 1986, we have measured radiocaesium in brown trout (Salmo trutta) and Arctic charr (Salvelinus alpinus), both of which are widely eaten in Scandinavia, in a lake contaminated by Chernobyl fallout3,4. We have measured radiocaesium in nearly 4,000 fish, taking samples 2-4 times every year from spring to autumn. We find that the decline in radiocaesium was initially rapid for 3-4 years and was then much slower. About 10% of the initial peak radioactivity declines with an ecological half-life of as long as 8-22 years.