Juvenile Atlantic Salmon Research Articles

New Fish Species in Water Bodies of Northeastern European Russia

Changes in the composition of ichthyofauna of northeastern European Russia under the effect of biological invasions are analyzed. It is shown that alien invader fish species appeared in the water bodies of the region as a result of acclimatization measures (the Far Eastern pink salmon and Pechora peled in the White Sea basin, the Northern Dvina sterlet and Siberian sturgeon in the Pechora River), intentional (the pike perch) or accidental introduction (the Chinese sleeper), and expansion of species from the Caspian Sea (the white-eye bream and asp) or the Baltic Sea (asp). The results of analysis of long-term changes in the freshwater ichthyofauna show that the appearance of alien fish species in the water bodies of northeastern European Russia has led to an increase in the diversity of ichthyofauna, trophic competition with native species, deterioration of the fishery status an increased mortality risk for juvenile Atlantic salmon, and potential aggravation of the epizootic situation in the lower reaches of the Northern Dvina River. At the same time, the successful acclimatization of the Pechora peled in the White Sea basin has made it possible to develop a scheme for organizing full-cycle lake coregonid farms with the formation of peled brood stocks and local whitefish polyculture. Sturgeon species naturalized in the Onega and Pechora rivers may subsequently diversify the list of commercial fish species.

Increasing temperatures accentuate negative fitness consequences of a marine parasite

Infectious diseases are key drivers of wildlife populations and agriculture production, but whether and how climate change will influence disease impacts remains controversial. One of the critical knowledge gaps that prevents resolution of this controversy is a lack of high-quality experimental data, especially in marine systems of significant ecological and economic consequence. Here, we performed a manipulative experiment in which we tested the temperature-dependent effects on Atlantic salmon (Salmo salar) of sea lice (Lepeophtheirus salmonis)—a parasite that can depress the productivity of wild-salmon populations and the profits of the salmon-farming industry. We explored sea-louse impacts on their hosts across a range of temperatures (10, 13, 16, 19, and 22 °C) and infestation levels (zero, ‘low’ (mean abundance ± SE = 1.6 ± 0.1 lice per fish), and ‘high’ infestation (6.8 ± 0.4 lice per fish)). We found that the effects of sea lice on the growth rate, condition, and survival of juvenile Atlantic salmon all worsen with increasing temperature. Our results provide a rare empirical example of how climate change may influence the impacts of marine disease in a key social-ecological system. These findings underscore the importance of considering climate-driven changes to disease impacts in wildlife conservation and agriculture.

Development of a Piscirickettsia salmonis immersion challenge model to investigate the comparative susceptibility of three salmon species.

Piscirickettsia salmonis, the aetiological agent of salmonid rickettsial septicaemia (SRS), is a global pathogen of wild and cultured marine salmonids. Here, we describe the development and application of a reproducible, standardized immersion challenge model to induce clinical SRS in juvenile pink (Oncorhynchus gorbuscha), Atlantic (Salmo salar) and sockeye salmon (O. nerka). Following a 1‐hr immersion in 105 colony‐forming units/ml, cumulative mortality in Atlantic salmon was 63.2% while mortality in sockeye salmon was 10%. Prevalence and levels of the bacterium in kidney prior to onset of mortality were lower in sockeye compared with Atlantic or pink salmon. The timing and magnitude of bacterial shedding were estimated from water samples collected during the exposure trials. Shedding was estimated to be 82‐fold higher in Atlantic salmon as compared to sockeye salmon and peaked in the Atlantic salmon trial at 36 d post‐immersion. These data suggest sockeye salmon are less susceptible to P. salmonis than Atlantic or pink salmon. Finally, skin lesions were observed on infected fish during all trials, often in the absence of detectable infection in kidney. As a result, we hypothesize that skin is the primary point of entry for P. salmonis during the immersion challenge.

Can aspects of the discharge regime associated with juvenile Atlantic salmon (Salmo salar L.) and trout (S. trutta L.) densities be identified using historical monitoring data from five UK rivers?

AbstractUnderstanding salmonid discharge requirements can help inform management to conserve wild populations in a changing climate. This study developed Bayesian hierarchical mixed‐effects models relating 0+ Atlantic salmon (Salmo salar L.) and trout (Salmo trutta L.) densities to different aspects of river discharge. Associations between these densities and nine hydrological variables representing the magnitude, frequency and duration of discharge events were evaluated using historical monitoring data from 36 sites on five rivers in England and Wales. All hydrological variables had weak associations with 0+ salmonid densities. More frequent high discharges between spawning and emergence were positively and negatively associated with 0+ salmon and trout densities, respectively. High discharges might increase spawning site availability for salmon and decrease egg‐to‐fry survival for trout. However, overall, only equivocal evidence was found regarding which discharge aspects affect juvenile salmonid densities. Therefore, a strategic review of juvenile salmonid monitoring programmes integrating environmental data collection is recommended.

Environmental conditions modify density‐dependent salmonid recruitment: Insights into the 2016 recruitment crash in Wales

Abstract Understanding the effects of density‐dependent and density‐independent factors on recruitment is often inhibited by difficulties quantifying their relative contributions in highly variable recruitment data. Use of data‐driven statistical methods with data that include one or more extreme recruitment events could help overcome these difficulties. Juvenile Atlantic salmon and trout abundances in Wales have declined over the last 2 decades, and 2016 was a notably poor recruitment year in rivers around southern Europe, including England and Wales. The 2016 recruitment crash coincided with extreme winter weather conditions, leading to speculation that unusually warm temperatures and high flows adversely affect salmonid recruitment and caused the 2016 crash, although this remains untested. We developed data‐driven statistical models to: (1) describe juvenile salmonid recruitment from density‐dependent and density‐independent factors; and (2) assess whether the density‐independent factors probably contributed to the 2016 salmon recruitment crash. We compiled salmon and trout young‐of‐year juvenile abundances from electrofishing surveys, egg deposition estimates and river flow and air temperature data from 2001–2017 for seven Welsh rivers, broadly representative of rivers around Wales. We used river flow and air temperature data to derive ecologically and behaviourally meaningful density‐independent explanatory variables. Salmonid recruitment in Wales was best described using density‐dependent and density‐independent factors, especially for salmon: after accounting for a concave relationship with egg deposition, salmon juvenile abundance was reduced under (1) warmer spawning temperatures that might inhibit spawning, and (2) higher flood frequencies during pre‐emergence and emergence that might washout eggs or alevins. Results were less clear for trout, perhaps because they are behaviourally more plastic. Our findings provide empirical support for general and predictable effects of temperature and flow during spawning and emergence on salmonid—especially salmon—recruitment in Wales. Furthermore, we suggest that the 2016 salmon recruitment crash was caused—in part—by particularly inclement spawning and emergence conditions, which could be more common under future climate change. Our findings suggest that future salmonid stock assessment models could include the effects of density‐independent variables on recruitment to improve their predictive power.

Effects of a temperature rise on melatonin and thyroid hormones during smoltification of Atlantic salmon, Salmo salar.

Smoltification prepares juvenile Atlantic salmon (Salmo salar) for downstream migration. Dramatic changes characterize this crucial event in the salmon's life cycle, including increased gill Na+/K+-ATPase activity (NKA) and plasma hormone levels. The triggering of smoltification relies on photoperiod and is modulated by temperature. Both provide reliable information, to which fish have adapted for thousands of years, that allows deciphering daily and calendar time. Here we studied the impact of different photoperiod (natural, sustained winter solstice) and temperature (natural, ~ + 4° C) combinations, on gill NKA, plasma free triiodothyronine (T3) and thyroxine (T4), and melatonin (MEL; the time-keeping hormone), throughout smoltification. We also studied the impact of temperature history on pineal gland MEL production in vitro. The spring increase in gill NKA was less pronounced in smolts kept under sustained winter photoperiod and/or elevated temperature. Plasma thyroid hormone levels displayed day-night variations, which were affected by elevated temperature, either independently from photoperiod (decrease in T3 levels) or under natural photoperiod exclusively (increase in T4 nocturnal levels). Nocturnal MEL secretion was potentiated by the elevated temperature, which also altered theMEL profile under sustained winter photoperiod. Temperature also affected pineal MEL production in vitro, a response that depended on previous environmental acclimation of the organ. The results support the view that the salmon pineal is a photoperiod and temperature sensor, highlight the complexity of the interaction of these environmental factors on the endocrine system of S. salar, and indicate that climate change might compromise salmon's time "deciphering" during smoltification, downstream migration and seawater residence.

Effect of PROPLEX DY and PROPLEX T on growth performance of Atlantic salmon smolts

Promising alternative proteins for Atlantic salmon include commercially available single cell proteins such as PROPLEX DY (yeast) and PROPLEX T (bacteria). To evaluate these products in the diets of Atlantic salmon, a digestibility trial with PROPLEX DY and two growth trials were conducted. In Trial 1, juvenile Atlantic salmon were fed diets containing between 0% and 15% PROPLEX DY. In Trial 2, juvenile Atlantic salmon were fed diets containing 0% PROPLEX products, 4% PROPLEX T, 5% PROPLEX DY, 4% PROPLEX T + 5% PROPLEX DY and 8% PROPLEX T + 5% PROPLEX DY. In Trial 1, Atlantic salmon fed the PROPLEX DY diets did not have significant growth differences compared to the fishmeal reference diet. Proximate composition did not vary except for ash which was lower for fish fed the 15% DY diet. In the Trial 2, the inclusion of trial ingredients did not affect growth or proximate composition. The fillets from fish fed diets containing PROPLEX DY had higher b* scores compared to the other dietary treatments indicating the fillets had a slightly yellower appearance. Both tested ingredients can be added to salmon diets without affecting performance if PROPLEX DY is kept below 15% and PROPLEX T below 8%.

Correction: RNA profiling identifies novel, photoperiod-history dependent markers associated with enhanced saltwater performance in juvenile Atlantic salmon

[This corrects the article DOI: 10.1371/journal.pone.0227496.].

Validation of Francis–Kaplan Turbine Blade Strike Models for Adult and Juvenile Atlantic Salmon (Salmo Salar, L.) and Anadromous Brown Trout (Salmo Trutta, L.) Passing High Head Turbines

The negative effects of hydroelectric power (HEP) on salmonid populations has long been recognized and studied. Downstream passage through turbines may potentially constitute a significant source of mortality for both juvenile and adult fish in regulated rivers. Numerical models have been developed to calculate turbine passage mortality based on the probability of collision with the turbine blades, but although widely used in management and conservation, their performance is rarely validated in terms of the accuracy and bias of the mortality estimates. In this study, we evaluated commonly used blade strike models for Kaplan and Francis turbines by comparing model predictions with observed passage mortalities for juvenile 13–27 cm and adult 52–94 cm Atlantic salmon (Salmo salar, L.) and anadromous brown trout (Salmo trutta, L.) acquired by acoustic telemetry. Predictions made for juveniles aligned closer with observed mortality for both Kaplan and Francis turbines (within 1–3% percentage points). However, the model severely underestimated the mortality of adult fish passing through Francis turbines, with up to 50% percentage points difference between predicted and observed mortalities. Furthermore, the model did not capture a clear negative correlation between mortality and discharge observed for salmon between 50–60 cm (grilse). We concluded that blade strike models are a useful tool for quantifying passage mortality for salmonid smolts passing large, high-head turbines, but that the same models should be used with care when trying to estimate the passage mortality of kelts in iteroparous populations. We also concluded that the major cause of passage mortality for juveniles is injury by collision with the turbine blade, but that other factors seem to contribute substantially to the passage mortality of kelts. Our study reports low mortality for smolts up to 27 cm passing through Kaplan and Francis turbines (0–12%), but high mortality for salmon over 50 cm passing though Francis turbines (56–81%).

Climate Change Impact on Salmon (Salmo salar) and Sea Trout (Salmo trutta) in the Salaca River, Latvia

Hydrometeorological conditions are important determinants of the distribution, abundance and growth of Atlantic salmon (Salmo salar) and sea trout (Salmo trutta) parr and smolt in rivers. Long-term monitoring of the temperate zone Salaca River was conducted to investigate long-term changes in water temperature and streamflow and their effects on juvenile Atlantic salmon and sea trout. The average water temperature in the Salaca River was found to increase over time, particularly in autumn, winter and spring. River discharge increased in winter and summer, which was caused by increased precipitation. Goals of our study were: 1) to examine trends in water temperature and discharge in the river, 2) to explore relations between hydrometeorological trends, on the one hand, and abundance, size at age of salmon and sea trout parr and smolt, as well as changes in smolt migration patterns, on the other. This was the first occasion that a study of this type had been carried out in the Baltic Sea basin. In summer, growth of salmon parr in all age groups in the Salaca River was reduced. However, the size of smolt was found to increase. Migration of sea trout smolt in the Salaca River took place earlier, and the end of both salmon and sea trout migration was also observed earlier. The age structure of salmon smolt changed, with the one-year- old age group dominating. Judging from the total numbers of salmon and sea trout smolt in the River Salaca, production of smolt in the river during the monitoring period increased.

Quantifying the relative importance of stock level, river temperature and discharge on the abundance of juvenile Atlantic salmon (Salmo salar)

AbstractFisheries scientists need to understand the relationships between river temperature, discharge and production of juvenile salmonids to inform evidence‐based management and regulation of rivers and to understand the potential effects of climate change. These relationships can be determined by characterising interannual variability in abundance and environmental conditions from long‐term monitoring data and assessing their inter‐relationships. Two major challenges are (1) the requirement to separate the relative effects of stock level and environment which both affect interannual variability in abundance and (2) obtaining long‐term environmental time‐series that do not suffer from temporal biases. This study built on recent advances in hydrological, river‐temperature and juvenile salmonid modelling to investigate the influence of temperature and discharge on interannual variability in Atlantic salmon (Salmo salar) fry (age: 0 years) and parr (age: 1 year) production. The study used a unique long‐term dataset (>50 years) with detailed age‐differentiated census data collected for multiple life‐stages. The study shows that most of the interannual variability in recruitment was explained by stock level. Discharge had a comparatively small effect on fry recruitment, but a greater effect than artificial stocking. Discharge had no discernible effect on parr recruitment. Temperature had no effect on recruitment of either life‐stage. This study suggests that salmon are well adapted to current environmental variability in natural upland rivers in Scotland, but reductions in discharge during spawning and emergence could negatively affect fry recruitment with consequences for regulation of river flows. The study highlights the importance of high‐quality census data for accurately determining the effects of environmental variability on recruitment.

Cortisol-Related Signatures of Stress in the Fish Microbiome.

Exposure to environmental stressors can compromise fish health and fitness. Little is known about how stress-induced microbiome disruption may contribute to these adverse health effects, including how cortisol influences fish microbial communities. We exposed juvenile Atlantic salmon to a mild confinement stressor for two weeks. We then measured cortisol in the plasma, skin-mucus, and feces, and characterized the skin and fecal microbiome. Fecal and skin cortisol concentrations increased in fish exposed to confinement stress, and were positively correlated with plasma cortisol. Elevated fecal cortisol was associated with pronounced changes in the diversity and structure of the fecal microbiome. In particular, we identified a marked decline in the lactic acid bacteria Carnobacterium sp. and an increase in the abundance of operational taxonomic units within the classes Clostridia and Gammaproteobacteria. In contrast, cortisol concentrations in skin-mucus were lower than in the feces, and were not related to any detectable changes in the skin microbiome. Our results demonstrate that stressor-induced cortisol production is associated with disruption of the gut microbiome, which may, in turn, contribute to the adverse effects of stress on fish health. They also highlight the value of using non-invasive fecal samples to monitor stress, including simultaneous determination of cortisol and stress-responsive bacteria.

Metabolic Rate Interacts with Resource Availability to Determine Individual Variation in Microhabitat Use in the Wild.

Ecological pressures such as competition can lead individuals within a population to partition resources or habitats, but the underlying intrinsic mechanisms that determine an individual's resource use are not well understood. Here we show that an individual's own energy demand and associated competitive ability influence its resource use, but only when food is more limiting. We tested whether intraspecific variation in metabolic rate leads to microhabitat partitioning among juvenile Atlantic salmon (Salmo salar) in natural streams subjected to manipulated nutrient levels and subsequent per capita food availability. We found that individual salmon from families with a higher baseline (standard) metabolic rate (which is associated with greater competitive ability) tended to occupy faster-flowing water, but only in streams with lower per capita food availability. Faster-flowing microhabitats yield more food, but high metabolic rate fish only benefited from faster growth in streams with high food levels, presumably because in low-food environments the cost of a high metabolism offsets the benefits of acquiring a productive microhabitat. The benefits of a given metabolic rate were thus context dependent. These results demonstrate that intraspecific variation in metabolic rate can interact with resource availability to determine the spatial structuring of wild populations.

Replacement of Fish Meal by Solid State Fermented Lupin (Lupinus albus) Meal with Latobacillus plantarum 299v: Effect on Growth and Immune Status of Juvenile Atlantic Salmon (Salmo salar)

Abstract The aim of this study was to assess quality of SSF (Solid State Fermented) lupin with Lactobacillus plantarum 299v, and its effects (on growth, feed utilization, digestibility and immunity) of juvenile Atlantic salmon (S. salar), when used as fish meal replacer. Five experimental diets were formulated to provide 40% crude protein and 21% dietary lipid (dry matter basis) with the raw or fermented lupin meal-based protein source replacing fish meal at 15% and 30%. Triplicate groups of fish (averaging 3.53 ± 0.05 g) were fed with experimental diets for 8 weeks. Fermentation process modified nutrient profile of lupin meal and enriched it with lactic, citric and acetic acids. Fish in the FL15% group showed a higher (P < 0.05) final body weight, weight gain, FCR, SGR, and PER compared to those of C group. Apparent digestibility coefficient (ADC) of protein and Nitrogen-free extract showed a significantly higher values in FL15% experimental group, compared to those shown in C group. Fish in the FL15% group showed a higher (P<0.05) lysozyme activity and leucocyte respiratory burst compared to that shown by fish samples in the C experimental group; phagocytic activity did not record differences among experimental groups. In conclusion, replacement of fish meal by raw or fermented lupin meal did not compromise growth, apparent digestibility coefficients and immune status of juvenile Atlantic salmon and even improve fish performance when supplemented at 15%.

The Effect of the Photoperiod on the Fatty Acid Profile and Weight in Hatchery-Reared Underyearlings and Yearlings of Atlantic Salmon Salmo salar L.

The influence of two light regimes, 16:8 h light/dark (LD 16:8) and 24:0 h light/dark (LD 24:0), in comparison to a usual hatchery light regime (HL), on the fatty acids content and weight gain in hatchery-reared underyearlings (at 0+ age) and yearlings (at 1+ age) of Atlantic salmon in the summer–autumn period was studied. The total lipids were analyzed by Folch method, the lipid classes using HPTLC, and the fatty acids of total lipids using GC. The increase in EPA and DHA observed in October in underyearlings and yearlings salmon (especially under LD 24:0) suggests they were physiologically preparing for overwintering. The changes in fatty acids and their ratios in juvenile Atlantic salmon can be used as biochemical indicators of the degree to which hatchery-reared fish are ready to smoltify. These associated with an increase in marine-type specific DHA and EPA, an increase in the 16:0/18:1(n-9) ratio, in correlation with a reduction in MUFAs (mainly 18:1(n-9)). These biochemical modifications, accompanied by fish weight gain, were more pronounced in October in yearlings exposed to continuous light (LD 24:0). The mortality rate was lower in experimental groups of underyearliings with additional lighting. Exposure to prolonged and continuous light did not affect yearlings mortality rate.

The effect of triploidy on the performance, gut microbiome and behaviour of juvenile Atlantic salmon (Salmo salar) raised at low temperature

Triploid Atlantic salmon (Salmo salar) aquaculture has been extensively considered for many years, but its development has so far remained very limited. This is mostly due to poorer farming performances of triploid fish (3 N) in comparison with their diploid (2 N) counterparts, with temperature seen as a key explanatory factor. Furthermore, there are many inconsistencies between studies and considerable knowledge gaps exist in triploid physiology and performance, especially at early life stages. The present work is a pilot experiment in which the temperature was kept constantly low (8 °C) during 149 days from 81 to 229 days post hatching (dph). We used a multi-trait approach with an emphasis on behaviour to investigate the performance of 3 N and 2 N Atlantic salmon. Specifically, we compared growth, survival, prevalence of malformation, gut microbiome, and behavioural traits such as swimming activity in response to a stressor, and boldness. Reared at a cold and constant temperature, juvenile triploid and diploid fish showed similar growth from 81 to 165 dph but a difference occurred at 221 dph. Interestingly, triploids outperformed diploids in terms of survival. The prevalence of deformity was low in both diploid and triploid fish and no lower jaw skeletal abnormalities were observed in live fish at the end of the experiment. Triploid and diploid fish displayed very similar swimming activity and boldness traits as well as a very similar gut microbiome. This multi-trait approach reveals that when raised at low temperature, triploid and diploid fish performed equally well, with even a better survival for triploids towards the end of the experiment. Our study emphasizes the need for developing different rearing protocols for diploid and triploid Atlantic salmon. Applying a lower temperature at early developmental stages in ectotherms inevitably slows their developmental rate and may allow fine tuning of key developmental structures, decreasing deformities at early and later life stages of triploids. Further research comparing different temperature profiles is needed to find the best compromise between growth performance and low malformation rate and to examine the long-term effect of low temperatures applied at an early stage of development. This is a crucial step to improve both triploid salmon performance and welfare.

RNA profiling identifies novel, photoperiod-history dependent markers associated with enhanced saltwater performance in juvenile Atlantic salmon.

Atlantic salmon migrate to sea following completion of a developmental process known as smolting, which establishes a seawater (SW) tolerant phenotype. Smolting is stimulated by exposure to long photoperiod or continuous light (LL) following a period of exposure to short photoperiod (SP), and this leads to major changes in gill ion exchange and osmoregulatory function. Here, we performed an RNAseq experiment to discover novel genes involved in photoperiod-dependent remodeling of the gill. This revealed a novel cohort of genes whose expression rises dramatically in fish transferred to LL following SP exposure, but not in control fish maintained continuously on LL or on SP. A follow-up experiment revealed that the SP-history dependence of LL induction of gene expression varies considerably between genes. Some genes were inducible by LL exposure after only 2 weeks exposure to SP, while others required 8 weeks prior SP exposure for maximum responsiveness to LL. Since subsequent SW growth performance is also markedly improved following 8 weeks SP exposure, these photoperiodic history-dependent genes may be useful predictive markers for full smolt development.

Effects of food intake on relaxin 3 (Rxn3) mRNA expression in the muscle of Atlantic salmon

The Rxn3 is a protein that belongs to the insulin‐like/relaxin superfamily and affect various physiological functions, including growth, food intake, and reproduction. Evidence in rodents suggests that Rxn3 is involved in the regulation of food intake and critical for proper development of muscle and bones. The roles that the insulin‐like/relaxin superfamily plays in regards to growth and metabolism is largely unknown in fish, including Atlantic salmon. Recently, we identified the mRNA sequence for the Atlantic salmon Rxn3 by screening the database using channel catfish insulin‐like 5 polypeptide mRNA sequence. Given the possible role of Rxn3 in muscle function and nutrient metabolism, changes in the expression of Rxn3 mRNA in response to the changes in food intake were examined. Juvenile Atlantic salmon (n=72, 6 fish per tank) were assigned to either receive food every day (control, n=4 tanks) or fasted for 14 days (fasted, n=4 tanks). Additionally, fish were fasted for 7 days and fed daily for the subsequent 7 days (refed, n=4 tanks). Muscle samples were collected from fish at the end of 14 days, and expression of Rx3 mRNA was measured using real‐time PCR. Expression of Rxn3 mRNA was similar among the three feeding treatments, suggesting that muscle expression of Rxn3 is not affected by changes in food intake. Given the preliminary nature of the study, further investigations are warranted to define the role of Rxn3 on the regulation of growth, food intake, and nutrient metabolism in Atlantic salmon.Support or Funding InformationThis project was partially funded by Kansas IDeA Network of Biomedical research Excellence (K‐INBRE) under the NIGMS/NIH grand P20GM103418.

Rapid plastic changes in brain morphology in response to acute changes in predation pressure in juvenile Atlantic salmon (Salmo salar) and northern redbelly dace (Phoxinus eos)

Teleosts exhibit inter- and intra-specific variation in the size and shape of their brains. Interpopulation differences in gross brain morphology among numerous teleost fish species have been observed and have been partially attributed to plastic changes in response to their environment, including predation. These differences manifest themselves macroscopically, potentially because teleosts retain the capacity for active neuroproliferation into adulthood. Building on previous work, showing chronic exposure to predation can affect brain morphology, we sought to determine whether these differences manifest themselves on a time scale shown to induce phenotypically plastic behavioural changes. In separate trials, we held northern redbelly dace (Phoxinus eos (Cope, 1861) = Chrosomus eos Cope, 1861) and juvenile Atlantic salmon (Salmo salar Linnaeus, 1758) in semi-natural conditions and exposed them to conspecific skin extract as a proxy for predation risk over 2 weeks. After exposure, their brains were excised, photographed, and analyzed for size (multivariate ANOVA) and shape (Procrustes ANOVA). Despite their brief exposure to simulated predation pressure, subjects from both species developed significantly different brain morphologies. Compared with controls, the Atlantic salmon exhibited a different brain shape and smaller optic tecta, whereas the northern redbelly dace had larger brains with more developed olfactory bulbs and optic tecta. Our results highlight the rapidity with which external environment can alter patterns of growth in the brain.

Ecology and movement of juvenile salmonids in beaver‐influenced and beaver‐free tributaries in the Trøndelag province of Norway

AbstractThere is concern that expanding beaver (Castor fiber) populations will negatively impact the important economic, recreational and ecological resources of Atlantic salmon (Salmo salar) and sea trout (Salmo trutta) populations in Europe. We studied how beaver dams influenced habitat, food resources, growth and movement of juvenile Atlantic salmon and trout on three paired beaver‐dammed and beaver‐free (control) tributaries of important salmon rivers in central Norway. Lotic reaches of beaver‐dammed and control sites were similar in habitat and benthic prey abundance, and ponds were small (<3,000 m2). Though few juvenile salmonids were detected in ponds, trout and salmon were present in habitats below and above ponds (comprising 9%–31% and 0%–57% of the fish collected respectively). Trout dominated control sites (79%–99%), but the greatest proportion of Atlantic salmon were upstream of beaver ponds (0%–57%). Growth rates were highly variable, with no differences in growth between lotic reaches of beaver‐dammed and control sites. The condition and densities of juvenile salmon and trout were similar in lotic reaches of beaver‐dammed and control sites, though one beaver‐dammed site with fine sediment had very few juvenile salmonids. Beaver dams did not block the movement of juvenile salmonids or their ability to use upstream habitats. However, the degree of repeated movements and the overall proportion of fish moving varied between beaver‐dammed and control sites. The small scale of habitat alteration and the fact that fish were able to move past dams makes it unlikely that beaver dams negatively impact the juvenile stage of salmon or trout populations.