Northern European Salmo trutta (L.) populations are genetically divergent across geographical regions and environmental gradients.

The salmonid fish Brown trout is iconic as a model for the application of conservation genetics to understand and manage local interspecific variation. However, there is still scant information about relationships between local and large-scale population structure, and to what extent geographical and environmental variables are associated with barriers to gene flow. We used information from 3,782 mapped SNPs developed for the present study and conducted outlier tests and gene-environment association (GEA) analyses in order to examine drivers of population structure. Analyses comprised >2,600 fish from 72 riverine populations spanning a central part of the species' distribution in northern Europe. We report hitherto unidentified genetic breaks in population structure, indicating strong barriers to gene flow. GEA loci were widely spread across genomic regions and showed correlations with climatic, abiotic and geographical parameters. In some cases, individual loci showed consistent GEA across the geographical regions Britain, Europe and Scandinavia. In other cases, correlations were observed only within a sub-set of regions, suggesting that locus-specific variation was associated with local processes. A paired-population sampling design allowed us to evaluate sampling effects on detection of outlier loci and GEA. Two widely applied methods for outlier detection (pcadapt and bayescan) showed low overlap in loci identified as statistical outliers across sub-sets of data. Two GEA analytical approaches (LFMM and RDA) showed good correspondence concerning loci associated with specific variables, but LFMM identified five times more statistically significant associations than RDA. Our results emphasize the importance of carefully considering the statistical methods applied for the hypotheses being tested in outlier analysis. Sampling design may have lower impact on results if the objective is to identify GEA loci and their population distribution. Our study provides new insights into trout populations, and results have direct management implications in serving as a tool for identification of conservation units.

Behavioral type, in interaction with body size, affects the recapture rate of brown trout Salmo trutta juveniles in their nursery stream.

Movement activity levels of wild animals often differ consistently among individuals, reflecting different behavioral types. Previous studies have shown that laboratory-scored activity can predict several ecologically relevant characteristics. In an experiment on wild brown trout Salmo trutta, spanning from June to October, we investigated how spring swimming activity, measured in a standardized laboratory test, related to relative recapture probability in autumn. Based on laboratory activity scores, individuals clustered into 2 groups, which showed contrasting patterns in the size-dependency of their recapture probability. Size had a slightly positive effect on recapture probability for passive fish but a clear negative effect on active fish. Our results show that the population structure in a cohort, in terms of relative proportions of behavioral types in different size classes, can vary over time. The results of this study could depend on either selective mortality or migration. However, selective disappearance of individuals with specific phenotypes, regardless of the mechanism, will have implications for trout population management, such as stocking efficiency of hatchery fish with high growth rates or maintenance of fishways past migration barriers.

Inactive trout come out at night: behavioral variation, circadian activity, and fitness in the wild.

Theory suggests that high activity levels in animals increase growth at the cost of increased mortality. This growth-mortality tradeoff has recently been incorporated into the wider framework of the pace-of-life syndrome (POLS) hypothesis. However, activity is often quantified only in the laboratory and on a diurnal basis, leaving open the possibility that animals manage predation risk and feeding efficiency in the wild by modulating their circadian activity rhythms. Here we investigate how laboratory activity in wild brown trout parr (Salmo trutta L.) associates with circadian activity, growth, and mortality in their natal stream. We found that individuals with high activity in the laboratory displayed high dispersal and cathemeral activity in their natal stream. In contrast, trout with low laboratory activity showed variation of activity in the wild, which was negatively related to the light intensity. Our results do not support the growth-mortality trade-off of the POLS hypothesis as highly active, fast-growing individuals showed higher survival than inactive conspecifics. These novel results show for the first time that active and inactive individuals, as scored in the lab, can show different circadian patterns of behavior in the wild driven by light intensity. This implies that studies conducted under a narrow range of light conditions can bias our understanding of individual behavioral variation and its fitness consequences in the wild.