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While evolutionary responses require heritable variation, estimates of heritability ([h.sup.2]) from wild fish populations remain rare. A 20-year molecular pedigree for a wild Scottish population of Atlantic salmon (Salmo salar) was used to investigate genetic contributions to (co)variation in two important, correlated, phenotypic traits: 'sea age' (number of winters spent at sea prior to spawning) and size-at-maturity (body length just prior to spawning). Sea age was strongly heritable ([h.sup.2] = 0.51) and size exhibited moderate heritability ([h.sup.2] = 0.27). A very strong genetic correlation ([r.sub.G] = 0.96) between these traits implied the same functional loci must underpin variation in each. Indeed, body size within sea ages had much lower heritability that did not differ significantly from zero. Thus, within wild S. salar populations, temporal changes in sea age composition could reflect evolutionary responses, whereas rapid changes of body size within sea ages are more likely due to phenotypic plasticity. These inheritance patterns will influence the scope of evolutionary responses to factors such as harvest or climate change and, hence, have management implications for salmonid populations comprising a mix of sea ages. Si une variation heritable est necessaire pour permettre des reactions evolutives, les estimations de l'heritabilite ([h.sup.2]) obtenues pour des populations sauvages demeurent rares. Un pedigree moleculaire de 20 ans pour une population sauvage ecossaise de saumons atlantiques (Salmo salar) a ete utilise pour etudier les contributions genetiques a la (co)variation de deux caracteres phenotypiques correles importants, a savoir <> (le nombre d'hivers passes en mer avant le frai) et la taille a la maturite (la longueur du corps juste avant le frai). L'age marin est fortement heritable ([h.sup.2] = 0,51) et la taille presente une heritabilite moderee ([h.sup.2] = 0,27). Une correlation genetique tres forte ([r.sub.G] = 0,96) entre ces caracteres indique que les memes sites fonctionnels doivent sous-tendre la variation des deux caracteres. En effet, la taille du corps pour des ages marins donnes presente une heritabilite beaucoup plus faible qui n'est pas significativement differente de zero. Ainsi, dans les populations sauvages de S. salar, les changements dans le temps de la composition des ages marins pourraient refleter des reactions evolutives, alors que des changements rapides de la taille du corps au sein de classes d'age marin sont plus probablement le fait d'une plasticite phenotypique. Ces motifs d'heredite influenceront la portee des reactions evolutives a des facteurs comme l'exploitation ou les changements climatiques, ce qui a des consequences pour la gestion de populations de salmonides composees d'un melange d'ages marins. [Traduit par la Redaction], Introduction Life-history traits such as age and size at first reproduction are typically expected to be under strong selection, given their close links with Darwinian fitness (Stearns 1992; Roff 2002). [...]

Metabolism defines the energetic cost of life, yet we still know relatively little about why intraspecific variation in metabolic rate arises and persists. Spatio-temporal variation in selection potentially maintains differences, but relationships between metabolic traits (standard metabolic rate (SMR), maximum metabolic rate (MMR), and aerobic scope) and fitness across contexts are unresolved. We show that associations between SMR, MMR, and growth rate (a key fitness-related trait) vary depending on the thermal regime (a potential selective agent) in offspring of wild-sampled brown trout from two populations reared for approximately 15 months in either a cool or warm (+1.8°C) regime. SMR was positively related to growth in the cool, but negatively related in the warm regime. The opposite patterns were found for MMR and growth associations (positive in warm, negative in the cool regime). Mean SMR, but not MMR, was lower in warm regimes within both populations (i.e. basal metabolic costs were reduced at higher temperatures), consistent with an adaptive acclimation response that optimizes growth. Metabolic phenotypes thus exhibited a thermally sensitive metabolic 'floor' and a less flexible metabolic 'ceiling'. Our findings suggest a role for growth-related fluctuating selection in shaping patterns of metabolic variation that is likely important in adapting to climate change. [ABSTRACT FROM AUTHOR]

With rapid global change, organisms in natural systems are exposed to a multitude of stressors that likely co‐occur, with uncertain impacts. We explored individual and cumulative effects of co‐occurring environmental stressors on the striking, yet poorly understood, phenomenon of facultative migration. We reared offspring of a brown trout population that naturally demonstrates facultative anadromy (sea migration), under different environmental stressor treatments and measured life history responses in terms of migratory tactics and freshwater maturation rates. Juvenile fish were exposed to reduced food availability, temperatures elevated to 1.8°C above natural conditions or both treatments in combination over 18 months of experimental tank rearing. When considered in isolation, reduced food had negative effects on the size, mass and condition of fish across the experiment. We detected variable effects of warm temperatures (negative effects on size and mass, but positive effect on lipids). When combined with food restriction, temperature effects on these traits were less pronounced, implying antagonistic stressor effects on morphological traits. Stressors combined additively, but had opposing effects on life history tactics: migration increased and maturation rates decreased under low food conditions, whereas the opposite occurred in the warm temperature treatment. Not all fish had expressed maturation or migration tactics by the end of the study, and the frequency of these 'unassigned' fish was higher in food deprivation treatments, but lower in warm treatments. Fish showing migration tactics were smaller and in poorer condition than fish showing maturation tactics, but were similar in size to unassigned fish. We further detected effects of food restriction on hypo‐osmoregulatory function of migrants that may influence the fitness benefits of the migratory tactic at sea. We also highlight that responses to multiple stressors may vary depending on the response considered. Collectively, our results indicate contrasting effects of environmental stressors on life history trajectories in a facultatively migratory species. [ABSTRACT FROM AUTHOR]