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Individual- and population-level responses to thermal change will be pivotal for species' resilience and adaptive responses to climate change. Thermal tolerance of ectotherms has been extensively studied under laboratory conditions, but comparatively few studies have assessed intra- and interpopulation variation under natural conditions or in situ. We measured field critical thermal maximum (CTmax) of brook trout (Salvelinus fontinalis) populations at twenty sites across Ontario, Canada, to assess their thermal tolerance in situ and examine potential factors underlying intraspecific variation in thermal performance. We modelled CTmax as a function of acclimation using short-term stream temperature data to assess interpopulation variation, and used full-season stream temperatures to calculate thermal safety margins (TSM) for each population. CTmax ranged between 27.41 and 30.46°C and acclimation periods between 4 and 40 days were strong predictors of site CTmax, aligning closely with lab-based studies. Seasonal temperature profiles varied substantially among sites, with mean 30-day stream temperature accounting for 66% of the among-site variation in CTmax. TSMs ranged between 0.51 and 15.51°C and reflected differences among site thermal regimes. Streams in watersheds with more urban or agricultural development had the lowest TSMs in addition to those that were fed by lake surface water. This work emphasizes the importance of locally based conservation and management practices that act at or below the population level, as local factors beyond acclimation temperature were partly responsible for variation in thermal tolerance and thus dictate the resiliency of brook trout under climate change. [ABSTRACT FROM AUTHOR]

Animals confront behavioral trade-offs whenever the movements required to attain suitable sites for reproduction or resource acquisition also increase exposure to predators. Hence, the consequences of such trade-offs should be considered in analyses of movement behavior. The objective of this study was to quantitatively link shifts in movement behavior of stream fish during the reproductive season to risk of mortality from land-based tetrapod predators. We tagged 30 brook trout with internal radio transmitters and extensively recorded both movement patterns and mortality from predation over the entire spatial range of the population. Our main result is that most of the individuals (19/30) had marked shifts in movement behavior, characterized by alternating periods of low and high movement rates, which were associated with marked shifts in mortality rate. Mortality rate increased from 0.016/day during periods of low movement rate (8.0 m/day; sheltering state) to 0.071/day during periods of high movement rate (91.0 m/day; searching state). Furthermore, individuals presenting no movement shift had low movement rate (9.6 m/day) and high mortality rate (0.096/day), in association with a preference for spawning ground. Mortality rates at the time of spawning were substantially greater than those previously reported in the literature and depended strongly on the movement state of the individuals and their use of spawning ground. Our quantitative analyses highlight the influence of within-individual variation in prey movement on risk of predation by land-based consumers. [ABSTRACT FROM AUTHOR]

A common dimorphism in life-history tactic in salmonids is the presence of an anadromous pathway involving a migration to sea followed by a freshwater reproduction, along with an entirely freshwater resident tactic. Although common, the genetic and environmental influence on the adoption of a particular life-history tactic has rarely been studied under natural conditions. Here, we used sibship-reconstruction based on microsatellite data and an ‘animal model’ approach to estimate the additive genetic basis of the life-history tactic adopted (anadromy vs. residency) in a natural population of brook charr, Salvelinus fontinalis. We also assess its genetic correlation with phenotypic correlated traits, body size and body shape. Significant heritability was observed for life-history tactic (varying from 0.52 to 0.56 depending on the pedigree scenario adopted) as well as for body size (from 0.44 to 0.50). There was also a significant genetic correlation between these two traits, whereby anadromous fish were genetically associated with bigger size at age 1 ( r G = −0.52 and −0.61). Our findings thus indicate that life-history tactics in this population have the potential to evolve in response to selection acting on the tactic itself or indirectly via selection on body size. This study is one of the very few to have successfully used sibship-reconstruction to estimate quantitative genetic parameters under wild conditions. [ABSTRACT FROM AUTHOR]

The importance of directional selection relative to neutral evolution may be determined by comparing quantitative genetic variation in phenotype ( QST) to variation at neutral molecular markers ( FST). Quantitative divergence between salmonid life history types is often considerable, but ontogenetic changes in the significance of major sources of genetic variance during post-hatch development suggest that selective differentiation varies by developmental stage. In this study, we tested the hypothesis that maternal genetic differentiation between anadromous and resident brook charr ( Salvelinus fontinalis Mitchill) populations for early quantitative traits (embryonic size/growth, survival, egg number and developmental time) would be greater than neutral genetic differentiation, but that the maternal genetic basis for differentiation would be higher for pre-resorption traits than post-resorption traits. Quantitative genetic divergence between anadromous (seawater migratory) and resident Laval River (Québec) brook charr based on maternal genetic variance was high ( QST > 0.4) for embryonic length, yolk sac volume, embryonic growth rate and time to first response to feeding relative to neutral genetic differentiation [ FST = 0.153 (0.071–0.214)], with anadromous females having positive genetic coefficients for all of the above characters. However, QST was essentially zero for all traits post-resorption of the yolk sac. Our results indicate that the observed divergence between resident and anadromous brook charr has been driven by directional selection, and may therefore be adaptive. Moreover, they provide among the first evidence that the relative importance of selective differentiation may be highly context-specific, and varies by genetic contributions to phenotype by parental sex at specific points in offspring ontogeny. This in turn suggests that interpretations of QST- FST comparisons may be improved by considering the structure of quantitative genetic architecture by age category and the sex of the parent used in estimation. [ABSTRACT FROM AUTHOR]

Abstract To examine constraints on evolution of larger body size in two stunted populations of brook charr (Salvelinus fontinalis ) from a single river in Cape Race, Newfoundland, Canada, we measured viability selection acting on length-at-age traits, and estimated quantitative genetic parameters in situ (following reconstruction of pedigree information from microsatellite data). Furthermore we tested for phenotypic differentiation between the populations, and for association of high juvenile growth with early maturity that is predicted by life history theory. Within each population, selection differentials and estimates of heritabilities for length-at-age traits suggested that evolution of larger size is prevented by both selective and genetic constraints. Between the populations, phenotypic differentiation was found in length-at-age and age of maturation traits, whereas early maturation was associated with increased juvenile growth (relative to adult growth) both within and between populations. The results suggest an adaptive plastic response in age of maturation to juvenile growth rates that have a largely environmental basis of determination. [ABSTRACT FROM AUTHOR]