The magnitude of ¢tness variation caused by maternal e¡ects and, thus, the adaptive signi¢cance of maternal traits may depend on environmental quality, generating crossing reaction norms among o¡spring phenotypes that shape life-history evolution. By manipulating intraclutch variation in egg size and comparing siblings we examined the maternal e¡ects of egg size on o¡spring performance and tested for the existence of reaction norms to environmental quality using the brown trout Salmo trutta. When sibling groups of small and large eggs were reared separately in a hatchery environment initial size di¡er-ences disappeared rapidly. However, in semi-natural environments and under direct competition, juveniles from large eggs experienced growth and survival advantages over siblings from small eggs. Moreover, distinct reaction norms existed, with the di¡erences in performance of juveniles from small and large eggs being most pronounced in the poorer growth environments. Our results provide the ¢rst direct evidence, to our knowledge, for a causal relationship between egg size and ¢tness-related traits in ¢shes, independent of potentially confounding genetic e¡ects. Moreover, they indicate that previous studies have been biased by experimental conditions that excluded competitive asymmetries and environmental variability. The existence of reaction norms indicates a shift in optimal egg size across gradients of environmental quality that probably shapes the evolution of this trait.

Here we critically review the scale and extent of adaptive genetic variation in Atlantic salmon (Salmo salar L.), an important model system in evolutionary and conservation biology that provides fundamental insights into population persistence, adaptive response and the effects of anthropogenic change. We consider the process of adaptation as the end product of natural selection, one that can best be viewed as the degree of matching between phenotype and environment. We recognise three potential sources of adaptive variation: heritable variation in phenotypic traits related to fitness, variation at the molecular level in genes influenced by selection, and variation in the way genes interact with the environment to produce phenotypes of varying plasticity. Of all phenotypic traits examined, variation in body size (or in correlated characters such as growth rates, age of seaward migration or age at sexual maturity) generally shows the highest heritability, as well as a strong effect on fitness. Thus, body size in Atlantic salmon tends to be positively correlated with freshwater and marine survival, as well as with fecundity, egg size, reproductive success, and offspring survival. By contrast, the fitness implications of variation in behavioural traits such as aggression, sheltering behaviour, or timing of migration are largely unknown. The adaptive significance of molecular variation in salmonids is also scant and largely circumstantial, despite extensive molecular screening on these species. Adaptive variation can result in local adaptations (LA) when, among other necessary conditions, populations live in patchy environments, exchange few or no migrants, and are subjected to differential selective pressures. Evidence for LA in Atlantic salmon is indirect and comes mostly from ecological correlates in fitness-related traits, the failure of many translocations, the poor performance of domesticated stocks, results of a few common-garden experiments (where different populations were raised in a common environment in an attempt to dissociate heritable from environmentally induced phenotypic variation), and the pattern of inherited resistance to some parasites and diseases. Genotype x environment interactions occurr for many fitness traits, suggesting that LA might be important. However, the scale and extent of adaptive variation remains poorly understood and probably varies, depending on habitat heterogeneity, environmental stability and the relative roles of selection and drift. As maladaptation often results from phenotype-environment mismatch, we argue that acting as if populations are not locally adapted carries a much greater risk of mismanagement than acting under the assumption for local adaptations when there are none. As such, an evolutionary approach to salmon conservation is required, aimed at maintaining the conditions necessary for natural selection to operate most efficiently and unhindered. This may require minimising alterations to native genotypes and habitats to which populations have likely become ad...

Why do highly fecund organisms apparently sacrifice offspring size for increased numbers when offspring survival generally increases with size? The theoretical tools for understanding this evolutionary trade-off between number and size of offspring have developed over the past 25 years; however, the absence of data on the relation between offspring size and fitness in highly fecund species, which would control for potentially confounding variables, has caused such models to remain largely hypothetical. Here we manipulate egg size, controlling for maternal trait interactions, and determine the causal consequences of offspring size in a wild population of Atlantic salmon. The joint effect of egg size on egg number and offspring survival resulted in stabilizing phenotypic selection for an optimal size. The optimal egg size differed only marginally from the mean value observed in the population, suggesting that it had evolved mainly in response to selection on maternal rather than offspring fitness. We conclude that maximization of maternal fitness by sacrificing offspring survival may well be a general phenomenon among highly fecund organisms.

Timing of breeding and offspring size are maternal traits that may influence offspring competitive ability, dispersal, foraging, and vulnerability to predation and climatic conditions. To quantify the extent to which these maternal traits may ultimately affect an organism's fitness, we undertook laboratory and field experiments with Atlantic salmon (Salmo salar). To control for confounding effects caused by correlated traits, manipulations of the timing of fertilization combined with intraclutch comparisons were used. In the wild, a total of 1462 juveniles were marked at emergence from gravel nests. Recapture rates suggest that up to 83.5% mortality occurred during the first four months after emergence from the gravel nests, with the majority (67.5%) occurring during the initial period ending 17 days after median emergence. Moreover, the mortality was selective during this initial period, resulting in a significant phenotypic shift toward an earlier date of and an increased length at emergence. However, no significant selection differentials were detected thereafter, indicating that the critical episode of selection had occurred at emergence. Furthermore, standardized selection gradients indicated that selection was more intense on date of than on body size at emergence. Timing of emergence had additional consequences in terms of juvenile body size. Late-emerging juveniles were smaller than early-emerging ones at subsequent samplings, both in the wild and in parallel experiments conducted in seminatural stream channels, and this may affect success at subsequent size-selective episodes, such as winter mortality and reproduction. Finally, our findings also suggest that egg size had fitness consequences independent of the effects of emergence time that directly affected body size at emergence and, in turn, survival and size at later life stages. The causality of the maternal effects observed in the present study supports the hypothesis that selection on juvenile traits may play an important role in the evolution of maternal traits in natural populations.