No abstract

Farm Atlantic salmon escape and invade rivers throughout the North Atlantic annually, which has generated growing concern about their impacts on native salmon populations. A large-scale experiment was therefore undertaken in order to quantify the lifetime success and interactions of farm salmon invading a Norwegian river. Sexually mature farm and native salmon were genetically screened, radio tagged and released into the River Imsa where no other salmon had been allowed to ascend. The farm ¢shes were competitively and reproductively inferior, achieving less than one-third the breeding success of the native ¢shes. Moreover, this inferiority was sex biased, being more pronounced in farm males than females, resulting in the principal route of gene £ow involving native males mating with farm females. There were also indications of selection against farm genotypes during early survival but not thereafter. However, evidence of resource competition and competitive displacement existed as the productivity of the native population was depressed by more than 30%. Ultimately, the lifetime reproductive success (adult to adult) of the farm ¢shes was 16% that of the native salmon. Our results indicate that such annual invasions have the potential for impacting on population productivity, disrupting local adaptations and reducing the genetic diversity of wild salmon populations.

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...

The breeding system of Atlantic salmon (Salmo salar) is shaped both by natural selection for offspring production and by sexual selection for access to mating opportunities. These evolutionary forces operate with differing intensities in the two sexes to shape their breeding behaviour and tactics. Female breeding success is largely dependent on egg production, access to breeding territories, and nest quality and survival. By contrast, male breeding success is largely determined by access to ovipositing females. As such, the breeding system of Atlantic salmon is similar to that of other members of the subfamily Salmoninae. However, early male maturity, a common pattern within the Salmoninae, reaches its greatest expression in both terms of frequency and magnitude of the mature male size difference in Atlantic salmon. Despite generalities, spawning populations of Atlantic salmon are not static, as they exhibit spatial and temporal variability in demography (e.g., spawner density, sex ratio, age at maturity, and body size). Events, both natural and anthropogenic (e.g., exploitation, habitat alteration, and climatic changes), affect this variability and ultimately shape the breeding system.