Sigurd Einum

SELECTION AGAINST LATE EMERGENCE AND SMALL OFFSPRING IN ATLANTIC SALMON (SALMO SALAR)

Abstract.— 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 organisms 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.

Highly fecund mothers sacrifice offspring survival to maximize fitness

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.

Experimental tests of genetic divergence of farmed from wild Atlantic salmon due to domestication

The genetic response of Atlantic salmon to culture is important in predicting the success of these fish in nature and their impacts on wild populations through competition and interbreeding. We compared a seventh-generation strain of farmed Atlantic salmon from Sunndalsora, Norway, with its principal founder population from the wild, the River Namsen. The fish were reared from eggs in a common environment and assessed for the extent of genetic divergence in several fitness-related traits. Morphology had diverged, as farmed juveniles showed more robust bodies and smaller rayed fins than the wild juveniles. Ecologically important aspects of behaviour also differed. Farmed juveniles were more aggressive in a tank environment typical of culture facilities, while wild juveniles dominated in a stream-like environment. Farmed juveniles were also more risk-prone, reappearing from cover sooner after a simulated predator attack. It was not surprising that growth performance was higher in farmed than wild juveniles, as the former had been subjected to intentional selection for this trait. Correlated responses to this selection may also explain the higher rate of smolting and lower rate of male parr maturity in the farmed than the wild salmon. Competition with wild juveniles, however, negatively affected the growth of the farmed juveniles, particularly under semi-natural conditions. Our results indicate that farming of Atlantic salmon generates rapid genetic change, as a result of both intentional and unintentional selection in culture, that alters important fitness-related traits.

Rapid parallel evolutionary changes of gene transcription profiles in farmed Atlantic salmon

Farmed salmon strains have been selected to improve growth rates as well as other traits of commercial interest but the 2 million farmed salmon escaping annually may enhance the risk of extinction of wild populations through genetic and ecological interactions. Here, we compare the transcription profiles of 3557 genes in the progeny of farmed and wild Atlantic salmon from Norway and Canada grown in controlled conditions, and demonstrate that five to seven generations of artificial selection led to heritable changes in gene transcription profiles, the average magnitude of the differences being 25% and 18% for at least 1.4% and 1.7% of the expressed genes in juvenile salmon from Norway and Canada, respectively. Moreover, genes showing significant transcription profile differences in both farmed strains (16%) all exhibited parallel changes. These findings, along with the identification of several genes whose expression profiles were modified through artificial selection, provide new insights into the molecular basis of parallel evolution, and suggest how gene flow from farmed escapees may affect the genetic integrity of wild populations.

Does within-population variation in fish egg size reflect maternal influences on optimal values?

Various models that assume correlations between maternal phenotype and offspring environment predict adaptive variation in egg size within populations. Here we conduct a comparative test of these models using published data on fish egg size. Intrapopulation variation in egg size was most pronounced in fish with demersal eggs and larvae (median coefficient of variation [CV] at family \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Atlantic salmon eggs favour sperm in competition that have similar major histocompatibility alleles

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Spatial distribution of limited resources and local density regulation in juvenile Atlantic salmon

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