Selection against late emergence and small offspring in Atlantic salmon (Salmo salar)

Timing of breeding and offspring size are maternal traits that may influenc e offspring competitive ability, dispersal, foraging, and vulnerability to predation and climatic conditions. To quantify the extent to which these ma ternal traits may ultimately affect an organism's fitness, we undertook lab oratory and field experiments with Atlantic salmon (Salmo salar). To contro l for confounding effects caused by correlated traits, manipulations of the timing of fertilization combined with intraclutch comparisons were used. I n 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 t he first four months after emergence from the gravel nests, with the majori ty (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 differ entials were detected thereafter, indicating that the critical episode of s election had occurred at emergence. Furthermore, standardized selection gra dients indicated that selection was more intense on date of than on body si ze at emergence. Timing of emergence had additional consequences in terms o f juvenile body size. Late-emerging juveniles were smaller than early-emerg ing ones at subsequent samplings, both in the wild and in parallel experime nts conducted in seminatural stream channels, and this may affect success a t subsequent size-selective episodes, such as winter mortality and reproduc tion. Finally, our findings also suggest that egg size had fitness conseque nces independent of the effects of emergence time that directly affected bo dy size at emergence and, in turn, survival and size at later life stages. The causality of the maternal effects observed in the present study support s the hypothesis that selection on juvenile traits may play an important ro le in the evolution of maternal traits in natural populations.