Effects of early experience on the reproductive performance of Atlantic salmon

To examine the link between early experience and subsequent reproductive performance, we experimentally manipulated the early experience of Atlantic salmon (Salmo salar). Salmon of a common genetic background were reared as juveniles either naturally in the river or artificially in a hatchery (sea ranched), depriving them of river experience, and then allowed to grow to maturity naturally in the ocean. At maturity the competitive and reproductive abilities of these fish were compared in four experiments quantifying reproductive success. Although levels of aggressive behavior were similar, the experience-deprive; sea-ranched males were involved in more prolonged aggressive encounters and incurred greater wounding and mortality than wild males. Furthermore, sea-ranched males were less able to monopolize spawnings and as a result obtained 51% the reproductive success of wild males across the experiments. This reproductive inferiority varied directly with the male density and bias in the sex ratio, reflecting the intensity of male breeding competition, A lower intensity of female than male competition was likely responsible for the lack of differences in breeding performance between sea-ranched and wild females. Sea-ranched females, however, produced smaller eggs than wild females, apparently in response to their higher juvenile growth rate. Differences in migratory behavior were also apparent, as sea-ranched males and females ascended the River Imsa later than wild fish. Our results indicate that early experience has implications for subsequent adult reproductive performance, affecting the development of specialized skills and traits important not only for early life, but also later life.