Circannual Cycle of Seawater Adaptability in Oncorhynchus nerka: Genetic Differences between Sympatric Sockeye Salmon and Kokanee

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Abstract
Anadromous sockeye salmon and nonanadromous kokanee (Oncorhynchus nerka) spawn sympatrically in locations across British Columbia, yet remain genetically distinct. To investigate the possibility that these differences are maintained by selection against hybrids of the two forms owing to differences in seawater adaptability, we raised pure and reciprocal crosses of Shuswap River sockeye and kokanee in controlled hatchery conditions and tested their seawater adaptabilities nine times over 2 yr. Sockeye, kokanee, and reciprocal hybrids all showed a seasonal cycle in their seawater adaptability, which was lowest in midwinter and highest in the spring. However, clear differences were evident among cross-types in the spring of their second year. Sockeye were the first to show increased seawater adaptabilities, hybrid groups next, and kokanee last. The observed differences could not be attributed to tank effects, or to differences in size, sex, or state of maturity.

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