A simple genetic mechanism may be partly responsible for maintaining violent cycles in abundance of some Fraser River sockeye salmon (Oncorhynchus nerka) stocks. If age at maturity is highly heritable, spawning runs in low cycle lines may have high percentages of age 5 spawners that tend to produce offspring that will return at age 5, due to production of low percentages of such spawners by preceding high cycle lines. Then even if each low cycle line has a high productivity per spawner, a large fraction of this productivity may be "lost" to the cycle line, in the form of age 5 recruits to other cycle lines. In the face of high fishing mortality, even a small relative loss from any cycle line may cause it to remain small relative to other lines. This model explains several observations, such as high productivity per spawner in low cycle lines, that cannot be explained by previous hypotheses involving depensatory predation or food supply. However, it is unlikely that genetic effects alone are responsible for the stability of cyclic patterns, unless low offcycle runs consist entirely of "temporal colonizer" genotypes that produce a high proportion of age 5 offspring as a dominant trait.

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