Simulation Studies of the Adams River Sockeye Salmon (Oncorhynchus nerka)

The pattern of abundance of sockeye salmon (Oncorhynchus nerka) of the Adams River since 1938 is: 1 year large or "dominant," 1 year small or "subdominant," and 2 years very small or "off." In recent years, the subdominant run has shown indications of becoming as large as the dominant. This pattern can be accounted for by a model system in which predation is not sufficient to influence the dominant run, but predators, benefited by the abundance of prey, remove such a large fraction in the 3 succeeding years that the pattern is retained. Buffering of predation by the dominant run accounts for the larger size of the subdominant. Patterns similar in form to those observed naturally since 1922 are produced over a narrow range of parameters, and suggest that the particular circumstances of the Adams run are necessary to the nature of the population fluctuation, but are not so particular that some other pattern could not have emerged with a different sequence of environmental conditions. Taking an array of results of a series of simulations suggests that the odds of developing the existing pattern were about 50:50, although the subdominant run would more commonly become dominant as well by 1951, rather than as late as the 1960's. The double dominance situation is stable in the model system. In the present circumstances it seems appropriate to explore the possible benefits of managing the fishery so as to enhance the growth in size of the subdominant run, and perhaps to consider predator-removal programs to enable development of large runs on all 4 years of the traditional cycle.