A commonly held assumption, that a unit of fishing effort catches a constant proportion of the fishable population at all population levels, is examined. Inspection of California's Pacific sardine, Pacific mackerel, and tuna fisheries leads to the conclusion that a nominal unit of fishing effort takes an increasingly larger proportion of the fish population as the population declines, and that the rate of change of the catchability coefficient relative to changes in population size is dependent on the nature of the specific fishery in question. This concept, a catchability coefficient which increases continuously in a declining population, contradicts the concept of "equilibrium catch" at all effort levels, predicted by Schaefer's model, and casts doubt upon the validity of certain controlled-entry fishery models.
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