Big Effects from Small Causes: Two Examples from Fish Population Dynamics

Abstract

1. A population which, before exploitation, includes upwards of 12–15 age-groups in appreciable quantities, is very sensitive to fishing. For example, even as little as 5% catch per annum eventually causes a major reduction in the relative weight of older fish in the stock. With unchanging recruitment the absolute size of the total stock also declines markedly, or even catastrophically, under moderate exploitation—so much so that it seems likely that recruitment must usually increase when the stock is first thinned, partly compensating for the removals.2. A population that produces its greatest sustained yield at a high rate of exploitation—say 75% or more at average recruitment—is then close to a point where recruitment will fall off rapidly if utilization becomes only slightly more intensive. This fact, and the aggravating effect of environmentally-caused variations in recruitment, suggest that any really close approach to the point of optimum yield will usually be too dangerous to be practical.