A simulation model of the sport-fishery for galjoen Coracinus capensis: an evaluation of minimum size limit and closed season

Abstract

An age-structured stochastic simulation model is used to evaluate minimum size limit and closed season restrictions in terms of yield, effort and stock size for the sport-fishery for galjoen Coracinus capensis for a projected 15-year period, on the assumption that effort is increasing at 6 per cent annually. It is argued that the female stock of the Western Cape is currently recruitment-overfished with F≫F _MSY , although the possibility that the stock is not recruitment-overfished is also considered. Simulated catch-at-age data for a variety of values of F show an insensitivity to the degree of stock depletion, a phenomenon ascribed to a strong size-selectivity effect. Closed-season and minimum-size options were evaluated against three possible effort-catch-rate (E-cpue) relationships, ranging from effort being independent of catch rate (cpue) to effort being entirely dependent on it. The present closed season of four months is ineffective at improving catches and counterproductive in that it reduces effort. The reasons for this failure are, first, that the closed season is at a time when galjoen are less "catchable" and, second, that, depending on the E-cpue relationship, fish not caught during the closed season attract additional effort during the open season. By contrast, the minimum size limit is extremely effective by increasing both yield and effort substantially, a result which is robust to differing E-cpue relationships and to the shape of the recruitment curve.