Modelling the effect of alternative management strategies on the catch and effort in the carpenter (Argyrozona argyrozona) linefishery

Abstract

In South Africa, linefishing increased in the mid-1800s and eventually reached a peak in the 1980s/90s. A legal linefishery emergency was declared in 2000 in accordance with the Marine Living Resources Act of 1998. This was a large blow for the commercially important industry. Nonetheless, the carpenter stocks had already been overexploited to such a high degree by linefishing, and also as part of trawling bycatch, that even at an effort less than half of the effort that produces a maximum sustainable yield, carpenter stocks were still found below the biomass found at the maximum sustainable yield. In order to explore three different management strategies for the carpenter stock on the offshore Central Agulhas Bank, an agestructured, non-stochastic model was set up. The influences of the following strategies - Total Allowable Catch (TAC), as well as revised minimum size limit and closed season – on a stock found at a dismal 19.2% of carrying capacity, are projected using this model on a monthly time-step. The objectives of this fishery are to achieve a biomass in the final year equal to the biomass found at a maximum sustainable yield (7243 t) and to maximise effort (employment) and catch. The overall recommendation for this stock was a reduction in minimum size limit from 35 cm to 30.48 cm in combination with a flexible TAC starting at 790 t. It is also recommended that even though models are important for fisheries management, no model should be taken in isolation without first considering life history traits and other important information about a stock.