Modelling the effect of marine reserves on the recreational shore-fishery of the South-Western Cape, South Africa

Abstract

A discrete, space-time, age-structured model of a fishery was developed to test the effect of marine reserves on yield of fish to recreational angling. The model was applied to three sympatric surf-zone species commonly targeted by shore-anglers. Movement rates of white steenbras Lithognathus lithognathus and galjoen Dichistius capensis, both of which are nomadic, were estimated from tag-recovery data. Because post-recruit blacktail Diplodus Sargus capensis are resident, a coefficient of passive larval diffusion was estimated from drogue-separation rates. Marine reserves cannot increase the yield-per-recruit of white steenbras, although the spawner-biomass-per-recruit responds positively. A "recruitment - spawner-biomass" function was applied to galjoen and blacktail. The yield of galjoen increased sharply as the size of area conserved increased. Small reserves, closely spaced, appeared to be optimal for blacktail. A range of reserve size and spacing combinations satisfied management criteria for the three fisheries. Because of the assumption of a uniform coastline, and the unknown extent of effort displacement by reserves, the results are too general to be of specific value, but they do suggest that management of multispecies fisheries by the use of marine reserves is a viable option. With the availability of site-specific data, the model can be applied to evaluate the effect of existing or proposed marine reserves.