A SIZE-STRUCTURED SIMULATION-MODEL FOR EVALUATING MANAGEMENT STRATEGIES IN GILLNET FISHERIES EXPLOITING SPATIALLY DIFFERENTIATED POPULATIONS

A length-structured simulation model is presented as a tool in decisio n making for gillnet fisheries management. The analytical model simula tes the fish population dynamics and impact of the fisheries, taking i nto account size-dependent spatial distribution patterns and migration of the fish. The daily recruitment of fish, at a theoretical starting length of 0 cm, is simulated with a variable temporal pattern, giving the model a high degree of generality and making it applicable to tro pical situations where recruitment is not restricted to a short period in time. The model is sex-differentiated, the growth of the fish is d etermined by their length and the dispersion over the length-classes i s controlled by the 'fractional boxcar train method'. Natural mortalit y is assumed constant for all exploited size classes. Fishing mortalit y is a function of fish length, based on a combination of gillnet sele ctivity curves for various mesh sizes. The fishing mortality for the m ost vulnerable size class is calibrated using estimates for the fishin g mortality per length-class from length-based cohort analysis. Spatia l distribution of the fish over different habitats is based on observe d patterns in size-dependent habitat occupation. The realistic model p rovides predictions on the progressive effects of management regulatio ns on numbers, size distributions and total biomass of the population and the catch in the various habitats. The commercial gillnet fishery for Oreochromis mossambicus (Cichlidae) in Tissawewa, a Sri Lankan res ervoir, is used as an example. Simulation results correspond with inde pendently estimated values for output parameters. Based on model predi ctions, conservation of this fishery is recommended by re-enforcement of the legal minimum mesh size of 76 mm stretched mesh, and a limitati on of fishing effort to the present level. The model also provides pre dictions on the effects of subsidiary small-meshed fisheries for minor cyprinids on the existing fishery for O. mossambicus. Model predictio ns take into account the size-dependent spatial distribution patterns of the fish and the spatial allocation of the fishing effort.