Growth and simulation modelling studies of rock-lobster (Jasus lalandii) and mussel (Aulacomya ater) populations and their interactions

Master Thesis


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University of Cape Town

The availability of the ribbed mussel, Aulacomya ater is thought to be the most important factor influencing the growth rate of the commercial rocklobster, Jasus lalandii on the southern African west coast. A range of growth rates represented by von Bertalanffy growth curves is estimated for both A. ater and J. lalandii. Five models of differing complexity based on the age distributions of these populations are formulated with the main purpose of investigating optimal harvesting strategies for i, lalandii. Mussel and lobster single-species models are developed with constant recruitment, as is a lobster model incorporating a linear stock-recruitment relationship. Two models in which lobster growth is influenced by mussel availability are presented, one in which there is constant recruitment and the other having a linear stock-recruitment relationship for lobsters. Data are compiled for three areas of differing J. lalandii growth rates and A. ater biomass. Two are rock-lobster fishing grounds, the Cape Peninsula and Dassen Island at which there is a small and large benthic biomass and corresponding "slow" and "overall" estimates for rock-lobster growth, respectively. The third area is a rock-lobster sanctuary, Robben Island where there is a large benthic biomass and rock-lobsters grow at a "fast" rate. The mussel, lobster and lobster-mussel models assuming constant recruitment are used to examine the effects various parameters have on population si2e composition. Growth and predation have marked effects on the si2e structure of the prey population whereas the size composition of the predator population is greatly affected by growth and harvesting. The simplest of the four lobster models, the single-species constant recruitment model produced. the most practical management information. The lobster model incorporating a stock-recruitment relationship is unstable while the lobster-mussel model with constant recruitment requires finetuning. Only simulations of the Cape Peninsula fishing ground produced valid results using the lobster-mussel model with a stock-recruitment relationship. Based on the results of the most reliable models, three main conclusions about rock-lobster harvesting strategies are made. An increase in fishing pressure is unlikely to be commercially beneficial because of the resulting decrease in catch ·per unit effort. If opened to fishing, the Robben Island sanctuary is likely to produce a sustainable yield of at least 340 tons annually with a small reduction (7 %) in the size of this population. Although results differ from area to area according to growth and harvesting rates, reducing the minimum catchable size from the present 89 mm carapace length to 70 mm or 80 mm is predicted to give a better overall harvest. The more conservative estimate of 80 mm minimum catchable size may cause the least change to the ecosystem, possibly produce a catch of more marketable si2e and may guard against "recruitment overfishing".

Bibliography: pages 86-95.