ENVIRONMENT, DISTRIBUTION AND GROWTH RATES OF WEST COAST ROCK-LOBSTER JASUS LALANDII (H. MILNE EDWARDS)

Summary

Growth rates of rock-lobsters Jasus lalandii varied considerably along the west coast of southern Africa. As small differences affect the productivity of individual fishing grounds, an attempt was made to identify the major environmental factors influencing growth. Six widely-spaced areas were studied over a period of nine years. Tagging and short-term holding experiments were used to measure rock-lobster growth rates, while diving and hydrological surveys provided detailed information on rock-lobster populations and their environment. Relatively rapid growth was found in the southern part of the Cape west coast (except for the extreme south, off the Cape Peninsula) where depths occupied by rock-lobsters extend to at least 50 m. Further north, rock-lobsters and associated benthos were limited to shallower depths and growth was generally slower. Food availability is probably the most important factor influencing growth rates, while differences in water temperature were not considered to be significant. Ribbed mussels Aulacomya ater (Molina) are the most important food source for rock-lobsters, and these occur in high densities throughout the geographic range of J. lalandii. However, most rock-lobsters are limited to feeding only on certain sizes of mussels, so that food may be limited even in areas of comparatively high mussel biomass. Density-dependent effects on feeding and growth are likely to be important, and these intensify in the northern areas of the coast owing to the restriction in depth range of the benthos. Extremely low dissolved-oxygen values (a phenomenon linked with the azoic muds off the central and northern coast) limit the northern extent of distribution of dense stocks of J. lalandii to about 25°20'S. Factors limiting the distribution of commercially exploitable stocks of J. lalandii at the southward edge of the range (Cape Point) are related to the distribution of phyllosoma larvae in offshore waters and to recruitment inshore of puerulus larvae.