Predation by West Coast rock lobsters (Jasus lalandii) on two species of winkle (Oxystele sinensis and Turbo cidaris)

Abstract

Rock lobsters, Jasus lalandii, may play an important role in structuring the communities on shallow reef ecosystems and have recently increased in density in the area between Cape Hangklip and Danger Point on the southwest coast of South Africa. This has probably resulted in increased predation on natural populations of smooth turban shells (Turbo cidaris) and pink-lipped topshells (Oxystete sinensis), both of which are proposed targets for small-scale commercial fisheries. This study examines the size selectivity, prey species preferences and consumption rates of rock lobsters feeding on these two winkle species. Rock lobsters showed a strong preference for small O. sinensis, and were limited to taking a maximum (or critical) size of prey, which increased linearly with rock-lobster size. By contrast, rock lobsters of all size classes tested were able to crack and consume a full size range of T. cidaris, and no size preferences were evident. When the two gastropods were offered simultaneously, rock lobsters preferentially consumed the thinner shelled O. sinensis. Consumption rates increased linearly with rock-lobster size. Population consumption rates indicate that rock lobsters will have a profound effect on winkle stocks in the area, even if winkles constituted less than 1% of the diet. This in turn suggests that commercial exploitation of these stocks is unlikely to remain sustainable.