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

Rock lobsters, Jasus lalandii, may play an important role in structuring th e communities on shallow reef ecosystems and have recently increased in den sity in the area between Cape Hangklip and Danger Point on the southwest co ast of South Africa. This has probably resulted in increased predation on n atural populations of smooth turban shells (Turbo cidaris) and pink-lipped topshells (Oxystele 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 thes e 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 lobster s of all size classes tested were able to crack and consume a full size ran ge of T. cidaris, and no size preferences were evident. When the two gastro pods were offered simultaneously, rock lobsters preferentially consumed the thinner shelled O. sinensis. Consumption rates increased linearly with roc k-lobster size. Population consumption rates indicate that rock lobsters wi ll have a profound effect on winkle stocks in the area, even if winkles con stituted less than 1% of the diet. This in turn suggests that commercial ex ploitation of these stocks is unlikely to remain sustainable.