Evidence for a positive relationship between juvenile abalone Haliotis midae and the sea urchin Parechinus angulosus in the South-Western Cape, SouthAfrica

Previous field observations have suggested an association between the urchi n Parechinus angulosus and juveniles of the abalone Haliotis midae. To test the generality and nature of this association, surveys were carried out at five sites between Cape Point and Danger Point in the kelp beds of the Sou th-western Cape, South Africa. These showed that both species occupy primar ily hard substrate, showing preferences for encrusting coralline algae. The y also confirmed a strong. positive relationship between urchins and juveni le abalone. Of the juvenile abalone sampled, more than 98% were found benea th sea urchins. All small(3-10 mm) and medium-sired (11-20 mm) juvenile aba lone were under urchins, whether on flat or vertical reef, or in crevices. A small proportion (similar to 10%) of larger juveniles (21-35 mm) was not found under urchins, and in these instances they occupied crevices instead. These findings are of particular importance in terms of their implications for the lucrative commercial abalone fishery in South Africa, indicating t hat urchins are of critical importance to the continued survival of viable abalone populations. There has been a dramatic decrease in natural populati ons of sea urchins over the past five years in the heart of the abalone fis hing grounds, and the present findings suggest that this will lead to recru itment failure of abalone. because juvenile abalone seem dependent on the u rchins. The long-term consequences for the industry may be crucial.