A PRELIMINARY INVESTIGATION OF ABALONE HALIOTIS-MIDAE RESOURCES ALONG THE TRANSKEI COAST, SOUTH-AFRICA

Abalone Haliotis midae populations have been exploited commercially off South Africa's Western Cape since 1949. More recently, fisheries for H. midae have developed along the east coast of South Africa, but there is very little information on the extent of the abalone stocks there. No H. midae are found north of the Mbashe River, but mean density between Wavecrest and the Mbashe River was 0,17 abalone . m(-2) of reef and the highest density was 0,84 abalone . m(-2) of reef. These densities are probably optimum habitat densities rather man average densities. Densities were generally lower at the northern end of the abalone range, but differences between the north and the south were not significant. At the northern limit, the population consisted mainly of small abalone. There was no significant difference in the numbers of abalone caught per diver per diving hour at the different stations, but catch rate (cpue) by mass of abalone was higher in the south. Highest cpue was 55 abalone (25 kg whole mass) per diver per diving hour. Sex ratios were 1:1 and abalone off the East Coast reached maturity at a size smaller than those off the South-Western Cape. Yield- and egg-per-recruit modelling indicated that reducing the current minimum legal size from 114 mm to 80 or 94 mm shell width would provide slightly better yields per recruit only at high values of natural mortality. However, at a smaller minimum legal size, egg production is reduced to below 50% at very much lower levels of fishing effort. Maintaining the current minimum legal size of 114 mm shell width ensures at least 30-50% egg production for virtually all realistic levels of fishing effort at all values of natural mortality investigated. The conclusion is that the stocks have rather limited potential for commercial exploitation.