INCIDENTAL CAPTURE OF ELASMOBRANCHS BY COMMERCIAL PRAWN TRAWLERS ON THE TUGELA BANK, NATAL, SOUTH-AFRICA

Species composition, sizes, trends in occurrence and catch rates of el asmobranchs caught by prawn trawlers on the Tugela Bank of Natal, Sout h Africa, were examined from May 1989 to June 1992. Seven endemic spec ies were recorded, the remainder having Indo-West Pacific or western I ndian Ocean distributions. Most sharks were between 0,5 and 1 m long a nd most of the Myliobatiformes were 0,5 m in disc width. The small siz es of trawl-caught elasmobranchs indicates that the Tugela Bank functi ons as a nursery area for several species. None of the species examine d exhibited diel patterns in frequency of occurrence, but Gymnura nata lensis, Himantura gerrardi, Dasyatis chrysonota chrysonota, Sphyrna le wini and the rhinobatids occurred more frequently in warmer months. On e species, Halaelurus lineatus, was more frequently taken during coole r months. Himantura gerradi, Sphyrna lewini, Mustelus mosis and Rhizop rionodon acutus were recorded more frequently in shallower trawls (20- 33 m) and G. natalensis and D.c. chrysonota more frequently in deeper trawls (33-45 m). Trawl-induced mortality was shown to be species spec ific, S. lewini having the highest mortality (98%). Based on catch rat es recorded and trawl fleet effort, it is calculated that 44 600 elasm obranchs were caught by Tugela Bank trawlers from 1989 to 1992. About 57% of these were returned to the water alive. These figures are compa red to catches made by recreational anglers and the Natal Sharks Board during the same period.