Incidental capture of elasmobranchs by commercial prawn trawlers on the Tugela Bank, Natal, South Africa

Abstract

Species composition, sizes, trends in occurrence and catch rates of elasmobranchs caught by prawn trawlers on the Tugela Bank of Natal, South Africa, were examined from May 1989 to June 1992. Seven endemic species were recorded, the remainder having Indo-West Pacific or western Indian Ocean distributions. Most sharks were between 0,5 and 1 m long and most of the Myliobatiformes were 0,5 m in disc width. The small sizes of trawl-caught elasmobranchs indicates that the Tugela Bank functions as a nursery area for several species. None of the species examined exhibited diel patterns in frequency of occurrence, but Gymnura natalensis, Himantura gerrardi, Dasyatis chrysonota chrysonota, Sphyrna lewini and the rhinobatids occurred more frequently in warmer months. One species, Halaelurus lineatus, was more frequently taken during cooler months. Himantura gerrardi, Sphyrna lewini, Mustelus mosis and Rhizoprionodon 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 specific, S. lewini having the highest mortality (98%). Based on catch rates recorded and trawl fleet effort, it is calculated that 44 600 elasmobranchs were caught by Tugela Bank trawlers from 1989 to 1992. About 57% of these were returned to the water alive. These figures are compared to catches made by recreational anglers and the Natal Sharks Board during the same period.