Biological and ecological aspects of the distribution of <i>Sepia australis,/i> (Cephalopoda: Sepiidae) off the south coast of southern Africa

Abstract

During the South Coast Biomass Survey in 1988, 49,4 kg (6336 individuals) of Sepia australis were caught between Cape Agulhas and Algoa Bay. A biomass index of 8031 of S. australis wascalculated for the area at that time. Largest catches were taken between about 20°E and 22°E, in waters of 10-11°C and 50-150 m depth. The overall sex ratio was 2M : 3F and mean individual mass was 6,47 g for males and 8,67 g for females. The largest animals were a mature male of 58 mm mantle length and a maturing female of 65 mm mantle length. Most of the animals trawled off the South Coast were maturing or fully mature in early winter and very few immature animals were found. Differences in mean mantle length and maturity stage of the animals in different areas were found to be correlated most strongly with water temperature but also with depth and longitude. Largest numbers and mean sizes of mature animals caught suggest that the main spawning grounds off the South Coast may be in deeper water on the western side of the Agulhas Bank. To the east the deeper water is warmer and the animals there may be at a disadvantage, as optimum temperatures and depths for spawning do not coincide.