Biological and ecological aspects of the distribution of Sepia australis (Cephalopoda: Sepiidae) off the south coast of southern Africa

ENGLISH 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 803 t of S. australis was calculated for the area at that time. Largest catches were taken between about 20oE and 22oE, 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. *********** AFRIKAANSE OPSOMMING: Gedurende die Suidkus-Biomassa-opname in 1988 is altesaam 49,4 kg (6336 individue) Sepia australis gevang. ‘n Biomassa-indeks van 803 t S. australis is toe vir die gebied bereken. Die grootste vangste is tussen 20°0 en 22°0 in water van 10-11°C, op 'n diepte van 50-150 m, gemaak. Die algemene geslagsverhouding was 2M: 3V en die gemiddelde enkele massa was 6,47 9 vir manlike en 8,67 9 vir vroulike diere. Die grootste diere was 'n ryp mannetjie met 'n mantellengte van 58 mm en 'n deeIs ryp wyfie met 'n mantellengte van 65 mm. Die meeste diere aan die Suidkus was 6f ryp 6f dee Is ryp in die vroe winter en baie min onryp diere is gevind. Verskille in gemiddelde mantellengte en stadium van rypheid in verskillende gebiede was mees konsekwent met watertemperatuur gekorreleer maar ook met diepte en lengteligging. Die grootste getalle en gemiddelde groottes van ryp diere dui daarop dat die hoof te lgebiede aan die Suidkus meer Wes, in dieper water, mag wees. Ooswaarts is die dieper water warmer wat die diere mag benadeel aangesien die optimum-temperature en diepte vir teling nie saamval nie.