The shoaling behaviour of pelagic fish and the distribution of seals and gannets off Namibia as deduced from routine fishing reports, 1982–1985

Abstract

Detailed fishing reports of the Namibian pelagic purse-seine fleet were used to elucidate circadian, lunar and seasonal variations in shoaling patterns of the South African pilchard Sardinops ocellatus, Cape anchovy Engraulis capensis and recruiting Cape horse mackerel Trachurus trachurus capensis in the northern Benguela system over the period 1982–1985. Data on the distribution of Cape fur seals Arctocephalus pusillus and Cape gannets Morus capensis per Fishing position were also evaluated. Pilchard and adult anchovy tend to form comparatively large and fewer shoals during the day than at night, but recruiting anchovy and horse mackerel showed no meaningful day/night differences in shoal sizes. Normally, pilchard and anchovy shoals descended during the day and horse mackerel during the night. Most pelagic fish catches were made in the coastal regions between 19 and 23°S, but the distributions of seals and gannets at sea differed somewhat and also were not the same as those of the fish.