Diel variability in bottom trawl catches and feeding activity of the Cape hakes off the west coast of South Africa

Abstract

The diel variability in bottom trawl catches and the feeding patterns of the Cape hakes Merluccius capensis and M. paradoxus were investigated during thirteen 28-h experiments in winter and summer between 1984 and 1990 within the same locality off the west coast of South Africa. Considerable variation was found in the diel differences in the catches, but day-time numbers and mass were generally greater than at night. Catch rates of M. capensis were significantly different in nine experiments whereas five experiments yielded significant differences in M. paradoxus. For small hake, especially M. paradoxus, euphausiids were eaten more frequently in summer and mesopelagic fish were of more dietary importance in winter. Hake-on-hake predation was a prominent feature in M. capensis, especially in summer when larger fish were caught at the study site. In the majority of cases, the size composition of M. capensis was not significantly different between the day and night catches. By contrast, diel size differences were generally found in M. paradoxus, but with no consistent pattern. There was little indication of any diel feeding periodicity in either hake species, with up to one-third of the hake examined with food in their stomachs having recently fed, regardless of time of day or night. The diet composition indicated that hake move into midwater at night in response to similar movement of their prey.