Feeding habits, daily ration and vertical migration of the cape horse mackerel off South Africa
Crustaceans, principally copepods and euphausiids, were most frequently found in the stomachs of Cape horse mackerel Trachurus trachurus capensis collected during demersal research surveys (1992–1995) in summer on the west coast and in winter on the south coast of South Africa. Fish (mainly pelagic) were infrequent (< 10%) in the diet of fish from both coasts. The feeding periodicity, rate of gut evacuation and vertical migration of horse mackerel were investigated from midwater and bottom trawl collections taken during five diel sampling periods between 1993 and 1995 at fixed positions on the South Coast. Horse mackerel feed only during the day, mostly in late afternoon prior to their ascent into midwater at around sunset. Based on the exponential rate of decline in stomach fullness throughout the night, the rate of gut evacuation by horse mackerel was estimated to be 0.22.h-1. Using the Elliott and Persson method, the daily ration was estimated as 3.8% of wet body mass. Horse mackerel appear to migrate vertically for reasons other than feeding, and selective advantages of this behaviour are discussed.