SEABIRD CONSUMPTION AND PRODUCTION IN THE BENGUELA AND WESTERN AGULHAS ECOSYSTEMS

Seabirds were estimated to consume about 430 000 metric tons of food per annum off south-western Africa (15-degrees-S to 28-degrees-E, from the coast to the 500 m bottom contour) in the 1980s. Of this some 34 per cent was eaten by Cape cormorants, 12 per cent by Cape gannets and 7 per cent by African penguins, species that breed in the region. Among the migrant seabirds, whitechinned petrels accounted for 12 per cent of overall consumption, sooty shearwaters 9 per cent, blackbrowed albatrosses 6 per cent and shy albatrosses 5 per cent. Consumption was higher in the austral winter than in summer because of the greater number of migrant seabirds in the region in winter Anchovy was the most important prey item, contributing about 31 per cent of all food eaten, followed by pelagic goby (18%), hakes (15%) and zooplankton (8%). Virtually all the hake component of the diet is scavenged from fishing vessels, and of the other main prey items only anchovy is targeted by commercial fishermen. It is estimated that African penguins decreased in abundance by about 400 000 birds between the mid 1950s and the mid 1980s and Cape gannets by 180 000, but that Cape cormorants increased by some 500 000 birds. The overall consumption by these three species decreased by about 35 000 tons wet mass between the two periods. Past estimates of consumption by seabirds off south-western Africa are reviewed. In the 1980s there were estimated to be, on average, about 2 million locally breeding seabirds and 3 million migrant seabirds in the region. The overall biomass of seabirds was about 5 000 tons live mass and the annual gross production about 700 tons live mass.