Are Cape Cormorants Phalacrocorax capensis losing the competition? Dietary overlap with commercial fisheries

Abstract

Cape Cormorants Phalacrocorax capensis breed in southern Africa’s Benguela upwelling system and in 2013 were listed as Endangered by the IUCN following a population decrease of >50% over their three most recent generations. This decrease was associated with reduced prey availability, particularly of the pelagic shoaling fishes Cape anchovy Engraulis encrasicolus and sardine Sardinops sagax. The diet of Cape Cormorants was examined from 929 regurgitated prey samples collected at 11 localities off west South Africa from 1988 to 2007. By number, anchovy was the most important prey in all years, except 2007, in which the geographical distribution of samples was limited. Overall anchovy contributed 85% of all prey items eaten and sardine 6%. However, from 1988 to 1997, sardine formed 93% of the diet by mass in the southwest. The proportional contribution of anchovy to the diet increased between 1988–1997 and 1998–2007, whereas that of sardine decreased. Anchovy eaten in the northwest were smaller than those caught in the central west and southwest were. Small, immature sardine were eaten in all regions, but mature sardine only in the southwest, the spawning ground of the western sardine stock. Anchovy and sardine are targeted by South Africa’s purse-seine fishery. Most anchovy and sardine eaten by Cape Cormorants had caudal lengths of 5–9 cm and 15–21 cm, respectively, and showed considerable overlap with sizes harvested by the fishery. This highlights the potential for competition between Cape Cormorants and the fishery for anchovy and sardine.