Prey consumption of Australasian gannets (Morus serrator) breeding in Port Phillip Bay, southeast Australia, and potential overlap with commercial fisheries

Abstract

Prey consumption of Australasian gannets (Morus serrator) breeding in Port Phillip Bay, Victoria, was investigated between 1997 and 2000 and the potential overlap between gannets and commercial fisheries in this region was assessed. Approximately 1000 gannets now breed in Port Phillip Bay annually and typically feed on inshore pelagic schooling fish species, such as pilchards (Sardinops sagax), barracouta (Thyrsites atun) and, to a lesser extent, anchovy (Engraulis australis), garfish (Hyporhamphus melanochir), mackerel (Scomber australasicus) and other species. Nutritional analyses of the major prey items in the gannet diet suggests that gannets may target prey species of higher energy density; however, variations in their representation in the gannet diet between breeding periods probably reflect changes in local availability. The daily energy requirement of individual gannets breeding in Port Phillip Bay, determined from activity patterns and activity-specific metabolic rates, was calculated as 4561 kJ d 1 . Using a bioenergetics model it was estimated that gannets breeding in Port Phillip Bay consume in total some 228.2 tonnes of prey during the breeding period, including 37.8 t of pilchard, 80.5 t of barracouta, 26.5 t of anchovy and 27.5 t of garfish. The potential foraging ranges of gannets during the breeding period were estimated at approximately 100 km. Within this zone all of the major prey items in the gannet diet are taken to some extent commercially, although only pilchards are specifically targeted as a fishery in large quantities. There is considerable spatial and temporal overlap in the harvesting of pilchards by both gannets and the commercial pilchard fishery, but, limited information on the abun- dance or availability of prey stocks inhibits quantification of this level of competition. Nevertheless, it is likely that the fisheries has the potential to negatively affect gannets. 2001 International Council for the Exploration of the Sea