ENERGY-EXPENDITURE AND FOOD-CONSUMPTION BY BREEDING CAPE GANNETS MORUS-CAPENSIS

Gannets (genus Morus) are piscivorous seabirds with a breeding distribution restricted to the cool to temperate waters of the mid latitudes. Field metabolic rates of breeding Cape gannets M. capensis and their chicks were measured using doubly labelled water. Energy expenditures of adults incubating or brooding chicks at the nest were 2090 kJ d-1 (24.2 W, 2.9 x basal metabolic rate, BMR). Metabolic rates of adults at sea were 4670 kJ d-1 (54.1 W, 6.5 x BMR). Overall field metabolic rates (FMRs), assuming birds spend half their time on land and half at sea, were 3380 kJ d-1 (39.1 W, 4.7 x BMR). Relative FMRs of adult birds were high compared with those of most other volant seabirds and approach those of high latitude seabirds that use mainly flapping flight, including the closely related northern gannet M. bassanus. Such high values probably reflect the high energy cost of sustained flapping flight and the plunge-diving foraging of gannets while at sea. Food consumption rates, calculated on the basis of the chemical composition of Cape anchovy Engraulis capensis and energy expenditure determined from CO2 production, averaged 691 g d-1 for incubating or brooding adults and 521 g d-1 for chicks. A pair of gannets successfully raising a chick would consume 246 kg of fish over the course of a breeding season. Total annual consumption of the gannet population in the southern Benguela upwelling system amounted to no more than 23 400 t. Of this 9000 t was accounted for by the commercially important anchovy. This probably represents no more than 1% of the total adult spawner biomass of this fish in the region.