Determining food requirements in marine top predators: a comparison of three independent techniques in Great Cormorants, Phalacrocorax carbo carbo

Assessment of food requirements is a key feature in the evaluation of the e cological status of the marine megafauna. However, this remains technically difficult because prey intake by marine top predators occurs mainly under water, out of sight. In this paper, we compare three independent methods cu rrently available for use in quantitative dietary studies: (1) time-energy budget; (2) stomach-temperature measurements; and (3) automatic weighing. T o this end, concurrent measurements were performed on Great Cormorants (Pha lacrocorax carbo carbo) breeding in Normandy. According to the time-energy budget method, breeding males required 690 g of fish while incubating, 1050 g when rearing small chicks, and 1350 g when rearing large chicks; corresp onding values for breeding females were 500, 760, and 970 g. These measurem ents are similar to estimates derived from automatic weighing data, which g ave a mean food intake of 540 and 390 g for incubating males and females, 1 150 and 830 g for those tending small chicks, and 1410 and 1010 g for those tending large ones, respectively. Stomach-temperature measurements, which can only be performed for birds raising small chicks, were lower (640 g fis h in males and 450 g in females) than those obtained using the other two me thods. We compare these results with former estimates obtained at the same study site and for other Great Cormorant subspecies and discuss the relativ e accuracies of the three techniques. Finally, we stress that better assess ment of the ecological status of marine top predators requires further tech nical improvements and additional investigations outside of the reproductiv e phase.