Feeding ecology of harbour porpoises, common and bottlenose dolphins and sperm whales in the northeast Atlantic
The diet composition of harbour porpoises (Phocoena phocoena), common dolphins (Delphinus delphis), bottlenose dolphins (Tursiops truncatus) and sperm whales (Physeter macrocephalus ) is described from stomach contents of animals stranded in Galicia (NW Spain), Scotland, Holland and Denmark between 1991 and 1996, and some older material (1976 - 1991). The present study also provides the first descriptions of population structure for cetaceans off Galicia. Diets of harbour porpoise in Scotland, Denmark and Holland consisted primarily of inshore demersal fish. There were differences between seasons and areas and to a lesser extent between sexes, age-groups and years. Whiting, sandeels, cod and viviparous blenny were among the most important prey. Common dolphins on the Galician coast took a wide range of inshore and oceanic fish and cephalopods. There were some seasonal differences in diet, but blue whiting, scad and sardine predominated all year round. Bottlenose dolphins off Galicia also took a wide variety of prey with blue whiting and hake being the most important. Sperm whales in the North Sea had been feeding almost exclusively on the oceanic squid Gonatus fabricii, although several other oceanic squid species were also eaten. Many of the prey species eaten by dolphins and porpoises are also the target of commercial fisheries. Dolphins and porpoises were mainly eating smaller size classes of fish than those taken by fisheries. The weight of prey eaten by sperm whale and harbour porpoise populations in the NE Atlantic was estimated from the dietary data and published estimates of population size and individual food requirements. Sperm whales could eat more than 1,000,000 tonnes of the oceanic squid Gonatus fabricii annually. The 95% confidence limits for porpoise consumption of whiting are as wide as 2,600 to almost 30,000 tonnes, depending on the assumptions made. This reflects the high level of uncertainty about population size and structure.