Diet of Cuvier's beaked whales Ziphius cavirostris from the North Pacific and a comparison with their diet world-wide

Cuvier's beaked whales Ziphius cavirostris are distributed world-wide and are recognized as vulnerable to anthropogenic noise. They are the most common cetacean to strand in temporal and spatial proximity to navy sonars. Cuvier's beaked whales are known for their extreme diving capabilities, but diet information, fundamental to understanding foraging at depth, is limited from most regions. We report on 11 441 prey items from stomach contents of 16 stranded or bycaught specimens collected between 1976 and 2016 across the North Pacific. Overall diet was composed of cephalopods, fish, and crustaceans, but was dominated by cephalopods. Thirty-seven cephalopod species representing 16 families contributed 98.0% by number and 87.7% by mass. The families Gonatidae (26.4% by number; 40.4% by mass), Octopoteuthidae (27.0% by number; 20.2% by mass) and Cranchiidae (27.2% by number; 10.7% by mass) were dominant. The majority of prey items (7997) were from an adult male stranded in California (USA) which contained 20 species from 10 families of cephalopods and fishes. Regional variation was suggested by a higher incidence of crustaceans from whales in the western Pacific, and fishes in the eastern Pacific. Our results combined with data in the literature reveal that world-wide, the most important cephalopod families in the diet are Cranchiidae, Gonatidae, Histioteuthidae, Octopoteuthidae, Ommastrephidae, Onychoteuthidae, Pholidoteuthidae, and Mastigoteuthidae, with Cranchiidae comprising important prey in all locations. While Gonatidae, Octopoteuthidae, and Cranchiidae are the dominant prey in the North Pacific, Histioteuthidae and Cranchiidae are most important in the North Atlantic. Knowledge of diet composition can be used to understand how whales utilize their habitat, in the calculation of nutritional requirements, and may also help to define the locations of important foraging grounds.