Prey selection of Dali's porpoise Phocoenoides dalli on the continental slope off the Pacific coast of Sanriku in winter

Abstract

We compared the diet of Dali's porpoise Phocoenoides dalli with taxonomic composition of fishes collected by a bottom trawl over continental slopes off Sanriku, northern Japan in winter. Stomach contents analysis revealed the porpoise fed mainly on mesopelagic micronekton, such as myctophids Diaphus theta and firefly squid Watasenia scintillans. Total weight and state of digestion of stomach contents showed that Dall's porpoise fed actively during the daytime. However, the stomach contents showed that gadid fishes, which dominated the bottom trawl samples, did not contribute to the diet. The micronekton consumed were diurnal vertical migrants, which migrate up to the epipelagic layer at night and down to greater depths during the daytime. These suggest that Dali's porpoise dives deeply to forage and feed selectively on mesopelagic micronekton with high energy content at deeper layers during the daytime.