Acceleration-triggered animal-borne videos show a dominance of fish in the diet of female northern elephant seals

Abstract

Knowledge of the diet of marine mammals is fundamental to understanding their role in marine ecosystems and response to environmental change. Recently, animal-borne video cameras have revealed the diet of marine mammals that make short foraging trips. However, novel approaches that allocate video time to target prey capture events is required to obtain diet information for species that make long foraging trips over great distances. We combined satellite telemetry and depth recorders with newly developed date-/time-, depth- and acceleration-triggered animal-borne video cameras to examine the diet of female northern elephant seals during their foraging migrations across the eastern North Pacific. We obtained 48.2 h of underwater video, from cameras mounted on the head (<i>n</i>=12) and jaw (<i>n</i>=3) of seals. Fish dominated the diet (78% of 697 prey items recorded) across all foraging locations (range: 37-55°N, 122-152°W), diving depths (range: 238-1167 m) and water temperatures (range: 3.2-7.4°C), while squid comprised only 7% of the diet. Identified prey included fish such as myctophids, <i>Merluccius</i> sp. and <i>Icosteus aenigmaticus</i>, and squid such as <i>Histioteuthis</i> sp., <i>Octopoteuthis</i> sp. and <i>Taningia danae</i> Our results corroborate fatty acid analysis, which also found that fish are more important in the diet, and are in contrast to stomach content analyses that found cephalopods to be the most important component of the diet. Our work shows that <i>in situ</i> video observation is a useful method for studying the at-sea diet of long-ranging marine predators.