Unravelling the mysteries of a mesopelagic diet: a large apex predator specializes on small prey

To gain insight into the foraging behaviour of deep diving seals, we developed a long-term jaw-motion recorder, which successfully measured the feeding attempts of four post-breeding female northern elephant seals for 5568days during migration in the north-east Pacific Ocean. Using the jaw-motion recorders in conjunction with satellite tracking data, we first reveal the three-dimensional fine-scale distribution of deep foraging activity in the north-east Pacific Ocean. A large number of jaw-motion events (2381758766 during 29254178 dives, per seal) were observed with diel patterns suggesting their dependency on small mesopelagic prey. Calculations using at-sea field metabolic-rate and the photographs concurrently obtained by the head-mounted camera indicated feeding on small mesopelagic prey (1020g) including lantern fish (F. Myctophidae). The foraging behaviour of the northern elephant seal contrasts with echolocating toothed whales, which make fewer feeding attempts, suggesting the whales forage more selectively. We hypothesize that the continuous diving mode exhibited by this seal could be attributed to their reliance on small prey and their less efficient passive sensors' for prey search, that is, their vision or whiskers to detect prey.