Foraging ecology of northern elephant seals

Sexual segregation in foraging is predicted from the great size disparity o f male and female northern elephant seals, Mirounga angustirostris. Our aim was to test this prediction by measuring diving and foraging behavior, for aging locations, and distribution of the sexes during biannual migrations i n the northeastern Pacific Ocean. Daily movements of 27 adult males and 20 adult females, during 56 migrations from Ano Nuevo, California, USA, were d etermined by Argos satellite telemetry via head-mounted platform transmitte r terminals. Diving records were obtained with archival time-depth-speed re corders attached to the backs of seals that were recovered when the seals r eturned to the rookery. Pronounced sex differences were found in foraging l ocation and foraging pattern, as reflected by horizontal transit speed and diving behavior. Males moved directly north or northwest at a mean speed of 90 +/- 27 km/d to focal foraging areas along the continental margin rangin g from coastal Oregon (534 km away) to the western Aleutian Islands (4775 k m away). Males remained in these areas (mean size = 7892 km(2)) for 21-84% of their 4-mo stays at sea. The predominance of flat-bottom dives in these areas suggests concentrated feeding on benthic prey. Migration distance and estimated mass gain were positively correlated with male size, and individ ual males returned to the same area to forage on subsequent migrations. In contrast, females ranged across a wider area of the northeastern Pacific, f rom 38 degrees to 60 degrees N and from the coast to 172.5 degrees E. Focal foraging areas, indicated by a reduction in swim speed to <0.4 m/s, were d istributed over deep water along the migratory path, with females remaining on them a mean of 3.5 d before moving to another one. Jagged-bottom dives that tracked the deep scattering layer prevailed in these areas, suggesting that females were feeding on pelagic prey in the water column. Females too k roughly similar initial paths in subsequent migrations, but large deviati ons from the previous route were observed. We conclude that there is habita t segregation between the sexes. Females range widely over deep water, appa rently foraging on patchily distributed, vertically migrating, pelagic prey , whereas males forage along the continental margin at the distal end of th eir migration in a manner consistent with feeding on benthic prey.