Abstract
In species exhibiting differential migration by sex and age, understanding what differences exist, and the adaptive reasons for these differences is critical for determining how demographic groups will respond to environmental variability and anthropogenic perturbations. We used satellite-telemetered movement and diving data to investigate differential migration and its ontogeny in a highly migratory North Pacific Ocean predator, the northern fur seal (Callorhinus ursinus; NFS), with a focus on understudied juvenile (1- to 2-year-old) animals. We instrumented 71 juvenile NFS in two years (2006–07 and 2007–08) at three major North American breeding sites and compared their migratory strategies with pups and adults. Although sexual dimorphism is strong in adult NFS, only weak differences in body mass between sexes were found in juveniles, which had similar body mass to pups (~3–4 months). However, unlike widely-dispersed pups, juvenile male and female NFS dispersed in different directions, and used different habitats characterized by distinct hydrography and prey assemblages during migration, similar to breeding adults. Juvenile diving behavior differed only modestly among habitats and between sexes, consistent with weak differences in body mass. Evidence of habitat sexual segregation by juvenile NFS contradicts previous hypotheses that physiological differences predominantly drive the ontogeny of differential migration.
Highlights
Long-distance annual migration is a common phenomenon in both marine and terrestrial species based upon individual adaptation to fluctuating resources[1]
Deployment durations for 68 satellite tags ranged from 27 d to 308 d (Table 1)
Juvenile data were compared with satellite tracking data from 124 adult females, 15 adult males, and 168 pups instrumented on PRB and BG in 12 separate years (Table 2), including 54 pups in 2006 and 18 adult females in 2006–07
Summary
Long-distance annual migration is a common phenomenon in both marine and terrestrial species based upon individual adaptation to fluctuating resources[1]. Differential migration occurs when separate age classes and/or sexes of animals exhibit varying migratory strategies and habitat selection[1,2]. Knowledge of differential migration within a species is key to understanding demographic trends, identifying sex and age-class specific critical habitats and developing effective conservation and management strategies for migratory populations. Adult male NFS from the EPS primarily remain in the Bering Sea and North Pacific Ocean, whereas adult females www.nature.com/scientificreports/. Pups from the EPS migrate throughout the central North Pacific Ocean and Bering Sea, foraging in predominantly offshore pelagic waters[15], whereas pups from SM mostly remain in the California Current during the migration[16]. It is likely that the lower survival of these age groups is related to the extended migratory phase of their life history
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