Migratory strategies of juvenile northern fur seals (Callorhinus ursinus): bridging the gap between pups and adults

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.

Movement patterns and activity levels are shaped by the neonatal environment in Antarctic fur seal pups

“…However, it is also possible that an effect of body condition could not be detected in our study because our model assumed that condition was constant between successive measurements, whereas in practice it will vary to some extent from day to day. Despite a number of studies having shown that sex-specific differences in activity, habitat use, foraging, and diving behaviour are established early in life in several pinniped species [13][14][15][16][17]46 , including Antarctic fur seals , our HMM did not uncover any obvious sex differences in activity. This is probably a consequence of the timeframe of our study.…”

“…A tracking study of European shags, for example, has shown that higher rates of juvenile mortality correlate with poor foraging proficiency [10], highlighting the importance of learning and memory for successful recruitment. Studies of several different pinniped species including Antarctic fur seals [11], New Zealand sea lions [12], Steller sea lions [13], northern fur seals [14] and grey seals [15] have also shown that sex-specific differences in foraging and diving behaviour become established early in life, implying that intrinsic factors may outweigh size-related effects associated with early sexual dimorphism on habitat choice.…”

Fluctuations in serum steroid hormone concentrations and body mass during growth and sexual maturation in captive northern fur seals (<i>Callorhinus ursinus</i>)

“…In wild female northern fur seals, age is often considered to be the key factor that determines puberty [15,17,54,57]. However, the results of this study suggest that a BMPB of 20 kg appears to be the threshold for puberty in captive female northern fur seals.…”

“…Females normally deliver a single pup in summer, nurse during the breeding season, and wean in early November. Then, the pups participate in migration for a few years until they become involved in reproductive activities on breeding islands [6,15,17,39,57]. Because northern fur seals have a distinct life history pattern consisting of terrestrial breeding and oceanic migration, the physiological changes associated with their annual reproductive cycle and ontogeny are of particular interest for understanding their environmental adaptations [27].…”

First-Year Survival of Northern Fur Seals (Callorhinus ursinus) Can Be Explained by Pollock (Gadus chalcogrammus) Catches in the Eastern Bering Sea

“…Fur seals established a new rookery in the early 1980s on Bogoslof Island, an active volcano on the southern margin of the Bering Sea about 40 km north of Umnak Island and 210 km southwest of St. George Island (Figure 1). Females that breed on Bogoslof Island and pups that are born there have the same migratory patterns and occupy the same habitats in the late fall, winter, and spring as Pribilof seals [20,24]. Adult females recruit to a harem occupied by a single bull, bear their young within two days of their arrival, are impregnated about 6 days later, and resume foraging at sea the following day [14,15].…”