Movements and foraging areas of naïve, recently weaned southern elephant seal pups

Authors


Bernie McConnell, Sea Mammal Research Unit, Gatty Marine Laboratory, University of St Andrews, St Andrews, Fife, KY16 8LB, Scotland. Tel: 01334 463280, Fax: 01334 462632, E-mail: b.mcconnell@smru.ac.uk

Summary

  • 1 Female southern elephant seals (Mirounga leonina L.) expend variable, often large, amounts of their stored body resources on their pups during lactation. There is some evidence that pups with higher weaning masses have a better chance of surviving their first year. But in order to understand what level of maternal investment is required to produce successful pups, we need to understand the behaviour and problems faced by naïve pups before nutritional independence.
  • 2 We used satellite telemetry to track 30 newly weaned pups on their first trip to sea from their natal site at Macquarie Island in 1995 and 1996. Track duration varied from 2 to 179 (mean, 77) days. Seven seals were tracked for the entire duration of their first trip.
  • 3 The movements were grouped into three phases. Phase 1 (mean duration 30 days) was characterized by rapid and directed dispersal from Macquarie Island at daily travel rates of up to 140 km day−1. Phase 2 (mean duration 67 days) consisted of slower travel rates (generally < 20 km day−1) where activity was often centred on localized patches up to 1900 km from Macquarie Island. This phase was sometimes interrupted by bouts of increased travel rate as the seal moved to another patch. Phase 3 (mean duration 42 days) consisted of prolonged increased travel rates as the seals returned to Macquarie or, in one case, Chatham Island.
  • 4 The routes of the tracks to the south-east were very similar. Simulated tracks based on a constant heading of magnetic east, at variable swimming speed, and modified by ocean current vectors produced a pattern similar to, but not identical to, the south-east tracks. The tracks to the west and south were more diverse and meandering.
  • 5 Based on a nearest neighbour analysis, neither sex, year nor weaning mass influenced Phase 1–2 or Phase 2–3 transition locations.
  • 6 Phase 2 tracks were associated in the south-eastern group with the Pacific Antarctic Ridge and in the south-west group, to a lesser extent, with the Indian Antarctic Ridge. The southern limits of Phase 2 tracks in the south-eastern group aligned with the southern Antarctic Circumpolar Circulation front.
  • 7 Using calculated estimates of body composition at weaning and estimates of the rate of utilization of body reserves for the period before animals reach phase 2 of their trip, we estimate that large pups will have reserves remaining to supply their needs whereas pups in the small group are approaching critical limits. However, these estimates are based on several assumptions and extrapolations. More information on body composition of pups at weaning and departure is needed along with behavioural information to clarify the value of maternal expenditure in terms of offspring survival.

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