Dr Andrew Trites
Behaviour of lactating stellar sea lions (eumetopias jubatus) during breeding season : a comparison between a declining and stable population in Alaska.
Female attendance patterns and activity budgets of Alaskan Steller sea lions (ieumetopias jubatus) were compared at two sites using scan sampling over two summer breeding seasons in 1994 and 1995 at Sugarloaf Island (a declining population) and Lowrie Island (a stable population). The goal was to document female behaviour and to determine whether there were behavioural differences between the two sites that were consistent with the hypothesis that Steller sea lions in the area of population decline were foodlimited. The perinatal period (time from birth of pup to the mother’s first feeding trip) averaged 10.1 days in the area of population decline compared to 8.0 days in the stable area, counter to initial predictions.
The first shore visit following the perinatal period was significantly longer in the area of population decline compared to the stable population. Females from both populations exhibited a diel haul out pattern where the majority of returns and departures to and from the rookery occurred between 1800 – 0600 hours. Similarly, the mean length of female foraging trips at both populations increased as their pups grew older, whereas shore visits became shorter. Foraging trips were significantly shorter in the area of population decline, again countering initial predictions. The mean length of maternal foraging trips in the area of population decline was 19.0 hours compared to 25.6 hours for the stable population. In contrast, shore visits and the perinatal period were significantly longer in the area of decline, again countering initial predictions. The mean length of shore visits for the declining population was 26.9 hours compared to 22.6 hours where the population was stable.
The average foraging-attendance cycle for both populations was 47.1 hours. Maternal attendance patterns responded in a similar way between years. Activity budgets for the proportion of time spent at sea during daylight observations were consistent with the maternal attendance results. Lactating females in the area of population decline spent less time foraging at sea (35.9 %) than females from the stable population (46.4 %). More time was spent resting ashore in the area of population decline (49.6 %) compared to the stable population (38.9 %). Females from the area of population decline spent more time suckling their pups and were twice as aggressive compared to the females from the stable population.
Overall, lactating females from both populations spent a consistent 85 % of their time foraging at sea and resting onshore. Mothers from both populations spent an average of 36.7 % of their total time with pups. A comparison of female Steller sea lions to other female otariid and phocid species showed that the partitioning of activities is related to differences in lactation strategies, social organization, predation, or thermoregulation. Behavioural observations of maternal attendance patterns and activity budgets are not consistent with the hypothesis that Steller sea lions at the declining site in the Gulf of Alaska have greater difficulty obtaining prey compared to the stable population in southeast Alaska.
Behaviour of lactating stellar sea lions (eumetopias jubatus) during breeding season : a comparison between a declining and stable population in Alaska. 1999. Milette, L.L.. Zoology. University of British Columbia, Vancouver. 67 pages (PDF)
Maternal attendance patterns of lactating Steller sea lions (Eumetopias jubatus) from a stable and a declining population in Alaska. Milette, L.L. and A.W. Trites. 2003. Canadian Journal of Zoology 81:340-348. (PDF)