February 28, 2024

Insights into the Timing of Weaning and the Attendance Patterns of Lactating Steller Sea Lions (Eumetopias jubatus) in Alaska During Winter, Spring, and Summer


Andrew W. Trites, Boyd P. Porter, Volker B. Deecke, Andrea P. Coombs, Michelle L. Marcotte, and David A.S. Rosen


Abstract: Behavioral observations of lactating Steller sea lions (Eumetopias jubatus) and their offspring were recorded at four haulout sites in Alaska to determine (1) whether sea lions wean during winter while they are 7 to 9 mo old and (2) whether sea lions using sites in the Gulf of Alaska (the declining endangered population) made longer foraging trips than sea lions in southeast Alaska (where the population appeared larger and healthier). Longer foraging trips are commonly thought to be an indicator of nutritional stress. Eight sets of behavioral observations were made using focal and scan-sampling techniques at haulouts from 1995 to 1998 during three seasons (winter, spring, and summer). Counter to expectations, we found no significant differences between haulout populations in the time that lactating Steller sea lions spent at sea or on shore. This suggests that lactating sea lions did not have more difficulty capturing prey from winter through summer in the area of decline compared to where sea lion numbers increased. Lactating Steller sea lions in both regions did make longer foraging trips in winter than they did in spring and summer. These changes in foraging patterns among seasons were consistent among all years and sites. The proportion of time that immature Steller sea lions suckled declined through the spring to early summer, suggesting that sea lions began supplementing their milk diet with solid food in the spring. We did not observe any sea lions weaning during winter; rather, most appeared to wean at the start of the breeding season when they were 1 or 2 y old. Sea lions observed in southeast Alaska during the late 1990s while population growth was slowing suggest that most males weaned at 2 y and that about 50% of females weaned at 1 y and the remainder at 2 y.

Key Words: Steller sea lion, Eumetopias jubatus, wean, forage, nutrition stress, Alaska

Document Type: Research article

DOI: 10.1578/AM.32.1.2006.85

Page Numbers: 85-97

Info SKU: Vol__32__Iss__1__Trites Category: