From Milk to Fish: The weaning of Steller sea lions in Alaska

Motherhood is a risky venture for Steller sea lions. The birthing, nursing and weaning of a pup requires an almost perfect set of conditions: a stable ocean climate, an abundant food supply and the good fortune to elude predators on land and at sea. Under the right circumstances, a mother’s hard work pays off and her pup survives its first year of life. This critical graduation phase, when the nursing pup begins to forage for itself, is known as the ‘age at weaning’.

Scientists do not know the exact age at which most Steller sea lions wean along Alaska’s unpredictable shores, but a new study seeks to fill this information gap and to provide a clearer understanding of the causes behind the decline of western Alaska’s Steller sea lions. Newly weaned pups are believed to be that age group that is most vulnerable to shortages in prey, which can lead to a physiological condition called nutritional stress. For much of the 1990s, researchers blamed nutritional stress for the widespread starvation of immature pups following weaning in western Alaska. The nutritional stress hypothesis remains a leading theory explaining population declines in this region.

Could an apparent absence of young be related to a critical time of year, such as winter, when young animals might have difficulty finding food? Are longer foraging trips an indicator of nutritional stress? To answer these and other questions, a team of researchers conducted a series of observations over four years (1995–1998) at four haulout sites across Alaska, surveying both stable and declining Steller sea lion populations. The results obtained by Boyd Porter (of the Alaska Department of Fish and Game), Dr. Andrew Trites, Volker Deecke, Andrea Coombs, Michelle Marcotte and David Rosen (each of the University of British Columbia) were published in the journal Aquatic Mammals.

Meaning in Weaning

Huddled for countless hours behind camouflaged blinds at each haulout site, the researchers surveyed the seasonal patterns of attendance (i.e., time on shore nursing vs. time at sea foraging) of mature females with pups (0–12 months) and yearlings (13–24 months). Yearlings appeared to nurse less as winter turned to spring and summer, which suggest a possible gradual weaning process in which the yearlings began to forage more and more frequently.

The study also found that most Steller sea lions weaned shortly before their first or second birthday and that weaning almost always appeared to begin in spring (April to May), before the start of the following breeding season. This result was none too surprising: each pup represents a major investment of time and energy to the mother and, the authors write, “It would not make evolutionary sense for a mother to wean her pup at a time of year that is not optimal for its survival.” Weaning a pup in summer just before the next breeding season would also allow the mother to return to the rookery in time to give birth and mate.

The researchers noted that males tended to wean later than females, and observed some pups nursing for as long as three years. This apparent ‘plasticity’ in the timing of weaning suggests that females in nutritionally stressed populations, such as those in western Alaska, may nurse their pups for an additional one or two years to enhance the pups’ chances of survival. But in so doing, these females also lose the opportunity to breed that year.

The researchers suggest that shifts in the timing of weaning may be a natural mechanism for population control with high proportions of pups suckling well beyond the first year indicating a population approaching its carrying capacity. In contrast, pups in a healthy and growing population might wean at just one year, enabling their mothers to breed again that same year. In the Gulf of Alaska and Bering Sea, young sea lions require a lot of energy to grow and may require their mothers to convert energy-poor pollock into energy-rich milk.

Seasonal Patterns

Of particular interest to the researchers was the way in which foraging times changed over the course of a year. Would sea lions have a harder time finding prey during winter than during summer? Could such a pattern lead to nutritional stress in areas of decline? The results were surprising.

“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,” the authors write. “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.”

It seems that the incidence of longer foraging trips in winter does not appear to foreshadow nutritional stress: this trend was observed in both stable and declining populations. Rather, it confirms the view that lactating sea lions make a greater investment in their young during winter than during spring or summer.

Despite the study’s insightful results, the logistics of conducting field research over an Alaskan winter made it difficult to collect consistent data at each haulout site, during each season and in each year.  The researchers therefore recognized the need to conduct year-round observations at just one or two locations. One of the study’s authors, Michelle Marcotte, embarked on just that: in September 2005 she completed field work in her year-long ‘Steller Watch’ study at a haulout in southeast Alaska and is currently analyzing over 12 consecutive months of data.

The combined results of these studies are certain to provide valuable new information about weaning among young Steller sea lions, and may hold clues to solving the mystery of western Alaska’s declining populations.

17 May 2006

Publications:
Insights into the Timing of Weaning and the Attendance Patterns of Lactating Steller Sea Lions (Eumetopias jubatus) in Alaska During Winter, Spring and Summer. Trites, A.W., B.P. Porter, V.B. Deecke, A.P. Coombs, M.L. Marcotte and D.A.S. Rosen. 2006. Aquatic Mammals 32:85-97. abstract Behavioral observations of lactating Steller sea lions (Eumetopias jubatus) and their offspring were recorded at 4 haulout sites in Alaska to determine: 1) whether sea lions wean during winter while they are 7-9 months 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 over 4 years (1995-1998) during 3 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 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. However, lactating Steller sea lions in both regions made longer foraging trips in winter than they did in spring and summer. These changes in foraging patterns between 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.