Surveying Sea Lion Diets in Southeast Alaska

Steller sea lion populations in western Alaska mysteriously and dramatically declined through the 1980s and 1990s.  During that same time, sea lion populations in Southeast Alaska increased. Although a number of factors may have contributed to these diverging population trends, a likely explanation may lie in dietary differences between these populations.

It may be, for example, that Steller sea lions in Southeast Alaska ate a wider range of prey and therefore had a more diverse diet. Or, perhaps their diet did not include a significant amount of low-calorie prey such as pollock. By collecting and analyzing fecal (scat) samples obtained from wild sea lions, scientists can determine the type and frequency of prey in the sea lion diet.

In a new dietary study, researchers sought to determine the diet of Steller sea lions in Southeast Alaska. They analyzed some 1,494 scat samples collected between 1993 and 1999 at 3 major breeding sites (rookeries) and 12 nonbreeding sites (haulouts). The study was recently published in the journal Fishery Bulletin by Andrew Trites and Arliss Winship (University of British Columbia) and Donald Calkins (formerly with the Alaska Dept. of Fish and Game, and now at the Alaska SeaLife Center).

Diversity in Diet
The authors report that a comparison of their dietary data from Southeast Alaska with dietary data collected from other regions of Alaska indicated that Steller sea lions consumed similar schooling species, most notably pollock, salmon, herring, sandlance, rockfish, and squid. However, in terms of frequency of occurrence, there were significant dietary differences between Southeast Alaska during the 1990s and regions where sea lions declined.

The researchers found that the sea lion diet was more diverse in Southeast Alaska during the 1990s than in any other region of Alaska, and may have had a higher energy content overall. They also discovered that the sea lions consumed the most diverse range of prey categories during the summer breeding season, and the least diverse during the fall.

These findings support a previous study that discovered a positive relationship between the diversity of summer sea lion diets and the rate of population change in areas of decline in the early 1990s. That is, the greater the diet diversity, the slower the rate of population decline. The authors suggest that sea lions enjoying a diverse, energy-rich diet may be less sensitive to the effects of ocean climate or human fisheries on any single prey population.

Another advantage of a diverse, energy rich diet is that it should be easier to find enough prey to meet daily energy requirements. Sea lions with a less diverse or less energy-rich diet would have to forage longer to consume enough food to meet their energy needs. This in turn might increase their risk of encountering killer whales or sharks, contributing to a further decline in the population.

Importance of Pollock
The scats collected during the 1990s revealed that pollock were the most frequently occurring food item at haulouts during the nonbreeding season, and were second in importance only to salmon at rookeries during the summer breeding season.

“Since the mid 1970s, pollock has been one of the most dominant species in the Bering Sea and Gulf of Alaska ecosystems,” the authors write. “Unfortunately, little is known about the distribution and relative importance of pollock in the Southeast Alaska ecosystem. It is unclear what role pollock and Atka mackerel stocks alone have played in the different trajectories of Steller sea lion populations.”

The authors note that pollock is not as dominant in the sea lion diet in Southeast Alaska as it is in other regions, and is usually part of a diverse diet that includes energy-rich prey such as herring or salmon. This does not appear to be the case in areas of decline in the Gulf of Alaska, where pollock has dominated a much less diverse diet.

A shift in the quantity, quality, or availability of key prey has dramatic consequences for sea lions, which have a daily energy requirement for swimming, foraging, nursing, and breeding. Understanding dietary differences between sea lion populations in western Alaska and Southeast Alaska may help to explain why one population declined while the other increased.

This study provides valuable insights into the composition of Steller sea lion diets in Southeast Alaska. In addition, it opens the door for future research into the relative importance of pollock in Southeast Alaska, and emphasizes the ongoing need to continue to assess and monitor sea lion diet as an indicator of population change.

September 17, 2007

Publication:
Diets of Steller sea lions (Eumetopias jubatus) in Southeast Alaska from 1993-1999. Trites, A.W., D.G Calkins and A.J. Winship. 2007. Fishery Bulletin 105:234-248. abstract Diet of Steller sea lions (Eumetopias jubatus) was determined from 1494 scats (feces) collected at breeding (rookeries) and non-breeding (haulout) sites in Southeast Alaska from 1993 to 1999. The most common prey of 61 species identified were walleye pollock (Theragra chalcogramma), Pacific herring (Clupea pallasii), Pacific sand lance (Ammodytes hexapterus), Pacific salmon (Salmonidae), arrowtooth flounder (Atheresthes stomias), rockfish (Sebastes spp.), skates (Rajidae), and cephalopods (squid and octopus). Sea lion diets at the three Southeast Alaska rookeries differed significantly from one another. Steller sea lions consumed the most diverse range of prey categories during summer, and the least diverse during fall. Diet was more diverse in Southeast Alaska during the 1990s than in any other region of Alaska (Gulf of Alaska and Aleutian Islands). Dietary differences between increasing and declining populations of sea lions in Alaska correlate with rates of population change, and add credence to the view that diet may have played a role in the decline of sea lions in the Gulf of Alaska and Aleutian Islands.