Prey Consumption Steller Sea Lions- Marine Mammal Research Consortium

Prey consumption by Steller sea lions in Alaska

Knowing how much prey are consumed is one of the keys to understanding the role that Steller sea lions play in their ecosystem. It is also important for determining the level of competition between Steller sea lions and commercial fisheries. In Alaska, Steller sea lions have been shown to prey on a variety of species of fish. Arliss Winship and Andrew Trites combined this information on the diets of Steller sea lions in Alaska with a model that estimates the amount of food that Steller sea lions require (link to "How much food do Steller sea lions need?" What's New article) to predict the amount of prey that Steller sea lions consumed in Alaska.

The species of fish that Steller sea lions consume have been shown to differ among regions of Alaska (see Diet Changes). Researchers at the US National Marine Fisheries Service found that Steller sea lions in the Aleutian Islands and the western Gulf of Alaska consumed primarily Atka mackerel and walleye pollock. Our research (link to diet work) has shown that Steller sea lions in southeast Alaska consume a larger variety of species of fish in the summer including more herring and salmon which are fattier than fish like walleye pollock. However, walleye pollock is an important part of the diet in southeast Alaska during the rest of the year.

Differences in the diets of Steller sea lions resulted in differences in the mass of prey that Steller sea lions consumed at different times of the year and in different regions of Alaska. Our model predicted that an individual Steller sea lion needed to consume a smaller amount of prey in southeast Alaska during the summer to meet its energy requirements than it did during other seasons because it was preying on high-energy species. Individual Steller sea lions in southeast Alaska also needed to consume a smaller amount of prey than sea lions in other regions of Alaska during summer.

The estimated total number of fish consumed by Steller sea lions differed among regions of Alaska. More fish were consumed in southeast Alaska, where there was a larger number of Steller sea lions than in other regions of Alaska. When all of Alaska was considered, our model predicted that the two fish species consumed in the largest quantities were Atka mackerel and walleye pollock. Steller sea lion predation on Atka mackerel may account for a large percentage of the natural mortality of Atka mackerel. The model also suggests that Steller sea lion predation accounted for a small percentage of the natural mortality of walleye pollock.

Further details about this study are contained in Winship and Trites (2003).