Steller Sea Lion Decline

In the 1960s, there were between 250,000 and 300,000 Steller sea lions worldwide. The Gulf of Alaska and Aleutian Islands historically contained the largest percentage of this population. In 1977, this area contained 74 percent of the global population, but by 1989 it dropped to 56 percent. In 1990, the Steller sea lion was listed as threatened throughout its range. Later studies of mitochondrial DNA from Steller sea lions suggest that there are at least two stocks, an eastern stock (California through Southeast Alaska) and a western stock (Prince William Sound and areas west). The decline was happening solely to the western stock and in 1997, the listing of the western stock was reclassified as endangered.

To adequately protect the species, scientists needed to understand the reason for the decline. Two main theories were initially put forth. In 1976 and 1977, a warming trend in the Gulf of Alaska and the Bering Sea caused changes in the structure of fish populations for these areas. Small oily fish were greatly reduced and larger but less nutritionally useful fish took their place. The diet of Steller sea lions changed from capelin, herring, eulachon, and sandlance to pollock, cod, and hake. The change would be as if a human diet switched from steak and eggs to popcorn. There was also concern that perhaps fishermen were competing with Steller sea lions for the resources that were available. Regulations to protect the sea lions included: a three-mile buffer zone to restrict boats from approaching rookeries and disturbing sea lions, a 10-mile “no trawl” zone around the Ugamak Island rookeries, and a 20-mile “no trawl” zone around six rookeries during the winter roe pollock fishery. These regulations allowed sea lions more available pollock at a time when it would be most nutritionally important for them, when female sea lions are pregnant and when the pollock was rich with roe.

Since the early 1980s when the decline became known, much research has focused on Steller sea lions. Studies conducted in Kenai Fjords National Park through the Alaska SeaLife Center have examined food availability, environmental change, disease, predation, and pollution. The work of Don Calkins, chief Steller sea lion scientist at the Alaska SeaLife Center, and his colleagues points toward nutritional stress as one of the causes of decline. In essence, reduced prey availability is causing a nutritional problem that decreases the fitness of young animals.

Since 2000, survey data indicates that the decline has slowed or stopped. The western stock has increased approximately 5.5 percent between 2000-2002. These trends continued between 2002-2004. These are the first reported population increases since the 1970s. The most recent published data for the 2006 Steller sea lion stock assessment reports a population estimated at 39,500.