Steller sea lion populations in the western Gulf of Alaska and Aleutian Islands declined nearly 80% from the late 1970s to the early 1990s. One possible reason suggested for the decline is reduced food availability, in particular a reduction in the abundance of pollock. As a result, the valuable commercial fishery for pollock was implicated in the Steller sea lion decline. Researchers at PWSSC began to study this possibility in 2000. The results showed that herring, rather than pollock, was the preferred food for Steller sea lions in Prince William Sound over the extended Alaskan winter period. Major differences in winter habitat between pollock and herring explain the preference. During winter, herring are distributed in protected bays and inlets in the upper 50 meters, while pollock are found in open, deep basins at depth over 200 m. The herring school densities are also much higher than pollock densities, so the ease of capture for the air-breathing marine mammals is far better for herring.

Steller sea lion populations in the western Gulf of Alaska and Aleutian Islands declined nearly 80% from the late 1970s to the early 1990s. One possible reason suggested for the decline is reduced food availability, in particular a reduction in the abundance of pollock. As a result, the valuable commercial fishery for pollock was implicated in the Steller sea lion decline. Researchers at PWSSC began to study this possibility in 2000. The results showed that herring, rather than pollock, was the preferred food for Steller sea lions in Prince William Sound over the extended Alaskan winter period. Major differences in winter habitat between pollock and herring explain the preference. During winter, herring are distributed in protected bays and inlets in the upper 50 meters, while pollock are found in open, deep basins at depth over 200 m. The herring school densities are also much higher than pollock densities, so the ease of capture for the air-breathing marine mammals is far better for herring.

The results of the PWSSC research also suggested that Exxon Valdez Oil Spill may have contributed to the Steller sea lion decline as a result of the collapse of the Prince William Sound herring population that followed the oil spill. The decline of Steller sea lion numbers between 1989 and 1999 was a function of distance from Prince William Sound (SSL Fig 1).

PWSSC researchers also found that the Steller sea lions forage primarily at night, and remain close to shore during day. High-technology infra-red sensors were used to study the night time foraging activity. The tendency of the sea lions to stay close to the beach during day may result from the potential predation by Orca whales.

Steller sea lion foraging on herring is particularly intense in late winter/early spring when the herring group together to spawn. Over 300 Steller sea lions have been observed feeding on the herring under these circumstances.

Scientists at PWSSC also work on herring with the Alaska Department of Fish and Game in Kodiak, Alaska. The objective is to compare herring populations and the role of marine mammal predation between the two locations.