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ANCHORAGE -- Alaska's endangered Steller sea lions feast on herring but ignore more abundant pollock, according to a report in the June 28 issue of the British science journal Nature.
The study is of great interest to Alaska's $700 million pollock industry, which federal regulators have restricted to protect sea lions. Some scientists theorize the Stellers might be losing a competition with fishing boats for food.
Pollock, the abundant, bland, white-fleshed fish targeted in Alaska's most valuable fishery are used to produce fast-food fish portions and a versatile protein paste called surimi.
The study by Gary L. Thomas and Richard E. Thorne of the Prince William Sound Science Center in Cordova combined sonar surveys of Pacific herring and walleye pollock with infrared scanning of foraging sea lions.
The goal was to observe prey abundance and sea lion feeding habits.
According to the brief article in Nature, sea lions were surveyed with sonar and infrared scans during March 2000 in Prince William Sound, including the important herring hangout of Rocky Bay at Montague Island.
"Despite the much greater abundance of pollock, the infrared system revealed that foraging by Steller sea lions was exclusively on herring and was conducted only at night," according to the article. "Foraging activity was intense on dense herring schools. Steller sea lions were often observed swimming side by side in a row of 50 or more individuals along the edges of a school, suggesting that they were herding the herring."
The article said herring tended to stay closer to the surface at night and deeper during the day, while pollock stayed deep all the time. Though Stellers can dive to the pollock, the herring are more accessible at night, and that might explain the foraging behavior of sea lions.
Scientists are not certain whether competition with fishing boats or ocean climate shifts are to blame for the steep decline in Stellers across Western Alaska, including in the Sound. Some suggest pollock are not as nutritious for sea lions as more oily fish such as herring and capelin.
The herring population is at a historic low in Prince William Sound and has not been harvested commercially since 1998, Thorne said Friday in an interview.
Thorne, a senior scientist at the independent, nonprofit center and a University of Washington faculty affiliate, said no fishing industry money was used in the study. Rather, he said, funding came from the Prince William Sound Oil Spill Recovery Institute, which was authorized by the federal Oil Pollution Act of 1990.
Thorne said the study doesn't get the fishing industry off the hook for the sea lion decline. However, it does raise questions about the relative role of pollock in the Stellers' diet and whether curtailing the pollock fishery is the right step.