The Steller sea lion : A declining species

抄録

Steller sea lions (Eumetopias jubatus) are the largest of the sea lion and fur seal subfamily and show a marked difference in size with males two to three times larger than females. Males can be as large as 1,120 kg and grow to 3.25 m; they are about the size of a Kodiak grizzly bear (Ursus arctos). Females average 250 kg and are approximately 3.2 m long. The Steller sea lion breeding range extends across the North Pacific Ocean rim, from the Kuril Islands and Okhotsk Sea, through the Aleutian Islands and Bering Sea, along Alaska's southern coast, and south to central California. Steller sea lions eat a variety of fishes and invertebrates. In Alaska, the principal prey is walleye pollock (Theragra chalcogramma) with Pacific cod (Gadus macrocephalus), Atka mackerel (Pleurogrammus monopterygius), octopus, squid, herring (Clupea harengus), flatfishes, and sculpins also consumed. There were reportedly over 300,000 Steller sea lions in the world in the late 1970s. Since then, the Alaskan sea lion population has plummeted to a small fraction of earlier levels resulting in the species being listed as threatened under the U.S. Endangered Species Act (ESA) in November 1990; the western stock was changed to endangered in 1997. Possible causes for the decline may include redistribution, changed vital rates, pollution, predation, subsistence use, commercial harvest, disease, natural fluctuation, environmental changes, and commercial fishing. The last two are now considered the most probable links to the decline. Steller sea lions may be affected by commercial fishing directly through incidental catch in nets, by entanglement in derelict debris, by shooting, or indirectly through competition for prey, disturbance, or disruption of prey schools. Current research is trying to determine the relationship between commercial fisheries and the decline and to monitor status. Management regimes include restrictions on incidental take, prohibition of shooting sea lions, no trawl buffer zones around some rookeries, and other measures.