Inter-decadal patterns of population and dietary change in sea otters at Amchitka Island, Alaska

After having been hunted to near-extinction in the Pacific maritime fur tra de, the sea otter population at Amchitka Island, Alaska increased from very low numbers in the early 1900s to near equilibrium density by the 1940s. T he population persisted at or near equilibrium through the 1980s, but decli ned sharply in the 1990s in apparent response to increased killer whale pre dation. Sea otter diet and foraging behavior were studied at Amchitka from August 1992 to March 1994 and the data compared with similar information ob tained during several earlier periods. In contrast with dietary patterns in the 1960s and 1970s, when the sea otter population was at or near equilibr ium density and kelp-forest fishes were the dietary mainstay, these fishes were rarely eaten in the 1990s. Benthic invertebrates, particularly sea urc hins, dominated the otter's diet from early summer to midwinter, then decre ased in importance during late winter and spring when numerous Pacific smoo th lumpsuckers (a large and easily captured oceanic fish) were eaten. The o ccurrence of spawning lumpsuckers in coastal waters apparently is episodic on a scale of years to decades. The otters' recent dietary shift away from kelp-forest fishes is probably a response to the increased availability of lumpsuckers and sea urchins (both high-preference prey). Additionally, incr eased urchin densities have reduced kelp beds, thus further reducing the av ailability of kelp-forest fishes. Our findings suggest that dietary pat ter ns reflect changes in population status and show how an ecosystem normally under top-down control and limited by coastal zone processes can be signifi cantly perturbed by exogenous events.