The Alaska Sea Otter's Disturbing Decline
By Robin Meadows

While the recovery of California sea otters has limped along for most of this century, until recently the Alaska otters had been rebounding beautifully. In the central Aleutian Islands, most otter populations had returned to historical levels by 1965. But now Alaska sea otters are declining at alarming rates. The population at Adak Island alone decreased by roughly a quarter each year during the 1990s.

What is causing this abrupt decline in the Alaska sea otter? Biologists have nixed a number of potential reasons. The otters arent running out of foodsea urchins, the Alaskan otters principal prey, have increased eight-fold at Adak Island during the otters decline. The otters arent reproducing poorlyjust as many pups are being born and surviving now as before the decline. The otters arent succumbing to diseases or toxinsthere has been essentially no increase in carcasses found on beaches. And, they arent leaving the central Aleutians for other parts of the region.

The surprising answer appears to be that killer whales are eating huge numbers of sea otters in Alaska, report Jim Estes of the Biological Resources Division of the U.S. Geological Survey and his colleagues in a 1998 paper in the journal Science.While there is no direct proof, Estes and his colleagues base their killer whale theory on solid evidence. First, although killer whales and sea otters have both lived in the Aleutian archipelago for a long time, no one had ever seen a killer whale eat an otter until 1991. Since then, a growing number of killer whales have been seen eating otters.Second, Estes and his colleagues compared sea otter populations in two areas of Adak Island: one with killer whales and the other without. The area with killer whales is Kulak Bay, which is open to the ocean. The area without killer whales is Clam Lagoon, which is inaccessible to whales.

The researchers found that while the sea otter population in whale-free Clam Lagoon did not change between 1993 and 1997, the population in Kulak Bay decreased by 76 percent. To make sure that the Kulak Bay otters werent just swimming into Clam Lagoon to escape the killer whales, Estes and his colleagues tagged otters in each population. They found that the two populations hardly mixed at all and that five times as many tagged otters disappeared from Kulak Bay as from Clam Lagoon (65 percent versus 12 percent).

But is it really possible for killer whales to have eaten enough sea otters to cause the sharp decline in the Aleutian archipelago? Estes and his colleagues analyzed the otter population living between Kiska and Seguam Islands, an area that spans about 400 miles. Before the decline, nearly 53,000 sea otters lived there. Based on a 78-percent drop over six years, the researchers estimated that about 40,000 sea otters were lost. This means that killer whales would have had to eat about 6,800 sea otters per year. Because a single killer whale can easily eat more than 1,800 otters per year, the researchers conclude that it is entirely feasible that the whales are causing the otter decline.

Why, then, would some killer whales suddenly start preying on sea otters? Estes and his colleagues blame the collapse of northwest Pacific populations of the whales preferred prey: Steller sea lions and other pinnipeds. Pinniped numbers have been declining there since the late 1970s, and sea lions sank to their lowest levels in the Aleutians in the late 1980s, which coincides with the onset of the sea otter decline.

And why have pinnipeds been decreasing in the northwest Pacific? The numbers of fish they eat have been decreasing, partly because fisheries have been increasing their efforts in the area, say the researchers.

Heres the story in a nutshell: Fewer fish meant fewer pinnipeds, which forced hungry killer whales to start eating sea otters. And thats not all there is to the story. The effects of the fish decrease have cascaded through the ecosystem. Fewer sea otters led to more sea urchins, which led to the overgrazing of kelp forests. And, ironically, kelp forests are nurseries for baby fish, which means that the decrease in kelp forests could contribute to the decrease in fish, making this a vicious cycle.

So, if Estes and his colleagues are right, then the best way to save sea otters from killer whales is to protect fisheries in the northwest Pacific.

ZooGoer 28(6) 1999. Copyright 1999 Friends of the National Zoo. All rights reserved.

 



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