Recently, sea otter populations in the Pacific Northwest have been a bright spot in otherwise troubling ecosystem assessments. A handful of sea otters reintroduced back into their historic homes in the 1970s have multiplied—to nearly 750 in Washington and an estimated 2,500 in British Columbia. Even the much-worried-over southern sea otters in California (which were, miraculously, never extirpated) has shown improvement. The California population now boasts over 2,800 animals, the highest number in at least 22 years.

But in Alaska, home to roughly 90 percent of the world’s sea otters, the picture is less rosy. Although populations in southeast Alaska appear to be relatively stable—despite some notable losses such as the Exxon Valdez oil spill—otter numbers are plummeting in southwest Alaska, along the Aleutian chain. In fact, the Center for Biological Diversity is suing the US Department of Interior to list the Aleutian sea otters under the Endangered Species Act. It’s a wonder they’re not already listed: they’ve probably suffered somewhere between 75 and 95 percent losses.

There are several main theories about sea otter woes in the Aleutians, though most people agree that the proximate cause is orca predation: for reasons that are not clear, killer whales have started hunting sea otters. According to one theory, when Bering Sea whale population declined (largely because of hunting), Aleutian orcas turned to sea lions and seals for food; when those marine mammals populations diminished, then killer whales looked to sea otters. Other theories run that orcas have run out of seal and sea lion prey because of depressed fish stocks (due to over-fishing), and changes in ocean temperature that are inimical to plankton (perhaps due to global warming).

In any scenario, the sea otter declines are a worrisome indicator that something is deeply awry in the 1,200 mile sub-arctic ecosystem of the Aleutians. But until we know more about the sea otters—what’s driving them toward extinction—and what’s causing the behavioral shifts in orcas—we cannot know for sure how serious the problem is. But it is quite conceivable that the sinking otters are a symptom of a much deeper ecological sickness affecting the Bering Sea and the North Pacific.