Are Shark Bites Killing California Sea Otters?

What's killing sea otters? Could it be whales?

Everybody loves sea otters. What's not to love, after all, with their adorable furriness and that whole "lying-on-their-back-and-cracking-open-a-clams-on-a-rock" thing that they do -- one of relatively few instances of tool use in the mammalian world.

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Alas, notwithstanding their almost magical ability to make humans go "awwwww," sea otters have, in different parts of their range and at various times over the past two and a half centuries, taken something of a beating.

Beginning in the mid-eighteenth century, Russian hunters worked east, across the Aleutian islands to mainland Alaska, decimating sea otter populations along the way in order to harvest the animals' pelts. Later, Spanish and British crews did much the same along the coasts of what are now Washington, Oregon, and California. By the time Russia, Britain, Japan, and the United States signed a treaty on the fur seal trade in 1911 that also banned sea otter hunting, the species' prospects looked bleak.

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Since then, however, sea otters have rebounded significantly in much of their range. In two areas, however -- the Aleutian Islands and California -- their populations are struggling.

In parts of the Aleutian chain, for example, some scientists have ascribed a sharp decrease in otter numbers to increased predation by killer whales which have shifted to sea otters as their preferred prey as Steller sea lions have become less available. (Of course humans may have had a hand in shifting this diet as over-fishing of pollock and other groundfish may have caused sea lion population declines -- but this scenario still remains controversial.)

In California, necropsies of some adult sea otters have revealed the presence of toxoplasmosis, caused by a parasite that cats carry and may have entered coastal waters via feline feces flushed through the sewage system. Other sea otters have been shown to have been afflicted by sarcocystis neurona, a parasite that begins its life in opposums. Some otters have also recently been stricken by toxic algae.

But researchers say that these causes alone do not explain the number of dead sea otters that have been found along the California coast in recent years. According to the United States Geological Survey, 237 dead otters were found in 2008;  232 in 2009; and as many as 304 last year, approximately 11 percent of the known population in the area.

According to Tim Tinker of USGS, the most likely cause of the increase in sea otter carcasses is sharks. Tinker told the San Francisco Chronicle that seal and sea lion populations have been increasing and spreading south; predatory sharks, especially great whites, have been heading south with them, bringing them into the range of sea otters. The sharks don't eat the otters, he pointed out, but bite them once, find them unsatisfying, and move on.

From the shark's perspective, the encounter is no big deal, but one shark bite is enough to finish off an otter, either directly or through infection.

"The otter population is already so small that a relatively small number of lethal bites is having, we believe, a pretty major impact on the sea otter population," he said.

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Photo by Mike Baird, via Wikimedia Commons