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Science & the Public

Otters and oil: Problems remain

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In the early days after the 1989 Exxon Valdez spill, some 350 Alaskan sea otters were rescued from oil-laden waters and shipped or airlifted to treatment centers. Cleaning and rehabilitation saved 197; they were then released back into the wild. These were the lucky ones. Overall, an estimated one-in-five of Prince William Sound’s 14,000 otters died from spill-related poisonings.

Twenty years later, otter populations throughout most of the Sound, especially its western reaches, have largely bounced back, notes James Bodkin, a wildlife scientist with the U.S. Geological Survey in Anchorage. But in areas immediately downstream from where the oil tanker ran aground, otters aren’t faring nearly as well.

Take Knight Island. Currents and foul weather carried the nearly 11 million gallons of crude oil that spilled from the Exxon Valdez along a 460-mile swath of water and beaches. The northern end of Knight Island was only about 35 miles away from where the tanker ran aground and directly in the path of the oil.

Sea otters long thrived around northern Knight Island, Bodkin says. But following the spill, “We took out 154.” Most were dead; those that weren’t were dispatched to rehabilitation centers, he recalls.

Once the island’s oil-blanketed beaches were scoured clean, biologists had assumed sea otters would appreciate the renovated real estate and move in. Yet today, only 100 otters cavort around northern Knight Island — and why their numbers are still depressed remains a puzzle.

But Bodkin and many others suspect residual oil and the animals’ foraging behavior both contribute.

At high tide, otters move in to the intertidal zone to dine. In a series of several dives, the animals excavate a shallow pit. By moving rocks, sand, and cobbles they unearth sediment-buried clams. They love clams.

However, thin layers of oil lay just below the sediment surface on some of these intertidal beaches, including those on northern Knight Island. Digging into the surface can break through to the oil, which tends to still be fresh — and, research has shown, surprisingly toxic. If otters hit oil, they’re invariably going to lick it off.

Such periodic, chronic ingestion of oil hasn’t been witnessed. At least not directly. But Bodkin says there is circumstantial evidence to suggest it occurs.

“We do see pits that they’ve dug on beaches where lingering oil occurs,” he told me. “We also have evidence of likely exposure,” he says — elevations in certain cytochrome P450 enzymes that vertebrates use to break down hydrocarbons. These elevated liver enzymes also show up in birds and fish that frequent the same intertidal zone.

“So when you ask about whether we see animals getting exposed — well, we see it chemically,” he says. This constitutes “strong evidence that these animals are in fact being exposed.”

Each exposure would be small. But their frequency could be high. Bodkin’s group has monitored the behavior of sea otters in the intertidal zone. On average, an otter will make 24 dives a day to feed in the intertidal sediment. “If we assume it takes three dives to make a pit,” Bodkin says, “they’re digging about eight intertidal pits a day.”

That may not sound like much, he concedes, but it you look at the population on this small area of northern Knight Island, “they’re digging some 200,000 intertidal pits a year. Over the past 20 years, the average population of otters here — 66 animals — would probably have together excavated more than four million pits.”

Demographic data also support the idea of a chronic poisoning of the otters, notes environmental chemist Jeffrey Short, who has studied spill effects in Prince William Sound since the accident — initially for the National Oceanic and Atmospheric Administration in Juneau, and since December as the Pacific science director for Oceana, a marine-conservation group.

Every spring, Short says, federal biologists walk the beaches of Prince William Sound scouting for carcasses of sea otters that died over the winter. Necropsies will indicate the age of each victim. “In healthy sea-otter populations, almost all of the mortality is in the very young or very old. But in the spill-impact areas, the mortality has been principally in prime-aged individuals.” And not just in the early years following the spill, but until maybe two or three years ago.

Data funded byExxonMobil disputes the pit poisoning of otters. In a Sept. 5, 2007, paper in Environmental Science & Technology, Paul Boehm of Exponent Inc. in Maynard, Mass., and his colleagues report that nearly all of the subsurface intertidal oil they identified on northern Knight Island was higher up the beach than otters had been clamming. Indeed, this team reported, the oil it found was well upland from clam habitat.

Michel Boufadel of Temple University has also been studying subsurface intertidal oil deposits. His data, due to be published soon, indicate that on five of the six spill-impacted beaches he’s been studying over the last two summers, Exxon Valdez crude residues occur throughout the intertidal zone, including the lower regions where otters feed.

If oil or something else is sickening sea otters, why don’t animals move to healthier neighborhoods? This has puzzled biologists. But research has begun showing that this species shows high fidelity to its home beaches. Relocate otters, Bodkin says, and they’ll swim many hundreds of kilometers to recolonize their home waters.

Next: Exxon Valdez killed future for some killer whales

Citations

Boehm, P.D., et al. 2007. Potential for Sea Otter Exposure to Remnants of Buried Oil fromt he Exxon Valdez Oil Spill. Environmental Science & Technology 41(Sept. 5):6860.

Raloff, J. 1994. An Otter Tragedy: Understanding the sea otter's vulnerability to oil has proved costly to all involved. Science News 143(March 27):200.
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