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Puzzled scientists probe cause of California otter deaths
22nd March 2003
The Mercury News
SANTA CRUZ, Calif. (AP)
Wildlife pathologist Melissa Miller and her team, clad in rubber gloves, hospital scrubs and big rubber boots, hovered around a stainless steel table where a California sea otter lay dead.
The 4-foot long, 65-pound adult male was found washed up on the rocks three days earlier at Pismo Beach, south of San Luis Obispo, and driven to a laboratory here for a necropsy, one of the numerous ways researchers are trying to figure out what's killing the threatened sea mammals.
California sea otters, with their luxuriously soft pelts, were hunted to near extinction by the early 20th century, but made a dramatic recovery, reaching a recent population high of 2,377 off the California coast in 1995.
Since then, however, their numbers have at best stagnated, and at worst are decreasing, along much of their range, which once extended all along the West Coast, and now is limited to California's central coast.
Despite concerted efforts to figure out what's killing them, it's not entirely clear. Sharks and orcas are among their few predators. Fishing nets have killed otters in the past, and may still, to some extent. Occasionally, one gets shot.
Many of those dying are adults in their prime reproductive years, from about 3 to 9 years old. The mammals live up to two miles offshore, mostly in the forests of kelp that are home to many species.
Finding out why they're dying, including looking at their environment, could give researchers tools to help the otters recover and keep a key stretch of the ocean healthy. The U.S. Fish and Wildlife Service, among other agencies, has been studying the problem intensely and is preparing a recovery plan to be unveiled soon.
``They are an important sentinel of what's happening in nearshore marine health,'' Miller said. ``It could have broader implications for other marine mammals and for us. We eat the same foods; we swim in the same waters.''
And without otters, the marine ecosystem can change significantly. For example, sea urchins are a favorite otter food, but when otters are gone, the urchins are unchecked and decimate kelp forests.
Scientists do what they can to understand the playful marine mammals while they're still alive. They tag some, count them each spring and fall, and study their eating and resting habits. They also study their corpses.
Every dead otter found gets some kind of exam, frequently here at the state Department of Fish and Game's Marine Wildlife Veterinary Care and Research Center, to try to determine the cause of death.
The exam on this male -- about as large as California sea otters come -- took hours. X-rays showed no trauma, just two toes broken a while ago, likely in a fight. Miller looked for gunshot wounds, shark bites, research tags, and any visible marks. No clues there.
Miller took samples from various organs and tissues and fluids, carefully labelling them for future reference. Like others whose corpses were found quickly, this one's hide was to be prepared for museums or schools to use in teaching.
The same parts of every examined otter are taken to build a standard bank of otter parts collected over many years. That gives scientists something to compare and refer to if they discover a new otter affliction. They also take photos for teaching purposes.
For the most part, this otter appeared healthy.
``He's in really good nutritional condition. His coat looks good,'' Miller said, ``what this suggests is he died acutely. If he had a chronic disease, he hid it well.''
Infectious disease seems to be one of the biggest culprits in otter deaths. Disease has always claimed otter lives, but recent research by Miller has shown that a protozoan parasite, Toxoplasma gondii, typically found on land, is more common in water than previously recognized and is finding its way into sea otters.
The parasite causes encephalitis that is killing some otters. More than 60 percent of the dead otters examined are found to have antibodies to this parasite.
Research by veterinarian Christine Kreuder shows that a high number are dying of heart disease, odd for a wildlife population. The heart disease is thought to be caused by a virus that inflames heart tissue, causing scarring that leads to heart failure. It can also happen when the immune system starts attacking itself, which could be triggered by bacteria.
But scientists also think the dead otters they retrieve -- mostly those that have washed up on shore -- are not representative of the entire otter population.
``All we really know is what we see washed up on the beach, and we're suspicious that's biased,'' Estes said. ``There's a concern that animals dying from certain reasons are less likely to be deposited on land.''
An example: Otters that drown in fishing nets and are discarded may sink and be eaten by predators. Also, otters that wash up from roughly Carmel south to Estero Bay can't be recovered because the shoreline is too rocky, leaving a 100-mile-long gap in the research area.
The otter on the examination table had very wet lungs. ``The immediate cause of death is drowning, but I don't know why,'' Miller said.
For the sea-going mammals, drowning is frequent, but it's often triggered by something else, such as being hit by a boat, and being hit by a boat can be caused by something else that makes an otter act funny.
Miller suspected the animal suffered from domoic acid poisoning. She sent a urine sample to a state-run lab in Davis, one of three in the United States that test for domoic acid and numerous other marine biotoxins.
Otters can contract domoic acid by eating shellfish that fed on toxic plankton. The toxin is suspected of killing hundreds of animals, from sea lions to dolphins to sea birds, along California's coast last year. But weeks after the examination, results came back negative for domoic acid poisoning. Other tests were inconclusive.
For now, the male on the table joins the many otters whose deaths are a mystery.
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On the Net:
USGS Western Ecological Research Center: http://www.werc.usgs.gov
California marine wildlife center: http://www.dfg.ca.gov/Ospr/marinewildlife
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