Sea Otters’ Deaths Cloaked in Mystery

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 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 along the West Coast but 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 in their prime reproductive years, from about 3 to 9 years old. They live up to two miles offshore, mostly in forests of kelp that are home to many species.

Finding out why they’re dying could give researchers the 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, is studying the problem and preparing a recovery plan.

“They are an important sentinel of what’s happening in near-shore marine health,” Miller said. “It could have broader implications for other marine mammals and for us.”

And without otters, the marine ecosystem can change significantly. For example, sea urchins are a favorite otter food, but when the 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.

Each dead otter found is examined, 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. Miller looked for gunshot wounds, shark bites, research tags, any visible marks. No clues there.

Miller took samples from various organs, tissues and fluids, carefully labeling them for future reference.

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 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% 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. It 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 believe the dead otters they retrieve 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 Carmel south to Estero Bay can’t be recovered because the shoreline is too rocky.

The otter on the table had very wet lungs. “The immediate cause of death is drowning, but I don’t know why,” Miller said.

For the seagoing mammals, drowning is frequent, but it’s often triggered by something else, such as being hit by a boat.

Miller suspected it suffered from domoic acid poisoning. She sent a urine sample to a lab in Davis, one of three in the United States that test for domoic acid and 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 the coast last year. But weeks after the examination, results came back negative. Other tests were inconclusive.

For now, the male on the table joins the many otters whose deaths are a mystery.