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UC Davis, state wildife health agency identify troubling trends in sea otter deaths

May 6, 2003

May 6, 2003

A new UC Davis analysis finds that adult sea otters in California in 1998-2001 died in unusually high numbers from newly recognized diseases and in geographic clusters -- all of which suggest that their coastal environment may be so substantially altered that the species could be in jeopardy.

California sea otter on exhibit at the Monterey Bay Aquarium. This animal, Rosa, was stranded as a pup, rescued and raised by the Monterey Bay Aquarium's Sea Otter Research and Conservation program, but was unable to return to the wild. Photo courtesy of Monterey Bay Aquarium/Randy Wilder
The findings are particularly worrisome in the wake of last week's report that a startling 100 southern sea otters have washed up dead on California beaches since January -- 100 deaths in a population that has been falling overall since 1995 and now stands at about 2,000.

The new analysis, done in collaboration with the California Department of Fish and Game, does not include those 100 otters, but its findings may help researchers understand their deaths.

"We are very concerned that the otters are dying so frequently of diseases. This indicates that the ecosystem is very unhealthy," said wildlife epidemiologist Jonna Mazet, director of the UC Davis Wildlife Health Center based at the School of Veterinary Medicine and leader of the campus' longstanding otter-research programs.

The new study identifies the most important causes of otter deaths in California and determines where otters are most at risk. It reviews 105 deaths of southern sea otters (Enhydra lutris nereis) -- all the beached carcasses found in California from February 1998 through June 2001, the most recent period for which complete data are available.

The lead analyst was Christine Kreuder, a veterinarian at the Wildlife Health Center and a doctoral student in epidemiology. Her chief collaborator was Melissa Miller, a UC Davis wildlife veterinarian who has been performing otter necropsies for Fish and Game since 1998.

The researchers' key findings:

In a healthy population, juvenile and aged otters should account for most deaths. But the UC Davis/Fish and Game analysis found that the largest proportion of dead otters -- 47 percent -- were 4 to 9 years old. That is the age when the animals should have been most healthy. These prime-breeding-age otters are necessary for population growth. Many deaths in this age group would make it difficult for otter populations to recover.

Nearly two-thirds of the otters -- 64 percent -- died of some form of disease. A surprising 38 percent of the disease group died from parasitic infections, including thorny-headed worms

(acanthocephalans) and protozoa (Toxoplasma gondii and Sarcocystis neurona).

The study even identified a new cause of death -- heart disease -- and researchers are pursuing the underlying cause.

The study was also the first to show that otters with Toxoplasma

infections were four times more likely to be killed by sharks than those without.

"This parasite invades the otter's brain and can cause neurological problems such as seizures," Kreuder said. "That could make the otters confused and disabled. They would be less able to evade sharks, more likely to swim to unprotected offshore waters, and more likely to shake and twitch, which attracts sharks." Shark attacks were clustered between Point Ano Nuevo and Santa Cruz.

Credit: Nancy Ottum, Wildlife Health Center,
UC Davis School of Veterinary Medicine.

Two geographic clusters of disease were identified. First, half of the otters found near the town of Morro Bay had died of Toxoplasma brain infections.

Protozoal parasites kill otters by causing encephalitis (fatal brain infections). The only known source of these parasites is the feces of two land mammals not native to California -- domestic cats for Toxoplasma and opossums for Sarcocystis. UC Davis has another study under way to find out how those parasites are getting from the land into the ocean; storm-drain water runoff is one suspect.

The second cluster was due to acanthocephalan worm infections, which killed five of the six otters found in a 1.25-mile stretch of southern Monterey Bay.

Sea otters become infected with acanthocephalans when their diet is unusually high in sand crabs, which carry the worms. In the otters, the worms migrate from the intestine to the abdominal cavity, causing fatal infections.

"Because we did not see these two causes of death distributed evenly along the coast, we know that otters are at high risk in these areas," Kreuder said.

In addition to Kreuder and Miller, the analysis was conducted by Jonna Mazet, who is Kreuder's doctoral adviser; David Jessup, a senior veterinarian at Fish and Game's marine wildlife center in Santa Cruz and Miller's supervisor; UC Davis veterinary pathologist Linda Lowenstine, parasitologist Patricia Conrad and epidemiologist Tim Carpenter; and Fish and Game biologists Mike Harris and Jack Ames.

The study was funded by the University of California Marine Council Coastal Environmental Quality Initiative, the Morris Animal Foundation and the PKD Trust.

Kreuder will present the group's findings on May 11 at the annual meeting of the International Association of Aquatic Animal Medicine in Waikaloa, Hawaii. The analysis is expected to be published this year in the Journal of Wildlife Disease.

On May 12, the spring otter count begins, led by the U.S. Fish and Wildlife Service.

The UC Davis Wildlife Health Center, a program of the School of Veterinary Medicine, has a longstanding sea-otter research program in collaboration with state, federal and local agencies. Research projects include studies of survival rates, disease and death; effects of oil exposure; and evaluating the accuracy of aerial censusing.

The University of California is one of the world's foremost research and teaching institutions, and UC Davis is the University of California's flagship campus for environmental studies. UC Davis is a global leader in environmental studies relating to air and water pollution; water and land use; agricultural practices; endangered species management; invasive plants and animals; climate change; resource economics; information technology; and human society and culture. One in six of UC Davis' 1,500 faculty members specializes in an environment-related subject.

Media contact(s):

* Sylvia Wright, UC Davis News Service, (530) 752-7704,

* Christine Kreuder, UC Davis Wildlife Health Center, (530) 752-4167,

* Jonna Mazet, UC Davis Wildlife Health Center, (530) 752-4167,