Researchers at the University of California, Davis found the sea otters are dying in geographic clusters and from newfound diseases - including a disease that makes them four times more likely to be killed by sharks.
Their study came before 100 more southern sea otters washed up on California beaches this year, further reducing a population of about 2,000 that has been dwindling since 1995.
The otters were hunted to near extinction for their luxuriously soft pelts by early in the last century. Their range once extended the length of the West Coast, but now is limited to California's central coast.
In a normal population, the young and old are the most likely to die. But nearly half of the dead California otters - 47 percent - were in their adult prime, between the ages of 4 and 9 years old. Those otters also are in their prime breeding years, multiplying the impact of their loss.
The researchers and the California Department of Fish and Game studied 105 adult sea otters, which are federally protected mammals, that died between February 1998 and June 2001.
Disease killed nearly two-thirds - 64 percent - of the animals.
Otters infected with one type of protozoa were four times more likely to be killed by sharks, the study revealed. The parasite invades the otter's brain where it can cause seizures, or make the animals confused or 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," said the study's lead analyst, Christine Kreuder, a veterinarian at the UC Davis Wildlife Health Center.
That parasite caused brain infections that killed half the otters found near Morro Bay. The shark attacks that can be a secondary cause of death, however, were clustered farther north, between Santa Cruz and Point Ano Nuevo.
The parasite is found in domestic cat feces, while a second parasite is found only in opossums' droppings. A separate UC Davis study is trying to track how those land-based parasites get into the water, though storm water runoff is suspected.
The study also said five of the six otters found along a 1¼-mile section of coastline in southern Monterey Bay died from eating a diet unusually high in sand crabs, which carry parasitic worms that can cause fatal abdominal infections.
Heart disease also is killing otters, the researchers discovered, though they're now looking for the virus or bacteria they think is causing the unusual heart problems.
The overall frequency of the disease deaths "indicates that the ecosystem is very unhealthy," said Jonna Mazet, who directs the university's Wildlife Health Center and leads its otter research programs.
It's time to do something about it, urged Steve Shimek, executive director of The Otter Project, a nonprofit organization that focuses on research and conservation of the sea otter.
"We know that the sea otter is in trouble. The (population) counts have gone down six of the last seven years. We also know we're losing our reproductive animals - you can't do that forever before you crash" the population, Shimek said.
"We know we have this problem. Can't we start working on the solution now?" he asked.
That means better sewage treatment and banning the use of problem contaminants on shore, and controlling the accumulations in harbor areas where otters also congregate, Shimek said.
The U.S. Fish and Wildlife Service in April released a long-delayed recovery plan that blames human activities, from fishing and boating to pollution and oil spills, for the species' decline.
Kreuder presented the findings at the International Association of Aquatic Animal Medicine annual meeting, and they are expected to be published in the July issue of Journal of Wildlife Disease. Results from the U.S. Fish and Wildlife Service's annual spring otter count are expected in June.
By Don Thompson