Sea otters, dead and alive, have been washing up on the California coast in record numbers for two springs, and scientists say they are closer to solving the mystery.
The principal culprits, the scientists say, are toxoplasma gondii and sarcocystis neurona, parasites that can cause fatal diseases in the brains and nervous systems of otters and other animals. Toxoplasma gondii, which can affect humans too, spreads its hardy spores through cat feces, while sarcocystis neurona is spread through opossum feces.
Toxoplasma gondii had been widely thought to be the more prevalent parasite. But in April, scientists confirmed that at least 11 of the 62 otters that were found washed up on the shore had died from exposure to sarcocystis neurona. The finding of the 62, most of which were dead, was a record for a month, surpassing the one-month record of 48 last year. In the 10 years before 2003, an average of 24 otters were stranded on the coast each April.
Some otters died of exposure to toxoplasma gondii and some of exposure to demoic acid, a toxin found in red tide algal outbreaks. Some otters were too decomposed to determine the causes of death.
The 11 otters killed by sarcocystis neurona were found in a cluster of 14 around Morro Bay, halfway between Los Angeles and San Francisco. Dr. Melissa Miller, a wildlife veterinarian and pathologist for the University of California, Davis, said she had never seen such a high number of otter deaths attributed to sacrcocystis neurona in such a small area along the coast.
Dr. Miller said she suspected that the otters encountered the parasite by eating shellfish that carried it. The shellfish, she said, could have picked up sarcocystis neurona from the runoffs of heavy rains in February. Runoff has been shown to transport cat feces infected with toxoplasma gondii to the ocean, where the spores are eaten by shellfish, and then by otters, she said. Her theory is that the water might have carried opossum feces in the same way.
Studies to investigate this theory are being conducted, she said.
Opossums are not native to California. According to the California Department of Fish and Game, they were probably imported from Virginia in the early 1900's so that people could hunt them.
''What's interesting about the opossum,'' Dr. Miller said, ''is that they're not even a native species. They're certainly naturalized, though, and not hard to find around here.''
Because the weather has largely been dry since May, fewer otters have washed ashore. But Dr. Brian B. Hatfield, a marine biologist with the United States Geologic Survey Western Ecological Research Center in San Simeon, said the increased numbers each year were troubling.
In 1981, fewer than 1,500 otters were counted off California, said Dr. Hatfield, an organizer of a survey that counts otters along the coast. Those counts, while an imprecise measure of population, rose steadily through the early 1990's but have stubbornly remained at 2,100 to 2,500 since 1995. Otters face threats from pollution, boats, fishing nets and occasionally even sharks as well as from parasites.
''I guess a lot of us are more concerned with elevated mortality over all,'' Dr. Hatfield said. ''Between 1985 and 1994, the mortality numbers were between 4 and 6 percent of our spring population count. Since then, it's averaged over 8 percent.''
Photo: A record 62 sea otters washed up on the California coast in April. Most were dead, and scientists believe that a parasite from opossum feces is to blame. (Photo by Gary Randall/FPG) Map of California highlighting Morro Bay.