June 15, 2004, Tuesday Late Edition - Final
Section F Page 4 Column 2 Desk: Science Desk Length: 651 words
In the Case of the Dead Otters, Evidence Points to Opossums
By CHRIS DIXON
Sea otters, dead and alive, have been washing up on the Californiacoast in record numbers for two springs, and scientists say they arecloser to solving the mystery.
The principal culprits, the scientists say, are toxoplasma gondiiand sarcocystis neurona, parasites that can cause fatal diseases inthe brains and nervous systems of otters and other animals.Toxoplasma gondii, which can affect humans too, spreads its hardyspores through cat feces, while sarcocystis neurona is spread throughopossum feces.
Toxoplasma gondii had been widely thought to be the more prevalentparasite. But in April, scientists confirmed that at least 11 of the62 otters that were found washed up on the shore had died fromexposure to sarcocystis neurona. The finding of the 62, most of whichwere dead, was a record for a month, surpassing the one-month recordof 48 last year. In the 10 years before 2003, an average of 24 otterswere stranded on the coast each April.
Some otters died of exposure to toxoplasma gondii and some ofexposure 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 acluster of 14 around Morro Bay, halfway between Los Angeles and SanFrancisco. Dr. Melissa Miller, a wildlife veterinarian andpathologist for the University of California, Davis, said she hadnever seen such a high number of otter deaths attributed tosacrcocystis neurona in such a small area along the coast.
Dr. Miller said she suspected that the otters encountered theparasite by eating shellfish that carried it. The shellfish, shesaid, could have picked up sarcocystis neurona from the runoffs ofheavy rains in February. Runoff has been shown to transport cat fecesinfected with toxoplasma gondii to the ocean, where the spores areeaten by shellfish, and then by otters, she said. Her theory is thatthe 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 CaliforniaDepartment of Fish and Game, they were probably imported fromVirginia in the early 1900's so that people could hunt them.
''What's interesting about the opossum,'' Dr. Miller said, ''isthat they're not even a native species. They're certainlynaturalized, though, and not hard to find around here.''
Because the weather has largely been dry since May, fewer ottershave washed ashore. But Dr. Brian B. Hatfield, a marine biologistwith the United States Geologic Survey Western Ecological ResearchCenter in San Simeon, said the increased numbers each year weretroubling.
In 1981, fewer than 1,500 otters were counted off California, saidDr. Hatfield, an organizer of a survey that counts otters along thecoast. Those counts, while an imprecise measure of population, rosesteadily through the early 1990's but have stubbornly remained at2,100 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 mortalityover all,'' Dr. Hatfield said. ''Between 1985 and 1994, the mortalitynumbers were between 4 and 6 percent of our spring population count.Since then, it's averaged over 8 percent.''
Images: Photo: A record 62 sea otters washed up on the Californiacoast in April. Most were dead, and scientists believe that aparasite from opossum feces is to blame. (Photo by Gary Randall/FPG)
Map of California highlighting Morro Bay.