SANTA CRUZ -- The recent deaths of at least 21 California sea otters have been traced to a poison produced by algae building up in freshwater sources that drain into the ocean, scientists from UC Santa Cruz and the state Department of Fish and Game said.

The three-year study is the first to prove a link between the ocean and the liver-attacking toxin microcystin that accumulates in Watsonville's Pinto Lake and other tributaries. The findings offer, as one researcher put it, "another piece of the puzzle" regarding mysterious sea otter deaths, but scientists also warn of public health implications for humans and pets if the algae increases unmitigated.

Melissa Miller, a senior wildlife veterinarian at the Department of Fish and Game Marine Wildlife Veterinary Care and Research Center in Santa Cruz, presented the results of a three-year investigation at the California and the World Ocean Conference in San Francisco on Friday. Afterward, co-author Raphael Kudela, a professor of ocean sciences at UCSC, said otters are at particular risk because they consume shellfish near estuaries affected by the algae.

Commercially harvested shellfish are not at risk because they are not likely to be sourced from areas where tributaries connect to the bay, he said. Water treatment procedures remove the microcystin from drinking water.


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"There is almost no chance that humans are going to be exposed to this toxin right now," he said. "If it keeps getting worse, maybe."

The research provides new insight into the reasons beyond starvation and disease that local sea otter populations have suffered in recent years. The mammal has been listed as threatened under the Endangered Species Act for more than 30 years.

"This adds to the fact that there is no silver bullet," said Steve Shimek, executive director of The Otter Project, an advocacy group in Monterey. "There are lots of things killing sea otters."

Kudela said microcystin was "pretty low down on the list" of possible causes for a rash of sea otter deaths in 2007 because "we didn't think there was any way for it to get into the ocean."

But after tissue from the livers of dead sea otters tested positive for microcystin poisoning, scientists zeroed in on tracking the algae's march to sea from places like Pinto Lake, where dog deaths might be linked to the poison.

"We suspect that dogs are getting poisoned when they drink contaminated runoff or get into the bacterial scum and then lick it off their fur," Miller said in a statement. "However, we haven't been able to do perform postmortem examinations yet to confirm microcystin poisoning in local dogs."

Testing of the lake, which drains into Corralitos Creek then into the Pajaro River, confirmed extensive amounts of algae with high amounts of the poison in 2007.

"We were able to track the toxin all the way down the Pajaro River to within one kilometer of Monterey Bay," Miller said. During the rainy season this year, researchers also detected microcystin in the outflows of the Salinas, Pajaro and San Lorenzo rivers. The toxin also was detected in water at the Santa Cruz wharf and at the mouth of the Pajaro River last year after the first significant rain.

"Now that we know, we can look into mitigating it," Kudela said.

There are no government regulations for exposure to microcystin, which even in low levels can damage the liver and possibly lead to liver cancer, scientists said. There is no formal system for routinely monitoring microcystin levels in water or shellfish.

Kudela said regional scientists will more closely monitor algae content in local freshwater bodies and outflow points to the bay and work with authorities to regulate exposure to the ocean and marine life. The key to reversing the algae buildup is to cut down on "nutrient loading" of the watershed caused by runoff from leaky septic tanks, industrial agriculture, lawn fertilizer and other chemicals, Kudela said.

Annual estimates released last month by the U.S. Geological Survey show that the number of sea otters off the West Coast dropped 3.6 percent from 2009 and the number of pups was down 11 percent to the lowest count since 2003.

The USGS and California Department of Public Health and State Water Resources Control Board also worked on the study. The U.S. Environmental Protection Agency has provided funding for monitoring Pinto Lake.