For 14 years, since they first reported that a disturbing proportion of deaths among rescued California sea lions were caused by metastatic cancer, researchers have been trying to pinpoint the source of the illness.

In 1996, Dr. Frances Gulland, the director of veterinary science at the Marine Mammal Center in Sausalito, and colleagues at the University of California, Davis, found that a striking 18 percent of deaths in stranded adult sea lions were the result of tumors in the reproductive and urinary tracts.

“It’s such an aggressive cancer, and it’s so unusual to see such a high prevalence of cancer in a wild population,” Dr. Gulland said. “That suggests that there’s some carcinogen in the ocean that could be affecting these animals.”

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