Cancer Kills Many Sea Lions, and Its Cause Remains a Mystery

Dr. Frances Gulland and Dr. Bill Van Bonn performing an autopsy on a young sea lion on Feb. 23 at the Marine Mammal Center in Sausalito, Calif. (Photo: Heidi Schumann for The New York Times)

The New York Times
By: Ingfei Chen
Published: March 4, 2010

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.”

The center has not observed the same syndrome in other seals.

Years of study have led researchers to think the answer lies not with any one culprit, but with several. Their research has added to a body of evidence concerning industrial contaminants in the ocean and their effects on the health of its inhabitants.

Sea lions have had to cope with a variety of challenges lately. There was the animals’ mass exit from Pier 39 in San Francisco late last year, which experts suspect was driven by a hunt for a better food supply. Also in 2009, the Sausalito mammal center had an unusually busy year. It took in a record 1,370 sick and injured California sea lions, and doctors found major problems in many, including malnutrition, parasitic diseases and bacterial kidney infections. Some had brain seizures from a toxic algae poisoning.

But the cancers are what Dr. Gulland found most worrisome.

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