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. Heidi Schumann for 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. One day last month, a volunteer rescue crew netted an ailing sea lion stranded on Stinson Beach and drove back to the hospital, which was newly rebuilt and reopened last summer. The thin, lethargic 200-pound young adult male had paralysis in its genital area and in its swollen hind flippers, clear signs of cancer. “It’s pretty distressing to see,” Dr. Gulland said. The veterinary team had to euthanize the animal. A post-mortem examination revealed not only cancer in the penis, but also tumors riddling the lymph nodes, lower spine, kidneys, liver and lungs. The disease typically starts around the penis in males and the cervix in females, then spreads. In an average year, the Marine Mammal Center sees 15 to 20 California sea lions with cancer. … Ordinarily, veterinary experts do not see much cancer in wild animals, but there has been little monitoring for the disease. Recently, however, cancer has emerged as a key concern for some endangered species, including green sea turtles, Attwater’s prairie chickens and Tasmanian devils, said Denise McAloose, a veterinary pathologist at the Wildlife Conservation Society in New York City. In addition, about 18 percent of dead, stranded beluga whales in the St. Lawrence River estuary in Canada were found to have intestinal tumors or other cancers, which have been linked to industrial pollutants. …

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