California sea lions are found from Vancouver Island, British Columbia to the southern tip of Baja California in Mexico, with a distinct population of at the Galapagos Islands. . They breed mainly on offshore islands, ranging from southern California's Channel Islands south to Mexico, although a few pups have been born on Año Nuevo and the Farallon Islands in central California. A third population in the Sea of Japan became extinct, probably during World War II.
“It’s such an aggressive cancer, and it’s so unusual to see such a high prevalence of cancer in a wild population,” said Dr. Frances Gulland, director of veterinary science at the Marine Mammal Center. In 1996, Gulland and colleagues at the University of California, Davis, found that 18 percent of deaths among stranded adult sea lions were related to cancerous tumors.
“That suggests that there’s some carcinogen in the ocean that could be affecting these animals,” Gulland said.
The first reports of sea lion cancer came 14 years ago, among rescued California sea lions. Today, the Marine Mammal Center sees 15 to 20 California sea lions with cancer each year.
“It’s pretty distressing to see,” Gulland said. During post-mortem examinations of the sick sea lions, doctors have found tumors in the animals’ genitals, lymph nodes, lower spine, kidneys, liver and lungs.
Few veterinarians monitor the incidence of cancer among wild animals. But about 18 percent of dead beluga whales stranded in Canada’s St. Lawrence River had intestinal tumors or other cancers that were linked to industrial pollutants. Among the California sea lion population, no diagnostic test for cancer exists.
Dr. Robert DeLong, a research biologist at the National Marine Mammal Laboratory in Seattle, has seen two to five sea lions a year with advanced tumors, out of some 100,000 animals in the Channel Islands, the birthplace for many California sea lions.
Environmental pollutants are prime suspects. From the late 1940s until the early 1970s, DDTs and PCBs were dumped in the Southern California Bight.
Jason McManus via Care2 and Mother Nature Network