Experts Blame Natural Causes in Dolphin Strandings

Disease is probably to blame for an unusual number of dolphin strandings along the California coast earlier this year, marine scientists said Tuesday.

In a peculiar episode that sparked a statewide hunt for a cause, 46 common dolphins washed ashore during a nine-week period ending June 4. Thirty-six were stranded along a 110-mile stretch of coast in Santa Barbara and San Luis Obispo counties.

Initially, environmentalists and some marine mammal experts speculated that an oil spill or some other pollutant was the culprit. Other suspected causes ranged from a toxic “red tide” of algae blooms to the use of sonar by research vessels mapping the topography of the ocean floor.

Now investigators believe the mammals died of natural causes after being drawn unusually close to shore by a tempting food source.

“We think the animals moved near the beach because of a food source, like squid, and then the ones who were afflicted with disease died and washed ashore as part of a natural event,” said Joe Cordaro, a wildlife biologist with the National Marine Fisheries Service.

Normally, common dolphins remain farther off the coast and die unnoticed. But if they were lured closer to shore by an attractive source of prey, currents could have washed dead and ailing animals onto the beach--creating the appearance of an unnatural event.

Scientists based their conclusion on necropsies that show the beached dolphins shared at least one common characteristic--symptoms associated with pneumonia, a condition that is usually secondary to another disease. Experts must await the results of tissue analyses to determine what other disease may have been involved.

Environmentalists, meanwhile, were hesitant to rule out pollutants as the cause. They suspect some of the dolphins were killed by a massive spill of petroleum thinner at a San Luis Obispo County oil field owned by Unocal Corp.

But Cordaro said other marine species also would have shown up sick or dead on the beach if oil or other contaminants were to blame.

The stranded dolphins included two species--the long-beaked and short-beaked common dolphin. Known to form exceptionally large schools, they are the most common cetaceans found off California, numbering about 240,000 in the state’s waters. Also beached during the nine-week period were three sperm whales; the cause of their deaths remains unknown.

In a typical year, scientists record fewer than 30 dolphin and whale strandings a year throughout the state. Thus, the unusually high number documented this spring caused alarm.

Of the 46 dolphins that washed ashore, 22 were alive. Eleven died shortly thereafter, three were pushed back into the surf by beach-goers, four died at Sea World, two remain at Sea World and two were rehabilitated and released into the wild.