A year and a half ago, hundreds of dead and dying sea lions washed ashore along the coast of Monterey Bay in central California. Now, scientists have determined what killed them: a potent toxin produced by microscopic algae, which climb up the food chain through fish and into marine mammals. The study, which appears in tomorrow's Nature, is the first to establish an unequivocal link between algal blooms and mammal deaths, and it suggests the poisonous pathway can endanger humans, too.
Biologists have long known that toxins produced by algal blooms--exploding populations of minute, marine algae--can accumulate in shellfish that graze on them. They had also suspected that algal blooms trigger some of the epidemics that periodically afflict sea lions and other marine mammals along California's coast. But they had lacked evidence that clearly linked sick animals to blooms, in part because the algae often vanish quickly, leaving little trace of their presence.
In April 1998, bay water samples routinely monitored by Chris Scholin, a molecular biologist at the Monterey Bay Aquarium Research Institute, first showed a tiny rise in an alga called Pseudo-nitzschia australis. By May, Pseudo-nitzschia australis numbers had soared, sea lions were convulsing and dying, and water samples contained traces of domoic acid, the alga's deadly neurotoxin. Further sleuthing revealed Pseudo-nitzschia australis remains and domoic acid in the bay's anchovies, which feed on algae by filtering water through their gills. Lesions found in the brains of dead sea lions were typical of domoic acid poisoning. The scientists also discovered Pseudo-nitzschia australis, domoic acid, and traces of anchovies in sea lion feces--damning evidence, they say, of a pathway of death from algae to anchovies to sea lions.
The incident should serve as a warning that fish, in addition to shellfish, can accumulate domoic acid at levels high enough to poison people, says one of the study's authors, Frances Gulland, chief veterinarian at the Marine Mammal Center in Sausalito, California. "If people had eaten those anchovies, the same kinds of things could have happened to them," Gulland says.
Other experts praise the team's work. "This is one of the most conclusive, compelling examples that's been brought to light about toxic algae affecting mammals," says JoAnn Burkholder, a marine ecologist at North Carolina State University in Raleigh. "Before this, there were always questions in the data."