Los Angeles ? Two dozen sick or dead sea lions and dolphins have washed ashore in Southern California in recent weeks, heralding the apparent return of a naturally occurring toxin that last year caused the second-largest marine mammal die-off in U.S. history.

The California sea lions and common dolphins have been found on beaches between Santa Barbara and San Diego and were believed to be victims of domoic acid poisoning. Last spring, the nerve toxin killed roughly 800 of the two species during a three-month period.

Domoic acid is produced by blooms of microscopic algae. The toxin is concentrated in filter-feeding animals, such as anchovies, sardines and shellfish, which are in turn eaten by marine mammals.

The toxin also causes a human illness, amnesic shellfish poisoning, which can be deadly.

Authorities are awaiting lab results that would confirm the presence of domoic acid. But the behavior of some sea lions, including head waving and seizures, suggests it is the culprit, as does the rate at which dead and dying animals have been found.

Last year, domoic acid was implicated in the deaths of 685 sea lions and 98 dolphins in Southern California. An additional 500 sea lions were sent to rehabilitation centers; about 60 percent survived.