SAUSALITO, Calif.—Record numbers of distressed sea lions have washed ashore in California for a second straight year, the latest example of a marine mammal facing severe problems amid what biologists say is overfishing and other human-caused strains on the world's oceans.

From January through May, a record 367 California sea lions have been admitted to the Marine Mammal Center here just north of San Francisco, nearly five times the average. In Southern California, more than 600 sea lions—more than twice the average—have been...