Adam Fox grabbed the nets and the cage from the back of his pickup truck. Something in the sand was disturbing the iconic Southern California scenery at the Santa Monica Pier on Friday morning.

A sea lion washed up on the beach just north of the Ferris wheel. It was alive but gravely ill and in a disoriented, near-comatose state. On the other side of the pier, another sea lion lied in the sand, a little more alert but still sick.

Mr. Fox, who works for a sea-mammal rescue group, rushed to help save the animals and transport them to get treated. Both sea lions were given ID numbers, which told a story.

They were tagged as 25-193 and 25-195 — the group’s 193rd and 195th patients of the year.

All along the Southern California coastline from San Diego to Santa Barbara, hundreds of animals — sea lions, dolphins, seabirds — are washing up on the sand either dead or seriously ill. Coastal researchers and officials say it’s become a marine-life crisis that has overwhelmed rescue organizations, distressed beachgoers and hurt California’s ocean habitat.

The cause is a neurotoxin produced by an algae bloom. The toxin, known as domoic acid, is harmless to fish but can be deadly to sea mammals. Fish carry the toxin, but if mammals and birds eat the fish, the toxin can poison them, causing seizures, making them behave erratically or putting them in a coma. The only treatment is to flush out the toxin and medicate the symptoms.