Unbeknown to them, sea mammals were ingesting domoic acid — a neurotoxin produced by the algal bloom. In the ecosystem, sea lions were perhaps hit the worst, suffering from seizures, brain damage, dehydration and muscle spasms as hundreds began to die.

The sick animals have been left feeling aggravated, which has led them to attack people and their pets walking by on the beach, researchers say.

“When they start to feel sick or incapacitated at all, it’s really stressful for a wild animal because they are prey,” said Michelle Berman Kowalewski, the founder of the nonprofit Channel Islands Cetacean Research Unit in Santa Barbara, Calif. “So if they can’t swim, if they can’t avoid obstacles, if they can’t avoid predators … that’s really scary for a wild animal.”

At least 500 sea lions and 100 dolphins have become sick from the algae on beaches in the Los Angeles and San Diego areas, wildlife officials said. And at least two dozen beachgoers have reported being bitten.

Harmful algal blooms — rapid increases in toxic-producing algae — occur almost every year and cause dozens of sea mammals to die in California, including particularly deadly occurrences last year and in 2015. Kowalewski said only a “perfect storm” of algae abundance and animals eating certain fish could cause an outbreak like this summer’s. Climate change and pollution can also cause harmful algal blooms, according to the National Oceanic and Atmospheric Administration.

Stemming from algae called Pseudo-nitzschia, the past month has brought the deadliest algae bloom researchers have seen in the state.

“It gives people sort of a sense of doom,” John Warner, the chief executive of the Marine Mammal Care Center in San Pedro, Calif., told The Washington Post.

Southern California wildlife care centers have become overcrowded. Kowalewski and Warner said their centers have treated over a hundred sea lions and dolphins this year — more than double the number they care for most years. Due to the lack of space, some sea lions have been left on shores, researchers said. But SeaWorld and local aquariums have volunteered to adopt some of the sick sea lions.

When treated, the sea lions are hydrated to help flush out the toxins. But Warner said about 30 percent of the sea lions he has treated have died. For those that survive, many could continue suffering health effects even after they’re released, wildlife officials said.

Humans can face health risks, too. The California Department of Health advised Santa Barbara County residents last month to avoid shellfish in case they contained toxins.

Beaches visitors have been cautioned to stay at least 50 feet away from sea lions. Many beaches have marked off sections that are off-limits to humans using bleach or chalk.

A 14-year-old told NBC 7 San Diego on Wednesday that he was celebrating the Fourth of July on the Del Mar, Calif., coast when a sea lion bit his right knee and butt.

“It looked really tired,” he told the news channel. “Its eyes were closed, and its mouth was twitching.”

This week, Warner said some recovering sea lions are waddling back toward the ocean, and calls about other sick marine mammals have decreased. Officials hope they’ve contained the contamination.

Researchers also hope the toxins won’t spread to other parts of the Pacific Ocean.

“The mortality rate will increase,” Warner said, “the longer this goes on.”