It’s sea lion breeding season, but hundreds of dead sea lion pups have shown up at Bay Area beaches this month, part of a worrying trend across the entire West Coast. And scientists don’t know why.

Patrick Robinson, the director of UC Santa Cruz’s Año Nuevo Reserve, told the Chronicle that a survey of Año Nuevo Island’s coasts in San Mateo County found over 300 stillborn or aborted sea lion fetuses in early May — an “abnormal event,” he said.

Similarly, more dead or stillborn pups, aborted fetuses, malnourished pups, and underweight adult females with dystocia — difficult births — were found at common sea lion colony breeding hotspots along the coast, from San Miguel Island in the Channel Islands all the way down to Mexico, per the LA Times.

Researchers are reportedly conducting tests on the fetuses for several factors including domoic acid, a toxin that has poisoned sea lion populations in the past, and bird flu, which has hit sea lion and elephant seal populations in South America. They haven’t received the results yet.

Another potential explanation could be the El Niño phenomenon, which brings warming waters and geographic shifts in schools of anchovies, sea lions’ principal food source. That could compel pregnant sea lions to swim farther for food and leave them malnourished, prompting premature births. Sharon Melin, a research biologist with NOAA, told the LA Times that up to 20%-30% of the approximately 25,000 pups born annually in the Channel Islands might be born prematurely during El Niño years.

As scientists continue to search for the cause of these deaths, animal rescue organizations like Sausalito’s Marine Mammal Center, which treats sick and injured sea lions, among other marine mammals, are gearing up for an increase in sea lion strandings. The center has reportedly admitted over 160 sea lions since the start of the year, slightly above the historical average.

Luckily, California’s sea lion population is still robust, scientists say. NOAA estimates it to be about 250,000 strong. Robinson said that “one bad year is not going to dramatically impact that,” but added that “the concern is if this is an environmental event that could affect other life as well.”

The public is advised not to intervene if they see a distressed marine mammal but to contact local marine life centers for trained rescuers.

Feature image via Unsplash/Joss Woodhead.