Sea lions on US west coast beaches that ingest domoic acid may face neurological problems that impair behavior and survival abilities, new study finds

The mystery of why sea lions have been stranding in droves on US west coast beaches in recent years is closer to being solved.

A new study suggests that sea lions have been eating crabs and small fish laced with the algal toxin domoic acid, which causes chronic seizures and brain damage, impairing the animals’ ability to navigate, eat and generally survive in the ocean.

Domoic acid was already a known cause of sea lion deaths, but the new study is the first to pinpoint how it affects behavior – and thus how it could indirectly lead to widespread declines in the population even when it doesn’t kill the animals.

By studying rescued sea lions at the Marine Mammal Center in Sausalito, California, researchers found that domoic acid bioaccumulates in sea lions as they eat large quantities of small fish until it causes significant brain damage, specifically to the hippocampus, a process which erodes memory.

“And not just short-term and long-term memory deficit, but probably more global deficits as well,” Peter Cook, the lead author said Monday. “What’s happening is probably a chronic thing – repeated exposure to domoic acid causes repeated seizures, which damage the brain.”

US west coast toxic algae bloom might be largest ever, say scientists Read more

Record numbers of sea lions have stranded in each of the past three years, according to the Marine Mammal Center. There are likely a number of factors, but Cook says domoic acid probably increases those strandings both directly, through damaging the animals’ navigation abilities, and indirectly, through pups being abandoned by mothers.

The findings provide critical new information about the impacts of domoic acid, which has increasingly devastated wildlife – and fishermen – in recent years.

The toxin is the reason why the season for Dungeness crab, a well-known and lucrative fishery along the west coast, has been delayed this year and has increasingly become one of the main impacts of warmer waters along the west coast as domoic acid-producing algal blooms grow larger and longer-lasting.

Those impacts have grown rapidly and recently.

In 1998, hundreds of sea lions experiencing seizures stranded off Monterey Bay, 75 miles south of San Francisco. At first, the reason for the seizures was a mystery. “People thought it was mercury poisoning,” said Kathi Lefebvre, a National Oceanic and Atmospheric Administration scientist who was not involved in the new research. She happened to be looking at the effects of domoic acid in fish in 1998 and thought the toxin might be the cause for sea lion seizures.

It turned out to be the first documented case of domoic acid poisoning in marine life. Those cases have grown since, and this year could be the worst ever. “Every year since – for the last 17 years – there have been sick and dying sea lions, sometimes in the hundreds,” Lefebvre said.

This year was the first in which a sea lion affected by domoic acid poisoning was reported north of California, she said. “What we’re most concerned about right now is this year we have had the likely largest ever recorded algal bloom producing domoic acid on the US west coast, spanning the largest geographic range.”

Historically, the toxic algal bloom would last just a few weeks, but due to warmer waters from climate change and this year’s El Niño weather phenomenon, this year’s bloom lasted for months. She said that persistence and its northward expansion makes studying the “sub-lethal chronic effects” domoic acid has on animals particularly important.