Algae Is Making Sea Lions More Aggressive in California

A toxin present in algal blooms is moving through the food chain, leading to the deaths of sea lions and dolphins

a sick sea lion
A sea lion receiving care at the Pacific Marine Mammal Center in Laguna Beach, California, shows signs of domoic acid poisoning. Allen J. Schaben / Los Angeles Times via Getty Images

Sea lions in California are becoming sick and aggressive toward people after consuming high levels of domoic acid, a neurotoxin found in algal blooms. Over just three to four weeks, the Marine Mammal Care Center, a nonprofit rescue organization in Los Angeles County, received more than 20 reports of the creatures biting swimmers at beaches, writes Rachel Barnes for

“It literally affects their brains, and the behavior of sea lions—especially when the concentrations of domoic acid are quite high—is drastically changed,” John Warner, the CEO of the center, tells NBC News’ Doha Madani. “They become symptomatic in ways that are just unpredictable in terms of their behavior—aggressiveness that we don’t normally see.”

Domoic acid enters the food web when fish eat toxic algae called Pseudo-nitzschia. The acid then accumulates in larger sea creatures when they gobble up high quantities of fish.

Since the bloom began in late May, hundreds of sick sea lions and dolphins have washed ashore in California, putting a strain on local organizations that respond to stranding events. The Channel Islands Marine and Wildlife Institute, which serves Santa Barbara and Ventura counties just north of Los Angeles, was receiving more than 200 calls per day in June about animals in distress, reports BBC News’ Madeline Halpert. The organization’s co-founder and managing director, Ruth Dover, tells CNN’s Rachel Ramirez that rescuers are “physically tired and emotionally drained,” from the constant stream of strandings.

“We thought last year’s domoic acid event affecting Santa Barbara and Ventura was tragic, but this year’s event is even more horrific,” Dover tells the publication. “Our beautiful beaches are littered with sick, dying and deceased sea lions and dolphins.”

Domoic acid can affect a variety of species of mammals, sea creatures and even birds. In 1961, a Pseudo-nitzschia bloom led to odd behavior in hundreds of seabirds in the coastal communities of Monterey Bay. This event may have inspired Alfred Hitchcock’s 1963 film The Birds—though at the time, the cause for the birds’ erratic behavior was unknown. 

Humans can also get sick from eating fish contaminated with domoic acid, though seafood sold commercially is tested for the toxin, per the California Department of Public Health (CDPH). Domoic acid poisoning—also called amnesic shellfish poisoning—can set in between 30 minutes and 24 hours after eating toxic fish. Symptoms include vomiting, headaches and dizziness in mild cases, or seizures, confusion, coma or death in serious ones, per the department. In mid-June, the CDPH issued a health advisory warning against eating sport-harvested mussels, clams or scallops from Santa Barbara County. 

Harmful algal blooms (HABs) often occur in warm or nutrient-rich waters. Many blooms are linked to high levels of phosphorus or nitrogen from fertilizer and sewer runoff. In the past 15 to 20 years, California’s Pseudo-nitzschia blooms have been increasingly threatening local wildlife and fisheries. These blooms tend to worsen in El Niño years, when increased rainfall leads to more runoff—and as of early June, meteorologists say El Niño is here. Climate change can also make blooms more frequent and severe.

This algal bloom appears to have been caused by wind-driven upwelling of nutrients in the water, which fuels the algae’s growth, according to a Facebook post from NOAA Fisheries West Coast.

Experts warn against approaching animals with signs of domoic acid poisoning, which, in sea lions, include unusual side-to-side head movements or keeping their heads extended back. But Warner tells NBC that people shouldn’t fear the animals.

“Sea lions, you know, they’re the ubiquitous California ocean wildlife… that people love. And this is not them,” Warner tells the publication. “So, I don’t want this to turn into people viewing sea lions like they do Jaws. ... This is not sea lions suddenly turning into evil people-biters. I don’t want people afraid of these animals going forward. This is a really sad event.”

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