Outbreak of domoic acid poisoning of sea lions on the California coast

The Marine Mammal Center said that the animals suffer from domoic acid poisoning, which is a neurotoxin produced by some types of marine algae, and is transmitted to sea lions through the fish they feed on, such as sardines and anchovies. The poison affects areas of the brain responsible for memory and behavior, leading to convulsions, tremors, abnormal movements, confusion and lethargy, and may cause the death of animals.

Dozens of sea lions were seen during the last week of August suffering from these symptoms, while they were washed up on the beaches of the San Francisco Bay area. The United States has recorded more than 100 cases of sick sea lions since the beginning of 2026, compared to an annual average of 70 to 90 cases.

Current cases are concentrated on the Monterey Peninsula and the adjacent Central Coast, near Monterey and Santa Cruz counties. Cases of poisoning began to appear earlier in the summer, before they increased significantly towards the end of August, which raised concerns among experts who have not yet been able to explain this late rise in cases.

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This phenomenon is linked to the proliferation of species of algae, including Pseudo-nitzschia, which produces domoic acid in certain marine conditions. Strong winds off the coast of California help cool, nutrient-rich water rise from the depths of the ocean, providing suitable conditions for algae growth and the spread of toxin up the food chain.

This phenomenon can occur during cold-water years due to strong upwelling, as well as during warm-water years, said Michael Milstein, a spokesman for the National Oceanic and Atmospheric Administration (NOAA). Experts are also looking into the impact of the El Niño phenomenon and rising sea surface temperatures in the Pacific Ocean on increased toxic algae blooms.

The danger of domoic acid is not limited to marine animals, as humans can be exposed to poisoning when eating contaminated seafood, such as mussels, oysters, crab entrails, sardines, and anchovies. Cooking or freezing seafood does not eliminate the toxin, and large doses can be fatal.

While cases of infected sea lions attacking humans have been documented during previous outbreaks, touching an infected animal does not lead to poisoning, and a sea lion bite is not the usual route of exposure to domoic acid. The center confirms that the poison leaves the animal’s body over time, but repeated exposure to it may lead to long-term and more serious effects.

The California coast is home to about 250,000 sea lions, compared to only about 1,500 in the 1920s, which makes monitoring and responding to these cases important to protect animals and beachgoers.