Toxic Algal Bloom Affecting California Sea Lions and Dolphins

Strong upwelling of nutrient-laden water from the deep ocean is driving a bloom of harmful algae along the Southern California coast. It produces a toxin that can poison sea lions and other marine mammals. The neurotoxin, domoic acid, does not appear to harm fish or pose a risk to swimmers. It can accumulate in shellfish such as mussels, oysters, and Dungeness crabs.

Strong upwelling is a common spring-to-summer occurrence, driven by winds that help draw cold water up from the deep ocean along the coast. This year a short-lived local marine heatwave first warmed coastal waters. This may have set up the algae for rapid growth, said Andrew Leising, research scientist at NOAA Fisheries' Southwest Fisheries Science Center.

The deep waters are rich in nutrients, so the wind-driven upwelling subsequently kicked in and fueled growth of the algae. In particular, it encouraged growth of the toxin-producing diatom Pseudo-nitzschia spp, which thrives in such conditions. Satellite maps of sea surface temperature anomalies display the upwelled water as blue streaks off the California Coast that are colder than usual this time of year. The same nutrients also nourish California's rich coastal ecosystem, including many fish and shellfish species that support commercial and recreational fisheries.

Domoic acid not only contaminates shellfish, but can poison marine mammals such as California sea lions that consume contaminated fish. The toxin accumulates in their bodies-causing seizures, other unpredictable behavior, brain damage, and death. Beachgoers and their pets should stay a safe distance from affected animals on shore and report them to the West Coast Marine Mammal Stranding Network at (866) 767-6114.

Another Outbreak Begins

The phone began ringing off the hook in late July 2024 for Channel Islands Marine & Wildlife Institute. The Institute serves Santa Barbara and Ventura Counties, responding to marine mammal strandings for the West Coast Marine Mammal Stranding Network. Last year the group responded to more than 650 affected marine mammals during a similar bloom that started in July and spread south as far as Los Angeles.

"The good news is that people realize something is wrong and know who to call," said Ruth Dover, Managing Director of the Channel Islands group, noting that the group is getting close to 100 calls a day. "The bad news is that many of these animals are really suffering." The group has rescued 45 sea lions exhibiting signs of domoic acid in the first 10 days of this harmful algal bloom-the third major annual bloom in a row.

"This is especially concerning because these blooms didn't use to be an annual event," she said. "Unfortunately, many animals are dying on the beach before we have a chance to rescue them. This indicates a high level of exposure to domoic acid."

Teams rescue affected sea lions and admit them to a rehabilitation facility. There, they can be treated with fluids to flush the toxin from their bodies and anti-seizure medication if needed. Teams cannot always reach all the affected animals in time, or their condition may be too advanced to respond to treatment; many have died.

The Marine Mammal Center in Sausalito has responded to 90 affected sea lions in San Luis Obispo County since July 29, most of them females. They have also responded to three common dolphins and four northern fur seals. These algae blooms have become a familiar summer occurrence off Central and Southern California. Their severity often depends on the strength of the upwelling that supplies nutrients to the algae.

"The upwelling helps make the California Coast such a rich ecosystem, but in this case there is another effect that we see playing out with marine mammals," said Clarissa Anderson, Executive Director of the Southern California Coastal Ocean Observing Network. The Network tracks domoic acid and the abundance of Pseudo-nitzschia along the California Coast and shares forecasts.

Cold Upwelling Dominates

Researchers including Anderson surveyed domoic acid events off California over the last 20 years and found that upwelling is the main driver. Last year the effects continued for a few months and spread south to affect animals in Los Angeles and farther south. While warmer waters can also foster growth of the toxic algae, in this case the main fuel is cold water from the deep ocean. It is so rich in nutrients that it speeds growth of the algae.

West Coast winds that usually blow at an angle to shore in the summer push surface water offshore, drawing up nutrient-rich cold water from the deep. NOAA Fisheries tracks the Coastal Upwelling Transport Index, a measure of the strength of the upwelling. We also track the Biologically Effective Upwelling Transport Index, which indicates the volume of nutrients the upwelling is importing into the ecosystem. It is not yet clear how upwelling will be affected by climate change, though models suggest that the southern California Bight could see a reduction in nutrient supply from upwelling.

"Upwelling makes the California Current Ecosystem what it is," said Michael Jacox, research scientist at the Southwest Fisheries Science Center. "The better we can predict how it will change months, years, or decades in the future, the better we can anticipate effects such as harmful algal blooms."

NOAA Fisheries is only one part of NOAA assisting with the response. The National Centers for Coastal and Ocean Science has a harmful algal bloom event response program. It provides immediate support to help state, tribal, and local officials manage events and advance the understanding of harmful algal blooms as they occur. For more information about the program, contact [email protected].

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