An outbreak of domoic acid linked to one of the largest harmful algae blooms ever recorded along the Southern California coast has led to the illness and deaths of thousands of marine mammals and birds in recent months, including California sea lions, common dolphins, and confirmed cases in at least two species of whales.

Results of postmortem analysis carried out on a juvenile female humpback whale and a subadult male minke whale confirmed the presence of high levels of domoic acid – a potent neurotoxin produced by harmful algal blooms – in multiple samples from both, including within their faeces, stomach contents, and urine.

The findings have supported suspicions that domoic acid toxicosis was the cause of death.

Like sea lions and dolphins, humpback and minke whales feed on small schooling fish such as sardines and anchovies, which are known to accumulate domoic acid. These new results reveal how the domoic acid event is right now reverberating through the marine food web, affecting more species as it continues.

This is the fourth consecutive year with a domoic acid outbreak off Southern California, an event that has – according to researchers and on-the-ground conservations – been occurring earlier and earlier each year with greater severity.

During this current ongoing domoic acid outbreak, four whales have stranded deceased in Orange and Los Angeles Counties: one humpback, one minke, and two gray whales. A juvenile female humpback stranded dead on January 24 this year in Huntingdon Beach, while – more recently – a subadult male minke whale stranded on April 6 after exhibiting abnormal swimming behaviour for several days in the Port of Long Beach.

The Pacific Marine Mammal Centre (PMMC) and the Ocean Animal Response & Research Alliance (OARRA) conducted necropsies and collected samples to determine the cause of death of these four whales. 

“Each of these whale deaths is heartbreaking – and also deeply telling,” said Keith Matassa, ceo and founder of the Ocean Animal Response & Research Alliance. “Whales are sentinels of ocean health, and their suffering signals a larger ecological imbalance driven by intensifying harmful algal blooms. These events are no longer isolated – they’re increasing in scope, scale, frequency, and severity.

“OARRA is at the forefront of marine mammal mortality response, working alongside partners to investigate, document, and better understand the impacts of these toxic events. We must listen to what the ocean is telling us and act decisively to support long-term monitoring, research, and collaborative response efforts.”

Experts suggest that one reason for the prolonged severity and increased frequency of these harmful algal blooms around Southern California could be the warming ocean temperature. Over the last two decades – coinciding with evidence of increasing temperatures – marine mammal toxin exposures have “increased significantly”.

“We typically see mass strandings of California sea lions, with occasional dolphins,  during domoic acid events,” said Dr Alissa Deming, veterinarian and vice president of conservation medicine and science at PMMC. “This bloom is negatively affecting record numbers of sea lions and dolphins – and now multiple whale species. The scale of this mortality raises serious concerns about ocean health.”

The successful necropsy and sample collection from these significant cases were made possible through the continued support of local and state agencies including City Officials, Marine Safety Departments, Beach Operations, Public Works, State parks, and the Department of Beaches & Harbours. Toxin analyses were made possible through NOAA’s WARRN-West programme at the Northwest Fisheries Science Centre.

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