Sea life along the Southern California coast is being decimated by what marine scientists are calling the deadliest and largest toxic algal bloom recorded. Also known as the red tide, this naturally occurring phenomenon has stained the shoreline and threatened the lives of thousands of marine animals and birds.
According to the National Oceanic and Atmospheric Administration (NOAA), the toxic algal bloom has affected Southern California for four consecutive years, but this year’s outbreak is the worst in terms of fatalities and geographic spread.
Unprecedented marine die-offs
In recent months, hundreds of whales, sea lions, dolphins, and brown pelicans have been reported to be sick or dead along the coast. By April 2025, NOAA had reported over 100 dolphins being affected, with 50 long-beaked common dolphins found stranded and unwell along Ventura and Santa Barbara beaches. Many died while others were euthanized to minimize their suffering. Within two weeks in April, another eight dolphins died on San Diego beaches, further alarming scientists. The outbreak has also spread south to San Diego County, where SeaWorld rescued 16 affected sea lions.
“The number of dead dolphins we’ve had this year is just off the charts. We’re all dealing with numbers within a two-month period of time that would normally represent the number of responses we would make in a year. It’s been the worst that’s on record in terms of how it’s affecting the animals and the different species of animals,” explained Glenn Gray of the Pacific Marine Mammal Center when speaking to The Weather Channel.
What’s causing the toxicity?
The toxic algal causing illness and death is a microscopic organism called Pseudo-nitzschia australis which produces a neurotoxin called domoic acid during bloom events. This toxin accumulates in tiny fish such as shellfish, sardines, and anchovies when they eat the algae. When marine mammals then feed on these contaminated fish, the neurotoxin attacks their hearts and brains which results in seizures, heart failure, and permanent brain damage.
“In the last four years, the impacts have been really heavy on the marine mammal side, both sea lions, some fur seals, and quite a few long-beaked common dolphins,” Clarissa Anderson, a Scripps Institution of Oceanography biological oceanographer, told The Guardian.
If humans eat the contaminated seafood, they contract amnesic shellfish poisoning (ASP) which causes vomiting, nausea, diarrhoea, and abdominal cramps within 24 hours. Within 48 hours, neurological symptoms occur which include headache, confusion, dizziness, short-term memory loss, seizures, abundant respiratory secretions, coma, cardiac arrhythmia, and eventually death.
Why is this happening more often?
Since domoic acid poisoning in marine mammals was first discovered in 1998, the outbreaks have worsened. In 2023 alone, over 1,000 sea lions died in California from domoic acid poisoning.
Marine experts note that the ongoing toxic algal bloom is fueled by several factors:
- Ocean upwelling winds push surface water away which allows colder and nutrient-rich water to rise to the surface. Those nutrients fertilize the surface waters and intensify algal bloom.
- Agri-industrial runoff: Scientists also believe the algal toxicity comes from chemicals used in human activities that flow through rivers and waste water runoffs into the ocean. This carries nutrients like carbon, nitrogen, and phosphorus which enrich the sea water and encourage the growth of the toxic algal.
- Climate change: Rising ocean temperatures cause more frequent and longer-lasting algal blooms. According to a 2022 study on Pseudo-nitzschia australis algal, worldwide warming temperatures are contributing to extreme toxic algal bloom events.
Economic toll and mitigation challenges
NOAA estimates that toxic algal bloom events cost the U.S. economy between US$10 and US$100 million annually through damage to marine life and ecosystems, disruptions to seafood supplies and threats to public health.
Controlling the toxic algal bloom is challenging, and according to a report by the U.S. National Office for Harmful Algal Blooms, prevention is the ideal approach. Currently, the most utilized measure is to sprinkle certain types of clay on ocean waters when blooms occur. As the dense clay particles sink, they capture some of the toxic algal cells and purify the water as they fall to the seabed.
