This blog was authored by Dylan Berk, San Diego Coastkeeper’s Spring & Summer 2025 Beach Cleanup Intern
Southern California’s coastal waters are facing an alarming surge in harmful algal blooms, poisoning marine life at unprecedented rates.
Once rare, domoic acid poisoning and harmful algal blooms in California have now been reported for four consecutive years (2021–2025)—a clear sign of shifting ocean conditions.
The number of dolphins, seals, sea lions, birds, and whales sickened by the toxic algal blooms in 2025 surpassed previous years’ records. Wildlife rescue organizations across the region received more than 100 calls a day during the height of the algal blooms in Southern California, and are at risk of maxing out their annual treatment budgets in the struggle to keep pace with the crisis.
This article explains why the harmful algal blooms in California were especially severe this year, what the impacts were to human health and marine life, and what to do in the event you encounter a distressed mammal affected by domoic acid poisoning.
What Causes Harmful Algal Blooms?
Harmful algal blooms are caused by an overgrowth of algae. In the ocean, seasonal upwelling brings cold, nutrient-rich water to the surface, accelerating the growth of Pseudo-nitzschia.
While upwelling is natural, it has intensified with rising ocean temperatures linked to climate change, creating optimal conditions for harmful algal blooms. [1]
Algal blooms are also caused by excess nutrients in the water, including nitrogen and phosphorus. These often enter the ocean from runoff, but it’s the increase in runoff from two particular events that has scientists particularly concerned: January’s devastating fires in Los Angeles and the increased pollution from the Tijuana River.
Why Is This Year’s Algal Bloom So Severe?
This year’s bloom was unusually intense, compared to blooms in recent years. Many researchers hypothesize that the particularly toxic blooms are linked to a “perfect storm” of possible environmental triggers throughout Southern California, right down to San Diego:
- Strong and early seasonal upwelling
- Nutrient-rich runoff from recent Los Angeles wildfires [2]
- Persistent pollution from the Tijuana River
Some scientists hypothesize that these factors combined to fuel the explosive growth of the Pseudo-nitzschia algae and unusually high levels of domoic acid in marine food webs this season.
The first reports of sick animals began in mid-February, according to NOAA. Previous studies have found elevated nutrient levels offshore following major wildfires, and this year’s bloom is likely amplified by the January fires in coastal Los Angeles.
How Do Algal Blooms Harm Wildlife?
Pseudo-nitzschia is a diatom—a tiny, single-celled alga that forms the base of the marine food chain. These organisms are essential to the ecosystem, nourishing plankton that in turn feed fish and larger animals.
However, when stressed, Pseudo-nitzschia produces a potent neurotoxin called domoic acid. Under the right conditions, blooms of this algae can rapidly grow and release large amounts of the toxin.
Marine mammals and seabirds are being poisoned by consuming fish that have ingested the toxic plankton, a process known as bioaccumulation, where toxins become more concentrated as they move up the food chain.
Are There Risks to Humans?
While some algae can be very harmful to humans, the recent Pseudo-nitzschia algal bloom poses no direct threat to swimmers or surfers. Ocean water remains safe for recreational activities.
However, the California Department of Public Health issued a recreational shellfish advisory in Santa Barbara, Ventura, Los Angeles, Orange, and San Diego counties, after domoic acid levels exceeded safety thresholds. These warnings apply only to sport-harvested mussels, clams, scallops, and oysters—not to commercially harvested shellfish, which undergo rigorous testing.
Domoic acid is the neurotoxin responsible for Amnesic Shellfish Poisoning (ASP), which can cause symptoms ranging from nausea and headaches to memory loss or, in rare cases, death.
The greatest risk to humans comes from interactions with wildlife that have been affected. Domoic acid impacts the brain and nervous system, causing disorientation or aggression in animals. Two people were bitten by poisoned sea lions in Southern California this year—neither incident occurred in San Diego. For your safety and theirs, it is always important to maintain at least 50 feet from all marine mammals—including sea lions, harbor seals, and dolphins—when appreciating them in the wild.
What Can You Do to Help?
We’re now seeing reduced cases of domoic acid poisoning in marine mammals, but that doesn’t mean it’s not still possible to find a sick seal, sea lion or dolphin. If you see a sick or stranded animal, keep your distance and report it to:
- SeaWorld Rescue: 1-800-541-7325
- Marine Mammal Stranding Network: 1-866-767-6114
Provide the animal’s location and, if possible, photos or videos. This helps responders assess the animal’s condition and determine the best course of treatment.Dyl
