Beachgoers in Half Moon Bay have been finding more sick and dead sea lions than usual this fall. Marine experts blame domoic acid, a natural neurotoxin produced by algae blooms, NBC Bay Area reported.
"We've seen a decline in the number of calls and responses since then except for San Mateo County; Half Moon Bay has been a primary hot spot the last few weeks," Marine Mammal Center spokesperson Giancarlo Rulli told NBC.
A record year
The Marine Mammal Center says it has documented more than 300 domoic acid cases this year, compared with roughly 75 to 90 in a typical year. It is the largest outbreak since the center was founded in 1975. Responses peaked in early September. On the busiest days the hotline took more than 600 calls, nearly four times the usual volume. Monterey, Santa Cruz and San Mateo counties saw the most activity.
Domoic acid comes from the alga Pseudo-nitzschia australis and builds up in sardines, shellfish and other small prey. In sea lions it attacks the brain and heart, causing lethargy, disorientation and tremors, and untreated cases can lead to seizures and death. Treatment includes anti-seizure medication, fluids and clean fish. Animals exposed repeatedly may stop responding to treatment. The outbreak follows a record leptospirosis outbreak in California sea lions in 2025, the center said.
Why it matters for people
The center calls sea lions a sentinel species, because they eat much of the same seafood people do. Data from outbreaks like this one helps health officials decide when to issue seafood warnings. In humans, domoic acid can cause amnesic shellfish poisoning, which can be life-threatening.
Seabirds are also struggling. International Bird Rescue has taken in about 300 sick birds since mid-August, mostly starving common murres, NBC reported. Officials ask beachgoers to keep their distance from stranded or dead marine animals, keep pets away, and report sick wildlife to the Marine Mammal Center or International Bird Rescue.