Sick Sea Lions on the Beach? What the Domoic Acid Bloom Means for Divers Right Now

If you have walked a Southern California beach this week, you may have seen it: a sea lion hauled out well above the waterline, head weaving, unresponsive or, worse, thrashing in a seizure. Dead and dying dolphins have shown up on the sand too. This is not random. It is a harmful algal bloom, and the toxin behind it - domoic acid - is worth understanding before your next dive.

Sea lions showing signs of domoic acid poisoning. Photo: U.S. Space Force (public domain).

What domoic acid actually is

Domoic acid is a neurotoxin produced by a bloom of microscopic algae called Pseudo-nitzschia. When conditions line up - the same warm-then-upwelling swings we have been tracking this summer - the algae explode in number. Small fish like anchovies and sardines eat the algae, and the toxin concentrates as it moves up the food chain. Sea lions, dolphins, and seabirds that gorge on those bait balls get a massive dose.

In mammals the toxin attacks the brain and heart. That is why stranded animals look confused, crane their heads, have seizures, or simply lie there exhausted. Some become unusually aggressive because they are neurologically impaired, not because they are tame.

What it means for divers in the water

You are unlikely to be poisoned by swimming through bloom water - the human risk comes from eating contaminated shellfish, not skin contact. The bigger, more immediate concern for divers is the wildlife itself.

• Give sick or erratic sea lions a wide berth. An impaired sea lion can be unpredictable and will bite. Do not approach one on the beach or corner one in the water.

• Do not try to rescue or move stranded animals yourself. Call a marine mammal rescue network and report the location instead.

• Expect more bait activity - and more predators. Blooms often ride with heavy bait schools, so keep your head on a swivel.

Your catch and the shellfish question

Domoic acid is the toxin behind Amnesic Shellfish Poisoning in people. It concentrates most heavily in filter-feeding bivalves - mussels, clams, scallops - and in the guts of fish. During a bloom the state issues biotoxin advisories and quarantines for recreational shellfish. A few sensible rules:

• Do not harvest mussels, clams, or other bivalves under an active quarantine or advisory.

• For fish, eat the fillet and discard the guts. The toxin loads up in viscera, not muscle.

• Cooking does not destroy domoic acid, so you cannot cook your way around a quarantine.

Where to check before you go

Bloom conditions change fast. Check the California Department of Public Health biotoxin information line and shellfish advisories, and follow local marine mammal stranding updates, before you dive or harvest anything.

Regulations, quarantines, and advisories change without notice. Always confirm current shellfish safety status with the California Department of Public Health and current fishing regulations with the California Department of Fish and Wildlife before you harvest or eat anything.