What is happening to the marine mammals in Southern California?

Recently, hundreds of sea lions and dolphins have washed up along the coast of Southern California - significantly more than usual. They have succumbed to domoic acid toxicosis, a deadly poisoning caused by the ingestion of domoic acid. Some of those that are still alive have become disoriented and have attacked multiple people so far.

To start, yes, domoic acid is very harmful to people. But no, it is not likely to be dangerous to people swimming or surfing in the water. The danger comes through accumulation via ingestion, which explains why marine animals are dying in concerning numbers these past weeks.

Bioaccumulation is a term we usually use to describe the mercury levels in fish such as tuna. Mercury makes its way into the water (an estimated 40% of the total amount coming from coal plants and chlorine production facilities) and into the smallest of organisms. They retain the mercury because they cannot remove it from their systems faster than they ingest it. Small fish eat these organisms, bigger fish eat the small fish, and even bigger fish eat those.

An example of both bioaccumulation and biomagnification of mercury. By Bretwood Higman, Ground Truth Trekking

When we consume fish higher in the food chain, we consume not just one fish's mercury, but all of the mercury they have stored from eating mercury-laden smaller fish. This process of concentration can occur across multiple trophic levels, meaning that instead of ingesting a minuscule amount, you end up consuming the accumulated toxins from many organisms. The term for this is “biomagnification.”

Domoic acid works similarly, but with a different source. It's produced by certain species of algae, primarily Pseudo-nitzschia. These microscopic diatoms create this neurotoxin, which then moves up the food chain. Filter feeders like mussels and clams consume the algae, small fish eat the filter feeders, and larger predators consume those fish. At each step, the concentration of the toxin increases.

Sea lions and dolphins, being at the top of this food chain, receive the highest dose. The acid binds to glutamate receptors in their brains, causing neurological damage, seizures, hallucinations, and often death. Humans can experience similar symptoms if we consume enough contaminated seafood, leading to Amnesic Shellfish Poisoning - named for the memory loss that sometimes accompanies it.

But why are we seeing such dramatic blooms of these toxic algae recently? Wind-driven upwelling plays a substantial role. Upwelling occurs when winds push surface water away from the coast, allowing deeper, nutrient-rich water to rise and replace it. These nutrients, particularly nitrogen and phosphorus, create ideal conditions for algal growth.

California's coast is a hotspot for upwelling events. Our dominant northwesterly winds push surface water offshore due to the Coriolis effect, drawing up that cold, nutrient-dense water from below. During spring and early summer, these winds often intensify, creating stronger upwelling events. The problem isn't the upwelling itself - it's a natural process that supports our rich marine ecosystem. The issue arises when additional nutrients from agricultural runoff, wastewater discharge, and other human activities supercharge the system.

While these events have occurred periodically throughout history, their increasing frequency raises concerns about the health of our coastal ecosystems. Marine mammal rescue centers from San Diego to Santa Barbara have been overwhelmed with stranded animals showing neurological symptoms. Unfortunately, while the sea lions taken in by the Marine Mammal Care Center in Los Angeles have a recovery rate between 50-65%, dolphins that have been stranded on the beach typically are in worse condition already, and their recovery rates are lower.

It's deeply concerning to see these marine mammals suffering. The sea lions and dolphins are experiencing the consequences of being at the top of a food chain in an ocean increasingly influenced by human activity. For now, keep your distance from the marine mammals and report any animals that are showing signs of poisoning.

Further Reading:

Report Animal (LA)

Report Animal (SD)

Domoic Acid

Biomagnification