OXNARD (AP) — The deaths of four dolphins and four sea lions found on a Southern California beach are likely linked to a natural neurotoxin called domoic acid.

Volunteers from the Channel Islands Marine and Wildlife Institute found the dead sea creatures Friday on Mandalay Beach in Oxnard.

The institute later confirmed the animals died of natural causes not related to fishing or a nearby power plant.

Marine biologist Tom McCormick tells the Ventura County Star that seasonal “red tides” bring plankton species that produce domoic acid. The toxin gets concentrated in smaller fishes, such as anchovies and sardines, which are eaten by larger animals like dolphins and sea lions. The result can be fatal.

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