The animals were discovered yesterday on Mandalay Beach in Oxnard.
The Ventura County Star provided photos of the creatures.
Their deaths are likely linked to a natural neurotoxin called domoic acid.
A marine biologist says that seasonal "red tides" bring plankton species that produce the acid.
Dolphins and sea lions likely ate the toxin that gets concentrated in smaller fishes such as anchovies and sardines.