Thousands of dead pelagic red crabs, also known as tuna crabs, washed up on the shore of Spooner's Cove in Montaña de Oro State Park this week.
Department of Fish and Wildlife Patrol Lieutenant Todd Tognazzini said this was the first report he's received of the red crabs this year.
The red crabs only appear on local shores every few years, with the last notable sighting being in 2003 or 2004, the same year as the last large El Niño event, Tognazzini said.
Todd Street, a frequent visitor to the area, said he was walking along the beach with his daughter on Friday when she noticed the thousands of crabs.
"It was a little strange to see that many of any animal dead," Street said. "My daughter started counting them. She got to 50, then 300 and then looked up and realized there were (thousands) more."
Derek Stein, and environmental scientist with Fish and Wildlife's Marine Invertebrate Project who's based in San Luis Obispo, said the tuna crabs are typically found much farther south, near Mexico, in warmer water.
"These crabs are not strong swimmers, and most likely were brought in by currents from the south," he said. "It may not be totally clear as to why they perish in masses, but one explanation is that they cannot withstand the cold water when they arrive on our coast."
Stein said they are typically only seen on the Central Coast and farther north during El Niño years. However, red crabs have reportedly washed up in large numbers in Monterey, on Catalina Island and in Southern California in other years, Tognazzini said.