Large numbers of red crabs were reported washed ashore in La Jolla Shores on May 30. But Linsey Sala, museum scientist and manager of the Pelagic Invertebrates Collection at Scripps Institution of Oceanography (SIO) assures this is a natural phenomenon that occurs off Baja California and also seen in past years off Southern California.
“It’s not really known why they strand, particularly; these are small, mainly swimming crabs that spend quite a bit of time in the water column. If they get entrained in certain strong, fast-moving currents or tides, they can get swept onshore pretty easily and don’t necessarily have the ability to crawl back into the water,” Sala said.
Last year, hordes of red crabs were found dead or alive on the beaches of Southern California, especially North San Diego County and Orange County. And although they don’t have an accurate count of them, Sala said marine biologists have seen them present in their samples throughout 2014-2016.
“Fishers have reported seeing them in the gut contents of their catch, and nearshore divers on the seafloor,” she explained. “They have a plausible explanation for their reappearance on our beaches: the crabs might have established a local population in our waters.”
The El Niño event of 1957-1959 brought red crab populations to Southern California, where they were noted through 1960. The phenomenon was studied by scientists who noticed the crustacean lingering near our coasts.
“The red crabs seemed to set up a population and they were here in higher numbers for another several years after the El Niño event. They can be brought in by northward flowing currents and inhabit these coastal waters, which can be a suitable place for them to live,” Sala said.
The May pelagic crab stranding was reported to SIO, said communications officer Britany Hook, but when scientists went out to study them, most had already been brought back into the ocean or eaten by predators.
Red crabs are a good food source for many local fish, sea birds and mammals, Sala explained. “Even some whales will eat them,” she said. In their native Baja California, they are called “tuna crabs” because they are a favorite food of several tuna species. However, these crabs are not recommended for human consumption, even though they don’t pose any threats.
Their known life span is about three years, and they are often deemed “grazers.” They feed on plant-like organisms such as microscopic algae, or animals like krill and other small crustaceans.