New study reveals cause of mysterious red crab strandings

The study could provide insight into other species as well.

From time to time, millions of pelagic red crabs, or tuna crabs, wash ashore on the U.S. west coast. It doesn't happen every year, but the mechanisms behind it have always been a mystery.

Until now, that is. In a recently-published paper, researchers have found atypical current are transporting them from their habitat off Baja California and onto sandy shores much farther north.

In addition to providing new insight into the stranding events, the study's authors created a seawater flow index that could help scientists identify abnormal current years and plan ahead.

Lead author Megan Cimino, a biological oceanographer at the National Oceanic and Atmospheric Administration (NOAA) and UC Santa Cruz through the Institute of Marine Sciences Fisheries Collaborative Program was inspired to study the crabs after biking past a stranding in 2018. At the time, she had "no clue what a red crab was," she said in a statement, "but it was very clear something different was going on in the ocean--something unusual."

EUREKALERT - red crabs

During pelagic red crab stranding events--like this one documented at a beach in Pacific Grove, California--the small red crustaceans wash ashore en masse in areas far north of their usual home range in the Mexican state of Baja California. (Stephanie Brodie)

Over the next few months, Cimino and her team compiled data on the crabs from various sources, including research surveys, videos, and social media reports, eventually painting a clear picture of the crabs' range and strandings between 1950 and 2019.

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The team then looked at ocean conditions. That's when they discovered crabs tend to roam outside their range when seawater flows from Baja California to central Cali. "The finding supports strong currents as the key indicator for the presence of the crabs over the other major hypothesis--that warm water brought by marine heatwaves and El Niño events causes the appearances," the authors say in a statement.

While the crabs are eye-catching, they aren't the only species displaced and washed ashore by atypical currents, and the index created for the study could help officials plan for future strandings.

"The index could be used as a kind of early warning system about what the ocean state is that year and whether we're going to expect southern species in northern regions," Cimino said. "That can help us plan and manage and give expectations for bycatch or different fisheries."

It could also help researchers make more accurate population estimates, as the index may help them broaden their search, Cimino says.

The full paper appear in Limnology and Oceanography.