Crabs May Signal a New El Nino

The invasion force began landing a few days ago, wave after wave of red pelagic crabs carried by warm currents from Baja and dumped unceremoniously on the beaches of Southern California.

A human tendency is to view such mass stranding--or other bizarre animal behavior--as a harbinger of rough weather or even a cataclysmic event.

In this case, the arrival of these thumb-length crabs is widely reported to portend an upcoming El Nino event, with its powerful winter storms, mudslides, home-bashing waves and mass die-offs of marine life.

Not so fast, scientists say.

“The mass stranding of red crabs is not sufficient evidence that an El Nino has arrived,” said Mark Ohman, a biological oceanographer at Scripps Institution of Oceanography in San Diego.

These crabs, he said, sometimes show up in non-El Nino years, too. They hitch a ride--probably involuntarily--on the warm plume of water that on occasion works its way north from Mexico.

Bill Patzert has been tracking this warm plume, along with other sea surface temperatures, using satellites from his station at the Jet Propulsion Laboratory in Pasadena.

Generally, he said, the state’s coastline is dominated by the California Current, a large, slow mass of water moving from north to south. Every so often, a California countercurrent kicks in.

“It sweeps all these crabs that are spawning, and they tend to wash up on the beach by the millions, or at least the tens of thousands,” Patzert said. The crab swarm, he said, “can occur during an El Nino but it doesn’t take an El Nino to make it happen.”

The red crabs, which look like tiny lobsters, began showing up on beaches in San Diego last week. Their tiny bodies turned some stretches of sand fiery red, as if a restaurant had laid out a massive seafood spread on the beach.

“They are so small, once they wash up on the sand, they cannot get back in the water and they kind of die,” said Jennifer Rogers, a San Diego lifeguard. She spotted even more in the surf. “They bite you a little bit in the water. It doesn’t hurt. They just pinch.”

Crab sightings reached as far up the coast as Surfrider Beach in Malibu, but most of the tiny crustaceans remained safely offshore. In previous migrations, they have been spotted as far north as Eureka.

These crabs, Pleuroncodes planipes, are sometimes called tuna crabs after the fish that love to eat them. They are a favorite snack of blue whales and other baleen whales.

For the first two years of their lives, these pelagic, or open-ocean, crabs swim freely in the water column, munching on plankton collected in micro-hairs on their legs. Only in the third year of life do they settle to the bottom, sometimes as deep as 300 meters.

“We know that their primary center of breeding is off south-central Baja California,” Ohman said. That makes them an indication of the northern movement of warmer water. But these tiny crabs may not be playing the role humans enjoy assigning to animals.

Scientists say people should not read too much into the crabs’ sudden appearance. But then scientists tend to discount the prescience of nervous dogs before earthquakes or, for that matter, Punxsutawney Phil’s celebrated insight into the length of winter. Red crabs tend to show up every six to 10 years, with or without an El Nino.

So far, Patzert sees little evidence of a brewing El Nino, which sends warm water surging north up the California coast from South and Central America.

The nation’s official forecasters at the National Oceanic and Atmospheric Administration last week noted only slightly higher than average temperatures in tropical Pacific waters, where El Ninos are born.

Their prediction is that a weak or moderate El Nino event may develop during the next six to nine months. If one does emerge, they said, the global impacts should be less than those experienced during the strong 1997-98 El Nino.