Climate changes and crab invasions

With summer’s warm tides came brigades of red tuna crabs, washing up by the thousand on the shores of several Southern California beaches. Locals were left questioning what brought upon the sudden crab invasion, leading millions of crabs to swim to their demise.

According to Corona del Mar High School’s AP Environmental Science teacher Kim Rapp, the crab phenomenon was a result of unusually warm ocean temperatures.

“Over the last couple years, we’ve had a warm ‘blob’ in the northwestern part of the United States and extending down to Southern California,” said Rapp.

She explained that the “blob” was a result of a high pressure ridge that raised the temperature of waters along the California coast about 5°F warmer than usual.

“We’ve been experiencing El Niño over the last year, which is a raise in the water temperature. It’s been known to cause the migration of warm water species, such as the red tuna crab,” Rapp explained.

Tuna crabs are a pelagic species, meaning they live neither close to the ocean floor nor near the shoreline. Their typical environment is further south, off the coast of Baja California, where the waters are warmer and the currents are weaker.

According to Rapp, the currents off the coast of Southern California are strong, consisting of a mixture of cold water from the North and warm water from the South. The site of these major currents is referred to as the Southern California Bight, responsible for great marine biodiversity. Tuna crabs are unable to swim in the swift currents of the Southern California Bight.

“As the water temperature off the California coastline warmed, the tuna crabs migrated north, getting caught in currents that they couldn’t swim out of. The currents carried them in, and they followed the warm water all the way to our shoreline,” explained Rapp.

The red tuna crab is not the only species that migrated toward warmer waters. Recently, there has been a large presence of tropical fish in local waters. Massive bluefin tuna and yellowtail, both of which are predators of the red tuna crab, have followed their prey to the California coast.

These temperature changes have altered the migration patterns of several species, which could have extreme repercussions on delicate marine food webs.

One example of the impact of rising ocean temperatures on food webs is the 1972 El Niño, which led to the collapse of an anchovy fishery off the coast of Peru.

“Photosynthetic rates can drop during an El Niño, and this dramatic drop can have impacts on the food supply of all organisms in the coastal ecosystem,” said Rapp.

During the 1972 El Niño, there was a sudden peak in the death rate of guano birds that nest along the Peruvian coast. There we no more anchovies left for the birds to feed on, leaving many birds to starve to death.

Another example is the increasing number of starving sea lions pups in California.

“The parents have to travel farther to search for food because of the temperature changes, leaving the pups to starve or die searching for food on their own. Since January, there have been over 1,400 stranded baby sea lions,” said Rapp.

Many of these pups are now under the care of local rescue centers, such as the Laguna Canyon Pacific Marine Mammal Center.

The tuna crabs suffered a similar fate, with millions of them washing up on the sunny shores of California to die. The slightest change in ocean temperature has killed off millions of crabs and other marine animals, setting the food web off balance. If these temperature changes continue, they could one day lead several marine species into extinction.