Monterey Bay’s pelagic red crab stranding events: the changes warming seawater could bring

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Pelagic red crab near the coast guard pier in Monterey

In the third week of May 2016, hundreds of thousands of pelagic red crabs washed ashore on Del Monte Beach in Monterey.

These lobster-like crabs like warm water and are usually found off the coast of Baja California further south. They show up in other places during El Niño weather patterns.

Other names for this bright red 13-centimeter crustacean are red crab, tuna crab, langostilla and, scientifically, pleuroncodes planipes. Adults tend to live on the continental shelf while their planktonic larvae, when released, are swept out to sea by the California Current. An opposing undercurrent at a lower depth eventually returns them to their continental shelf home. They feed on plankton collected in micro-hairs on their legs.

The pelagic red crabs might be the most abundant species of their size in the California Current, occupying a similar niche as krill in the Antarctic. Red crabs are prey for tuna, swordfish, yellowtail amberjack and sharks; and they feed gray, Bryde’s and blue whales and loggerhead turtles.

When stranded they form an abundant food source for gulls and other seabirds whose prey species are otherwise diminished during El Niño years.

The stranding events might follow swarming in order to spawn or result from currents or winds dragging the animals to the coast in waters unfamiliar to them.

In 1859, the pelagic red crabs were first ‘discovered’ after a large stranding in Monterey Bay. Other years when warm currents generated during El Niño events carried pelagic red crabs to Monterey Bay were 1959, 1969 and 1982-1983. After that they didn’t return for over thirty years until October 2015.

Besides the May-stranding, 2016 saw pelagic red crabs turning up during October and December as well. They were back again in Monterey Bay in 2017, on January 6 at Fisherman’s Shoreline Park and on March 9, with a stranding on the beach at Lovers Point in Pacific Grove.

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Pelagic red crab in Monterey

The local wildlife did not seem to be that familiar with the ‘alien’ red crabs. When I visited the area a couple of months after the May-stranding, the red crabs were still present, but the southern sea otters that I watched for hours catching prey, mostly crabs, completely ignored the very conspicuous red-ones seated on or swimming between the green-and-yellow-and-brown leaves of the kelp.

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Southern sea otter eating a crab in Monterey Bay

On the other hand, some 20 blue whales were spotted off of Moss Landing around the time of the May-stranding. While they are traditionally krill gorgers, they feed on pelagic red crabs off Baja and it’s quite possible they followed their food all the way to Monterey.

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Blue whale surface feeding off the Californian coast

The thicker layer of warmer water that brought the pelagic red crabs, prevented the upwelling of nutrient-rich water from the deep that normally takes place in the spring- and summertime around Monterey Bay.

As a result the commercial squid season that had opened in April produced no catches. Fishermen had to travel to the California-Oregon border further north to fish for squid and squid prices soared. The red tuna crabs were, however, recently spotted as far north as Oregon.

The appearance of pelagic red crabs is indicative of the intrusion of southern sub-tropical waters into central California. Their arrival is considered a rare but natural oceanic occurrence rather than a disruption caused by humankind. It gives us, however, a preview of the effects and changes global warming could bring. If the red crabs start showing up more frequently – and they have the last year – they might be an indicator of largely-anthropogenic climate change.

 

References:

https://en.wikipedia.org/wiki/Pleuroncodes_planipes

http://www.santacruzsentinel.com/article/NE/20160523/NEWS/160529891

http://www.sanctuarysimon.org/monterey/sections/other/sporadic_crabs82.php

http://sanctuarysimon.org/news/2017/01/pelagic-red-crabs-enter-monterey-bay-1959-1969-1982-1983/

http://www.mercurynews.com/2017/03/12/mystery-surrounds-frequent-return-of-the-red-crabs-to-monterey-bay/

http://www.earthwindow.com/crabs.html


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