Puget Sound Braces for the Worst: A Green Crab Plague

After eating its way up the Pacific Coast, the notorious invader is starting to dig its claws into an important Washington fishery.

Credit: CSIRO

We’ve been expecting them, the green crabs. Scientists have watched the European invaders creep up the Pacific Coast for decades, and now they’ve finally arrived in Puget Sound, off northwestern Washington.

Volunteers found Puget’s first green crab in a baited trap on August 30, and five more have been spotted since. That might not sound like a big deal―and the coming winter may put an end to any remaining upstarts―but scientists fear that this could be the beginning of a spiny scourge, one that has ravaged ecosystems and fishing industries across the world.

The International Union for Conservation of Nature lists the green crab among the 100 worst invasive species in the world, and the crustacean’s range now stretches from its native waters of northern Europe and northern Africa to Australia, Russia, South Africa, Argentina, and both coasts of North America.

“Green crabs are the rats of the sea,” says University of Washington researcher Sean McDonald, a member of Washington Sea Grant’s Crab Team, which was launched last year after the state called for a green crab watch on the shoreline. “They’re incredibly resilient.” The crustaceans swim in both fresh and salty waters, and native predators rarely recognize the invaders as prey. When matched for size, green crabs often outcompete local (and much meatier) species like the Dungeness crab, a Pacific Northwest delicacy.

As an invader, the species has fine-tuned its European claw skills to handle American prey. The green crab explosion on the East Coast is costing the local shellfish industry $22 million each year. “It eats about everything,” marine ecologist Mark Bertness told the journal Science in 2013. “In terms of biodiversity, it’s hell on wheels.”

A bed of eelgrass
Credit: Colin Faulkingham

Green crabs forage for clams in coastal mudflat ecosystems. As they do, the crabs uproot beds of eelgrass, which protects small marine animals like young salmon and provides refueling stations for migrating birds. Within a year of Maine’s 2012 green crab spike, more than half of the eelgrass cover in Casco Bay disappeared.

Across the country in Puget Sound, it’s unclear how exactly the eelgrass ecosystem would respond to a green crab attack. Species listed as threatened by U.S. Fish and Wildlife in Washington, such as Newcomb’s littorine snails, Pinto abalones, and Olympia oysters, would all be at risk during such an invasion.

Puget Sound’s scant green crab population isn’t yet considered established. “But, by virtue, founding populations start small,” McDonald says. You might be in the clear if you eliminate the pioneers, he says, but once the crabs reach critical mass, as in Maine, efforts at eradication are a losing battle.

When they do strike, it’s ugly. Maine clammers have described the crabs as “boiling out of the water.” A local scientist told the Bangor Daily News, “There are so many crabs, it’s really unsettling.”

Video courtesy P. Sean McDonald/Washington Sea Grant Crab Team

Maine has developed nightly bounty hunts to kill the countless crabs, but so far hunting, trapping, and other management efforts have all proved unsuccessful. Only the region’s harsh winters have kept the crabs in check.

In fact, green crabs often need just a couple of ingredients to kick off their ecological takeover: warm water and a ride.

For centuries, the crabs have piggybacked on the movements of people. They first voyaged to the U.S. East Coast in the 1800s within the ballast water of ships. The first green crabs to make it to the West Coast likely arrived in 1989 as stowaways within lobster or bait-worm shipments to the San Francisco Bay Area. “They’ve been leapfrogging their way up the coast ever since,” says McDonald. Populations now dot the Pacific coastline all the way up to British Columbia.

Puget Sound as seen from Richmond Beach Park
Credit: pfly/Flickr

Washington’s Crab Team has been closely monitoring Puget Sound ever since the species was first seen off the state’s coast nearly two decades ago—when their larvae floated for hundreds of miles on what McDonald calls “a nice, comfy conveyor belt” created by El Niño’s warm winter weather.

This year’s El Niño conditions seemed to have a played a part in Puget’s invasion, too. Most female green crabs expel their eggs in spring. If water conditions are salty and warm at that time, the eggs develop into larvae. “They need warm enough currents to keep them alive until they find someplace to settle,” says Allen Pleus of the Washington Department of Fish and Wildlife.

Pleus thinks that Puget’s crabs came either from California or from a neighboring population in Vancouver Island’s Sooke Harbor, where green crabs were detected in 2012. He says water circulation patterns in Sooke Harbor may be keeping its crabs close to shore and not carrying them out to sea. The crabs have been living there for years, but ideal conditions for them to disperse didn’t appear until the winter of 2014, when Washington experienced record-breaking warmth.

Green crabs grow faster in higher temperatures, too, and the time between generations shortens. Making matters worse, the faster young crabs mature, the shorter the period they are vulnerable to predators.

Warm waters may have also spurred the outbreak in the Northeast. Since 2004, the Gulf of Maine has been heating up faster than 99.85 percent of the world’s oceans. The Maine coast began experiencing its green crab explosion during the extreme marine heat waves of 2012 and 2013.

Along with bounty hunting, management efforts in Maine and other parts of the world have included trying to market green crab meat and turning the crabs into a savory stock. Washington’s management plan involves bolstering methods to predict, detect, and eradicate, as well as getting the public to be on the lookout for the invaders.

The incoming winter is expected to be mild, McDonald says, and his volunteer team is ready to monitor Washington’s inland waters. “If ever there was a time to make sure we have traps in the water and people combing the shorelines looking for [crab] molts,” blogs the Crab Team, “that time is now.”

Whether it works or not, well, we’ll have to wait until spring to find out.

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