An alien invader detected in Washington State is making waves across the border in British Columbia, raising concerns the aggressive European green crab may be conquering new waters.

Last week, citizen scientist volunteers caught a European green crab in Westcott Bay off San Juan Island, in Puget Sound.

That might not seem like a big deal; the invasive crab has been present on the outer coast of Washington and B.C. since the late 90s. But this is different.

"They've not been found in the Salish Sea or the inland waters," said Thomas Therriault, a scientist with Fisheries and Oceans Canada in Nanaimo, B.C.

"This is a fundamental shift in their distribution, if they are indeed established in those waters."

Green crab

A European green crab found in Willapa Bay, on the outer coast of Washington State. Green crabs have been established on the West Coast of Washington and B.C. since the late 90s. (P. Sean McDonald/Washington Sea Grant)

Monitoring programs haven't found them in B.C.'s Strait of Georgia. The nearest established population is the Sooke Basin.

Scientists are now trying to figure out: is the lone male crab a one-off, or the first sign of a population that's been undetected until now?

Raving mad crab

An inland invasion would be bad news for aquaculture and native species alike, said Therriault.

The green crab has a reputation to be among the worst invasive species in the world and it has already wreaked havoc on Canada's Atlantic coast, ripping up eelgrass beds and competing with native crabs and lobsters for food and shelter.

Even it's Latin name, Carcinus maenas, has been translated as "raving mad crab."

It's a voracious predator, using strong claws to crush the shells of mussels, oysters, clams, snails and juvenile crabs in its intertidal home.

"They're an aggressive, hardy crab and they'll out-compete our native species for sure," said Gail Wallin, executive director of the Invasive Species Council of B.C.

If the green crab does get established in the Strait of Georgia, that would be concerning for aquaculture operations that use the intertidal areas — such as ones in Baynes Sound on Vancouver Island, said Therriault.

They also spell trouble for eelgrass beds. En route to dinner, the green crab has a tendency to clip the eelgrass, which provides important nursery habitat for young salmon and many other species.

Green Crab

European green crabs aren't always green, but they do have five distinct spikes on each side of their back shell. This crab was photographed in Maine, where the species is also invasive. (Emily Grason/Washington Sea Grant Crab Team)

'Prevention is key'

If anyone spots a European green crab in the Strait of Georgia, the DFO wants to know, said Therriault.

"Clearly this new report suggests that we need to be vigilant in our monitoring."

Detecting them early will give fisheries managers more options, he said.

The public can also play a role in preventing the spread of the alien invaders, said Wallin.

Green crabs have a long larval stage, when the crabs are microscopic and can travel long distances in ocean currents.

There's little people can do about that — but they can reduce the chance juvenile crabs are hitching a ride, said Wallin.

"You want cleaned, drained and dried gear. You don't want to be moving the larval stage in your ballast water, or in your bilges. You don't want it on your fishing gear."

"Prevention is key," said Therriault. "Once they're well-established … then eradication becomes virtually impossible in the marine realm."