In Depth
Science
Hungry alien
The invasion of Europe's green crabs
Last Updated September, 24, 2007
By Peter Gullage, St. John's, CBC News
The green crab is a voracious bottom feeder. (CBC)
An environmental crisis is bubbling quietly below the surface of the bay in a southern Newfoundland village.
Last month, residents of the tiny fishing community cuddled up at the northern reaches of North Harbour, Placentia Bay reported sighting hundreds of unusual crabs to federal fisheries officials about a month ago.
The invaders have been identified as European green crab, an invasive species that has already altered coastal environments around the Maritimes, British Columbia and the east coast of the United States.
Scientists from the federal Department of Fisheries and Oceans, Memorial University and the Newfoundland and Labrador Department of Fisheries are surveying the damage in North Harbour.
"This is green crab ground zero," Cynthia McKenzie, a DFO marine biologist, said while standing on the beach as divers, just offshore, hand bags of green crabs to researchers in a small boat.
McKenzie is blunt in her assessment of what's happening. The green crab are "taking over", according to invasive species expert.
"The fact that they are so well established is a concern," she said.
The scientists are using several methods to catch and count the invaders: Nets are pulled through the water, crab pots are baited and divers are chasing green crab along the ocean bottom.
The results are troubling because they show the aggressive green alien crab are displacing the native red rock crab and eating just about everything else along the ocean bottom.
Top-100 list
The Invasive Species Specialist Group, a University of Auckland affiliated research group, based in alien-species wracked Australia, identifies the crab as one of the world's hundred most invasive species.
Originally confined to the European and North African Atlantic coasts, the green crab spread to the coasts of North America, South America, Australia, South Africa and North Pacific Asia with the rise of sea trade in the 18th century, travelling in ship ballasts, cargo holds or on crates.
Government divers check the bottom of Placentia Bay to determine the extent of green crab infestation. (CBC)
In the mid-90's, researchers in California found the crab had reduced competing predatory and fish-farm populations. They discovered the crab thrives in a wide range of coastal environments and can endure high and low water temperatures, and salinity-levels that would kill most other salt-water organisms.
In areas where human activity has reduced seawater salinity, such as estuaries and port-lands, the green crab easily out-competes other seafloor predators.
Oakley Johnson, a lifelong fisherman and North Harbour resident, conducted his own green crab research.
"We threw a pot over the side of the wharf just to see what was down there," Johnson said. "We got them in the hundreds in the one pot … We went so high as 450 in the one pot."
Bye-bye rock crab
In one net, McKenzie's team pulled in dozens of green crabs of varying sizes. Some are smaller than the nail of a child's pinkie nail, while fist-sized adults are found stuck together, still copulating in the net despite being pulled into the beach.
Fishermen around Placentia Bay figure crab larvae were carried into the bay in the ballast water of the hundreds of foreign oil tankers that visit the nearby refinery at Come By Chance each year.
McKenzie said it is impossible to determine what kind of ship may have carried crabs into the bay but there's no doubt the invasion is the result of human activity.
The preliminary research shows local species are paying the price. The green crab are displacing their red-hued cousins, the rock crab. That species is nowhere to be found in the "ground zero" corner of North Harbour.
"There was always a lot of them clams. I don't think there's either one left in North Harbour," Oakley Johnson said. "You can go down there now, and when the water's low it's just the same as looking in a field that somebody dug up with a shovel."
Not just Newfoundland
In Prince Edward Island, Mike and Tami Martell have all but given up on the eel fishery because of green crabs.
"It's been a terrible experience," Tami Martell explained.
They first noticed a few of the crabs in 1997. About five to seven crabs turned up in 35 eel pots that were set in the water near their home. By 2005, the green crab population had exploded, making eel fishing impossible.
Last year, the family tried to get back into the business but found the situation had only worsened. The Martells discovered as many as 1,500 green crab in each eel pot … and a disturbing sight.
"When the green crab gets in they'll attack the eel and clean it right to the bone," Tami Martell said.
In Newfoundland, with North Harbour being eaten from the ocean bottom up, the race is on to figure out what to do in these early days of a disaster.
"There's been a lot of discussion about fishing them out — fishing them really, really hard right now — while we know where they are and while they are contained to get as many as we can," McKenzie said.
"But the other method that we're going to have to discuss is what we're going to do along the shoreline where we have all the juveniles. If we don't get the juveniles off the coastline, then in the next year they'll be out again."
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