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Fisheries and Oceans Canada/The Green Crab: Alien Invasion

CANADA - The green crab (Carcinus maenas) is invading Canada's coasts, ruining prime habitats for shellfish stocks and nurseries for juvenile fish by its burrowing, and eating just about everything that lives in its new home.

A native species of Europe and northern Africa, it has hitchhiked to new locations around the world in ships' ballast waters and on ocean currents, fishing gear and pleasure boats.

Photo: Marketwire
It has been a pest in the Maritimes from as early as the 1950s, and since the late 1990s on the West Coast. In August 2007, green crabs were reported for the first time in Newfoundland, at North Harbour in Placentia Bay, by sharp-eyed fisherman, Earle Johnson.

Scientists at the Northwest Atlantic Fisheries Centre, part of Fisheries and Oceans Canada (DFO), in St. John's, have been preparing for just such an invasion for about two years.

In fact, DFO, the provincial Department of Fisheries and Aquaculture, and the Newfoundland Aquaculture Industry Association had already developed a guide to aquatic invasive species. Its distribution at a Placentia Bay Integrated Management meeting enabled Johnson to identify the unusual crabs in his fish box and told him whom to alert.

Within days of Johnson's report, an eleven-person team, led by DFO research scientist Cynthia McKenzie with members from Memorial University and the Department of Fisheries and Aquaculture, was mobilized and working in Placentia Bay to determine the extent of the infestation. What they found at North Harbour, according to McKenzie, was "ground zero for green crabs."

There were no other crabs and no juveniles of any species to be found. An empty clam bed had only depressions marking where the clams had been dug up and consumed by the green crabs. When the team checked the beach, they found juvenile green crabs, representing next year's population, in numbers too numerous to count. North Harbour had been completely taken over by green crabs.

The green crab is aggressive and fast, and has a voracious appetite. It can easily out-compete native crabs. On its menu are clams, mussels, oysters, scallops, and possibly even lobster. In its native habitat, the green crab has natural predators that keep it in check, but in its new homes it has none. Its eating patterns are the sea-based equivalent of a scorched earth policy, with few survivors left after an infestation takes hold. It is a prolific breeder, with a female capable of spawning up to 185,000 eggs per year.

In addition to eating all its new neighbours, the green crab burrows into the seabed, damaging the roots of plants such as eelgrass that define the habitat, causing established beds to float away. Two features make it unique in the crab world and add to its indestructibility: it can rotate its claws over its back, thus being able to defend itself from predators attacking from behind, and it can live out of water for up to a week - in full sun, no less. It can even survive fresh water, with no problem. Little wonder that the green crab has been dubbed the "cockroach of the sea."

For two weeks, the science team travelled to harbours all around Placentia Bay, following a set drill to assess the extent of the invasion. At each site they would put out modified whelk pots, baited with squid, at the wharf and leave them for thirty minutes. While they waited to see whether the pots would attract green crab, team members talked to the local fish harvesters to see whether they had observed any green crabs while working.

If green crabs were found off the wharf, then further methods (including beach seines and divers from Memorial University's Ocean Sciences Centre) were deployed to determine the population. The good news was that they found only small numbers of green crab in other areas in Placentia Bay - for the moment. North Harbour, though, is clearly the epicentre of the infestation with a well-established population, and it is expected that the green crab will spread from there in future years.

The green crab has had a significant impact on fisheries in the Maritimes, but, due to differences in environment such as water temperature, terrain, currents and other factors, its potential impact on Newfoundland and Labrador fisheries has yet to be fully determined. There are concerns, however, about its effect on habitat and commercial species.

The intense, rapid-response fieldwork of the science team, in addition to surveying the population of green crab, began to build a better understanding of how the green crab responds and adapts to Newfoundland conditions. The beach survey showed that the juveniles hide under seaweed and rocks on the beach. Diver surveys showed the males are just offshore and the females farther out. The work also provided a pretty good snapshot of what will be high-risk areas for infestation: crucial elements are a good food supply and a long rocky beach to serve as a safe nursery for the juveniles. "Know the enemy" is the critical first step in any battle.

And the battle is only beginning. Unfortunately, when green crabs appear in a region it is impossible to completely eradicate them, but, because of the early report from Johnson and the rapid response survey, it might be possible to control their population and, thereby, limit the damage they inflict. The 2007 work in Placentia Bay created a baseline for monitoring the spread and growth of the green crab, and similar population surveys will be an essential step in subsequent years.

The survey also raised a range of questions that will shape future research. For example, there is a need to determine the interaction between the green crab and lobster, as there is some concern that the crabs may eat lobster larvae and indications that mature lobster eat green crabs. More needs to be known about its behaviour in the cold waters and long winters of Newfoundland. As well, precise knowledge of the currents and eddies in and around North Harbour is essential to understanding how the green crabs could spread in future years to other points in Placentia Bay.

For the short-term, McKenzie points out that probably the most important legacy of the Placentia Bay work is that it "created quite a stir" locally and raised much-needed awareness about the green crab.

There is no doubt that the communities around Placentia Bay are now going to be vigilant about sightings of green crabs, and they are coming together to figure out mitigation strategies. Already there is talk in North Harbour of "fishing out" the green crab, and enlisting children and community groups to clear out the juveniles on the beach.

In the bigger picture, McKenzie points out how important a "culture of curiosity" is going to be for the future. With aquatic invaders hitchhiking to all parts of the globe, it is critical that people watch for new creatures turning up in their neighbourhoods. As she sums it up, "We don't know what will be coming into our waters in the future that conditions here would let take off and take over."