2002 October Report of the Commissioner of the Environment and Sustainable Development

October 2002 Report—Chapter 4

Case Study 4.5—The green crab is eating its way up Canada's coasts

The green crab, otherwise known as the cockroach of the sea, invaded the coast of North America at Cape Cod more than a century ago. By the 1950s, it had colonized in the waters of New Brunswick. It is likely that it invaded British Columbia in 1998 through warm tidal currents due to El Niño. The green crab not only preys on native crabs, clams, oysters, and mussels and occupies their habitat but also eats the same food as crabs, lobster, and many seabirds. A single green crab can eat 40 clams in a day. It also carries a parasite that is harmful to the eider duck, whose downy feathers have been prized for generations as insulation and bedding material.

The demise of the softshell clam fishery in northern New England and Nova Scotia in the mid-1950s was associated with green crab. In California the green crab was also blamed for losses of Manila clams as high as 50 percent.

The green crab is aggressively colonizing along Canada's east coast, putting Canada's clam, mussel, and oyster industries at risk. The landed value of Atlantic clams, mussels, and oysters was about $57 million in 2000. The landed value of Atlantic lobster, which scientists believe may also be threatened, was over $500 million in 2000.

On the west coast, the Strait of Georgia is believed to be suitable habitat for green crab. The landed value of native clams and crab in British Columbia was approximately $25 million in 2000. Dungeness crab is the most important commercial crab species in British Columbia. Roughly 222 fishing vessels and their crews rely on it and thousands of crab fishermen from 33 coastal First Nations communities depend on it. Recreational crabbers are estimated at between 10,000 and 20,000.

This omnivorous, aggressive and opportunistic intruder has left native populations of shellfish decimated in its wake.

Photo: Glen Jamieson, Fisheries and Oceans Canada

Source: Dr. Andrea Locke, Fisheries and Oceans Canada

Distribution of the European green crab on Canada's east and west coasts.

Source: Fisheries and Oceans Canada

The undersea plant codium is a threat

Scientists believe that codium smothers native mollusks, interferes with the reproductive cycle of the sea urchin, and drives out eelgrass that is habitat for eel. It also crowds out native kelp, which is prime habitat for lobster and other commercially valued species.

In Canada, codium is found along the coast of British Columbia, including the Queen Charlotte Islands and Vancouver Island. It was first reported on the east coast in Nova Scotia, in the late 1980s, and has since been discovered in the coastal waters of Prince Edward Island.

Codium is thought to have significant impacts on the lobster, oyster, kelp, and sea urchin industries and it may also affect eel.

In 2000 the value of the Atlantic sea urchin industry was more than $7 million. Eel catches brought in about $700,000.

Codium is an invasive form of algae that can cause major devastation to local habitat, affecting native species of kelp, eelgrass, sea urchin, oysters, and lobster.

Source: John Pearse, University of California, Santa Cruz

Distribution of codium in the southern Gulf of St. Lawrence

Source: Dr. Andrea Locke, Fisheries and Oceans Canada