QUEBEC — Marine scientists are on high alert for a crab invasion in the St. Lawrence River.

A single Chinese mitten crab, one of the most invasive species in the world, was found in the river near Quebec City this fall. It is the first discovery of the crustacean in Canadian waters where conditions are right for it to thrive, according to some scientists.

The prolific crustacean plays havoc with shore-based fisheries by overwhelming nets. It also digs tunnels deep into fragile riverbanks, accelerating erosion.

Alarmed federal scientists are calling on fishermen and boaters to report any more sightings of the crab which has spread rapidly around the world from China.

"It is very invasive, it can reach incredible numbers in a very short, sudden population explosion," said Yves de Lafontaine, an aquatic ecosystems analyst with Environment Canada in Montreal.

The critter can travel more than 500 kilometres in its four-year lifetime, reaching as far as Cornwall, Ont., if it establishes a breeding population in the St. Lawrence.

Named for the dense patch of dark hair on some of the crabs' claws, mitten crabs live most of their lives in fresh water but breed in salt water, making the St. Lawrence River a potentially perfect home base.

Over several decades, the voracious crawler has moved to England, Germany and California. The crab often travels in the ballast of ships, but scientists in California believe it was intentionally released in the San Francisco Bay.

The crab is a delicacy in China, especially prized for the aphrodisiac qualities of the female crab's ovaries. The crab is secretly served at Chinese restaurants in the Bay area, according to Kathy Hieb, a marine biologist with the state Department of Fish and Game.

The crab was first discovered in California in 1994. By 1998, millions of mitten crabs crowded the bay and the rivers that feed into it. They travelled hundreds of kilometres inland.

The crabs clogged water intake pipes at two major pumping stations that move water to parched areas of southern California.

The crabs are attracted to organisms that cling to manufactured underwater structures, so they often foul up fishing gear, docks and water intakes.

"We also got major complaints from recreational fishermen," Hieb said. "Apparently it is particularly adept at stealing bait."

Hieb said the crab's numbers peaked in 1998. Scientists are trying to find out the cause of recent declines.

The single St. Lawrence mitten crab discovered in September now has a new home in a tank in the Aquarium du Quebec in Quebec City.

The species was also found in Lake Erie in the 1970s. The crab could not establish a breeding population without access to salt water and never caused significant damage.

De Lafontaine said the crab found in the St. Lawrence is probably not the only one in the river, but he does not know if a breeding population has been established.

"It would be unbelievable that we were lucky enough to pull out the only crab in the river," de Lafontaine said.

In smaller water systems, like the Thames River in England, the crab can be fished to control populations. That probably wouldn't work in the huge St. Lawrence, de Lafontaine said.

Hieb suggested the cold water of the St. Lawrence may hinder population growth.

Fisherman Bernard Cote found the crab in his nets on the south shore of the St. Lawrence, across from Quebec City.

Cote normally catches eels for export to South Korea and sets another net to keep track of fish populations for the Quebec City aquarium. The net has trapped 40 species of fish, including sturgeon, catfish and eels.

He was checking his nets at low tide in September when he found his first crab.

"It was a big surprise," Cote said.

Cote said the crab could pose a long-term threat to the future of his fishery, but he is trying not to worry until others are found.

De Lafontaine said the crab could cause problems like another unwelcome foreign species that arrived in the St. Lawrence River and Great Lakes more than a decade ago.

The European zebra mussel upset the ecological balance of the region, clogging pipes, clinging to underwater structures and becoming a toxic food source for freshwater fish.

The mussel has caused billions of dollars in damage, according to U.S. authorities.