TORONTO — A worrisome fish discovered in Lake Ontario has again raised the spectre of invasive species in Canada as authorities across the country struggle to keep various imported pests at bay.

Toronto conservation officials doing a routine survey of the fish population at the mouth of the Don River recently discovered a grass carp. The large, grey fish with black-edged scales is one of four invasive Asian carp species authorities fear could upset the delicate ecology of Canada's Great Lakes.

"All those things we value most about the Great Lakes could and would change if invasive species continue to become established," said Nicholas Mandrak, a research scientist with the federal Department of Fisheries and Oceans.

None of the four Asian carp - grass, bighead, silver and black - are believed to have become entrenched, but authorities are on alert for a full-blown assault.

"It's a lot easier to deal with invasive species by preventing them from being introduced in the first place because it's much more difficult to control them after they've become established," said Mandrak.

Ballast-water monitoring and an electrical barrier that repels fish in a canal connecting the Mississippi River to Lake Michigan are just two initiatives in place to protect the lakes against invasive species, he said.

Conservation authorities hope this latest grass carp was on its own, perhaps released into an area waterway by would-be diners who purchased it at a local market but opted not to eat it.

The long-term effects of invasive species on the native environment are impossible to predict, authorities say, but there have already been some awful surprises.

In the Great Lakes, the zebra mussel is being blamed for a fatal botulism outbreak that has swept through waterfowl species. Early research has indicated that birds pick up the toxin when they eat fish that feed on the infected, fingernail-sized mollusks.

The mussels are believed to have arrived from foreign waters as stowaways in the ballast water of seafaring freighters.
"Undoubtedly there will be additional negative impacts that we have not foreseen," said Mandrak.

Grass carp have been discovered previously in the Great Lakes, the first in 1985 in Lake Erie. Commercial fishing nets have since captured three others in 1989 and 1998 in Lake Huron, but each is believed to have been an isolated occurrence.

Several bighead carp have also been discovered in the Canadian waters of the Great Lakes; the black carp and silver carp have yet to make an appearance.

Even the seemingly innocuous goldfish, a type of carp, is making a mess of our native water systems.

"We couldn't guess, not realistically, the damage these species are doing," said Beamish. "It's huge."

The grass carp, for example, has an aggressive habit of ripping through underwater vegetation and reproducing quickly, threatening the food and habitat of native wildlife.

And various agencies now spend $25 million to $30 million each year to control sea lampreys, said Bill Beamish, a University of Guelph professor. The lampreys, known for clinging to other fish and literally sucking the life out of them, originally arrived in Canadian waters in the 1960s through the newly opened St. Lawrence Seaway.

Canadian waters aren't the only trouble spot.

Across the country, authorities are shredding native forests in an effort to contain multiple-front invasions of such pests as the Asian longhorn beetle, the emerald ash borer and the brown spruce longhorn beetle.

Of the 441 species listed at risk by the Committee on the Status of Endangered Wildlife in Canada, about two-thirds can attribute their decline to invasive species, said Mandrak.

When grass carp are raised for live sale at markets they are typically sterilized, but experts caution the procedure is not always successful. A breeding pair released into the water system could spell disaster.

The species is widespread throughout the United States, the result of accidental escape from aquaculture and intentional stocking. Grass carp were also introduced in Alberta as a way of controlling aquatic plants and escaped into the wild.

There are about 26 non-native fish species in the Great Lakes, according to a 2001 study by McGill University researcher Tony Ricciardi. Mandrak said the Canadian waters of the Great Lakes are only home to about 100 different native fish species.