In this Jan. 5, 2006, file photo, round gobies swim in an exhibit of the kinds of invasive species threatening the Great Lakes ecosystem at Chicago's Shedd Aquarium. (M. Spencer Green/Associated Press)Invasive species that have reached the Great Lakes in ballast tanks of oceangoing ships may be costing the surrounding region in the U.S. about $200 million a year, American researchers said Wednesday.

The scientists from the University of Notre Dame and the University of Wyoming limited their study to the economic impact in eight U.S. states surrounding the Great Lakes, but said Canada has also suffered similar losses economically from invasive species.

The study says 57 of the 84 invasives that became established in the lakes after the St. Lawrence Seaway opened in 1959 were transported in ballast water. Among them are zebra mussels and two fast-spreading fish, the Eurasian ruffe and the round goby.

The environmental impact of these species has been well documented. Zebra mussels, for example, consume large amounts of phytoplankton, reducing the food supply for many species of zooplankton, which are important sources of food for smaller fish. The decline of these smaller fish has a negative impact on larger fish further up the food chain, such as adult salmon, trout and walleye.

Likewise, fish like the Eurasian ruffe and the round goby have been able to outcompete smaller fish such as whitefish and yellow perch for food, often displacing these species and again disrupting the populations of salmon, trout and walleye.

Zebra mussels are also known to filter more contaminants out of the water, but animals that consume the mussels are then likely to carry those contaminants with them.

The authors attempted to link these known impacts to economic indicators for sport and commercial fishing, wildlife viewing and use of water for municipal systems and industry.

Sport fisheries were the hardest hit, the study found.

The authors estimate that the annual losses to sport fishing in the eight states bordering the Great Lakes at more than $123 million annually, while commercial fishing losses amounted to just $2 million.

Impacts on commercial fishing were based on the reductions in weight of commercial harvests attributable to invasive species, while sport fishing was measured based on reductions in the number of person-days spent sport fishing.

University of Windsor biology professor Hugh MacIsaac, the director of the Canadian Aquatic Invasive Species Network, said the economic numbers make sense given his group's own work studying the problem.

He said while no Canadian numbers are available, it's likely the direct impact in Canada would be similar, although the losses here would be greater for commercial rather than sport fisheries, since commercial fishing is more prominent in Canada.

Reductions in wildlife watching — measured by the drop in paying visitors to wildlife refuges — accounted for $47 million in annual losses, the authors said.

Invasive species can also impact water systems, the authors said, as they can clog and damage intake pipes. The 826 facilities in the region reported added costs of $27 million.

MacIsaac said while the study looked at direct losses, it doesn't take into consideration the full costs of the impact of invasive species, only those costs that are readily quantifiable.

"There are reports of waterfowl dying in the U.S. as a result of changes in the Great Lakes, but how do you put an economic cost on a dying loon?" said MacIsaac.

With files from the Associated Press