The US Supreme Court has refused to hear the Great Lakes states' appeal to close shipping locks to stymie the on-going incursion of Asian carp.

Michigan, Minnesota, Ohio, Pennsylvania, and Wisconsin are suing the Army Corps of Engineers to provide greater protection to prevent the fish from entering the Great Lakes. While this suit continued, the five states sought an injunction to have the Corps close locks on waterways that connect the Mississippi River with Lake Michigan.

The federal government said that the efforts proposed by these states would detract from the long-term strategy of the Corps. This is the third time such an appeal has been rejected.

The EPA currently classifies the Asian Carp as an invasive species. The Corps and the concerned states agree that the carp are a serious threat to the ecology of the Great Lakes area, as well as its $7-billion sport-fishing market. The disagreement pivots around whether the current schemes in place are enough.

In 2002, The Army Corps of Engineers installed an electric-current barrier in the Chicago Sanitary and Ship Canal, the only navigable link between the Great Lakes and the Mississippi River drainage basins. The Great Lake states, evidently unsatisfied, have proposed that this link be permanently destroyed.

More recently, in early 2009, when the electric barriers were deactivated for maintenance, the Illinois Department of Natural Resources dumped 2,200 gallons of rotenone – a toxin to fish that is said to be harmless to humans – into the canal. The $3 million action produced 90 tons of dead fish, among which only one Asian carp was found.

Given the extreme measures being taken, you might wonder what it is about these carp that makes them so terrifying?

First, let's define the fish in question. 'Asian carp' is a catchall for four distinct species: the Bighead, Silver, Grass, and Black carp. Though their names might signify otherwise, these species do closely resemble each other, tending to have the same lurid tarnished-silver scales. The four species vary in size, but are large compared to native American freshwater fish. They can weigh anywhere from 60 to 110 pounds, and range from 40 to 60 inches in length. All four are known to inhabit the Mississippi River Basin, which eventually connects to the Great Lakes.

In the Mississippi and other American waterways, Asian carp "have left a trail of tremendous destruction," says Charlie Wooley, deputy regional director for the Fish and Wildlife Service, Minneapolis. Wooley says the carp's previous activity in other environments demonstrates its ability to "literally take over an ecosystem."

Wooley told the Monitor of the carp's two major threats. The first is a food problem. Asian carp don't eat other fish, but because of their voracious appetites (consuming up to a third of their body weight per day) they could easily out-compete native fish that rely on specific sources of food. Each type of carp prefers a different food – varying from grass to plankton to snails and mussels –making their attack on the ecosystem somewhat multi-pronged. Moreover, these sources lie at the bottom of Great Lakes' food chain. Changes at the foundation tend to reverberate through the entire ecosystem.

Then there's the Asian carp's fecundity. Female Bighead carp, for example, can carry up to 1 million eggs in a lifetime, much more than most native fish. They also reproduce rapidly, says Wooley. Once introduced, the Asian carp is difficult to stop.

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If the Asian carp does take hold in the Great Lakes, the ecosystem will no doubt do what ecosystems do best: adapt. After all the term "invasive species" is, by definition, relative, often marking a transitional phase as a species establishes itself in a new ecosystem. For instance, most earthworms in the United States are descended from those transported overseas by humans. But today, earthworms are widely regarded as ecologically essential.

Whether the economy adapts to the Asian carp, however, remains to be seen.