This was just on NBC news, I will look for a link, but here is the story as told on the news.
If you remember the snakehead fish that was found in the lakes outside of Washington DC, well apparently another "alien" fish has been identified and its actually much more dangerous than the snakehead. The Asian Carp, was originally imported to clean up the mud at the many catfish farms around the Mississippi river.
However, the fish has apparently escaped and is THRIVING in the Mississippi. The fish has a perculiar habit of jumping out of the water when it hears the sound of motor boats and frequently hits the people in the boat. The fish has multiplied and is destroying the ecosystem by eating all of the food that the natives usually eat. So far it has gone as far north as the Illinois river and is less than 15 miles away from the Great lakes. If it gets into the great lakes, it is likely that the entire ecosystem will be destroyed.
They are protecting the lake by use of an electrical field which the fish normally wont cross, however they have already found one in the lake.
Here is one article, but not the thing I saw: