Recent research indicates that even a handful of Asian carp, between just ten and twenty fish, could establish an ecologically damaging population in the Great Lakes. 

From ScienceDaily.com: 

“Published this week in the Biological Invasions journal, research from Professor Kim Cuddington of the Faculty of Science at Waterloo indicates that the probability of Asian carp establishment soars with the introduction of 20 fish into the Great Lakes, under some conditions…

 

‘This species will have a huge impact on the food web,’ says Professor Cuddington. ‘Not only is it a fast-growing fish physically, but the population itself grows very quickly. A female can lay well over a million eggs a year, and with no known predators present in the Great Lakes, the Asian carp could dominate the waters and impact fisheries.’…