Asian carp species of fish are considered to be invasive in North-America. Because they haven't evolved with the local ecosystems, there's no good natural mechanisms to balance their populations, and so they threaten native fish species.
Things were already scary enough, but scientists have made a new discovery that could complicate things further:
[Scientists] have documented for the first time that an Asian carp species has successfully reproduced within the Great Lakes watershed. [...] An analysis of four grass carp captured last year in Ohio's Sandusky River, a tributary of Lake Erie, found they had spent their entire lives there and were not introduced through means such as stocking, according to researchers with the U.S. Geological Survey and Bowling Green State University.
Grass carp are among four species imported from Asia decades ago to control algae and unwanted plants in controlled settings such as sewage treatment lagoons. They escaped into the wild and have spread into the Mississippi and other rivers and lakes.
Asian carps can be very fast breeders and eat large amounts of plankton. They could out-compete the locals and put significant pressure on their survival.
Because all sub-species of Asian carps require similar conditions to breed, this finding means that they could all potential reproduce in Great Lakes tributaries, including the most worrisome species, the bighead and silver carps.
A few years ago, scientists believed that perhaps two dozen rivers in the Great Lakes watershed offered good spawning habitat. But the grass carp analysis and other recent findings suggest the number may be considerably higher [...]
The Obama administration has spent nearly $200 million to shield the lakes, focusing primarily on an electrified barrier and other measures in Chicago-area waterways that offer a pathway from the carp-infested Mississippi River watershed to Lake Michigan. Critics say more is needed and are pressing to physically separate the two systems.