Image Via Flicker User Mike Licht, NotionsCapital.com

South of Chicago, electrodes stretch across the bottom of a canal that links the Mississippi River with Lake Michigan. They create an electric field—the only thing protecting the Great Lakes from the menace of three species of Asian carp.

The silver, black and bighead carp escaped from aquaculture lagoons in Arkansas in the 1990s and have since been making their way steadily northwards. Along the way, each carp has been vacuuming up 18 kg of plankton per day. Already, they have devastated sport and commercial fisheries in the lower Mississippi. Now they’re threatening similar damage to the US$4.5 billion Great Lakes fisheries.

Fully-grown carp can be 1 m long and weigh as much as 50 kg, too large for most predators to attack. Moreover, silver carp have a habit of leaping out of the water when they are startled, striking boaters and even knocking one unfortunate jet-skier unconscious.

It may sound like the plot of a B movie, but the US government is spending hundreds of thousands of dollars each year on the electric field to keep these fish out of the Great Lakes. If this barrier fails, Asian carp will join nearly 200 other aquatic invaders currently thriving in the Great Lakes basin, a list that includes zebra mussels, sea lampreys, spiny water fleas, round gobies and dozens of other fish, invertebrates and plants.

AN INVASION CRISIS

Aquatic invaders are arriving in the Great Lakes at a rate of one species every seven months, says
McGill aquatic biologist Anthony Ricciardi—the highest rate ever recorded in a freshwater eco-system. They are hitching a ride in the ballast water of ships, escaping from water gardens or fish farms, travelling up canals or being negligently or deliberately released.

While not all the aliens that arrive become established, those that do can wreak havoc in the absence of natural predators, diseases, or competitors to keep them in check.