Bighead and silver carp imported from Asia decades ago have infested the Mississippi River and many of its tributaries.

WSJ: Study: Asian carp unlikely to stow away in barges

Bighead and silver carp gobble enormous volumes of plankton, a crucial link in the aquatic food chain, while silver carp sometimes collide with boaters by hurtling from the water when startled.

WSJ: Asian carp spawning areas wider than expected

"These results support the conclusion that bighead and silver carp have not yet become established in Minnesota, " said Steve Hirsch, director of the Ecological and Waters Division for the Department of Natural Resources.

WSJ: Study finds Asian carp numbers low in Minnesota

Croix rivers found little evidence of bighead and silver carp, according to a study released Thursday that concludes that while the invasive fish are present in Minnesota, their numbers are likely relatively low.

WSJ: Study finds Asian carp numbers low in Minnesota

Of particular concern are silver and bighead carp, which gorge on plankton microscopic plants and animals that virtually all fish eat at some point.

WSJ: Report: Asian carp may have reached Great Lakes

He defended Illinois' efforts to block the advance of silver and bighead carp toward the lakes by hiring commercial fishermen and operating an electric barrier, but acknowledged more needs to be done.

NPR: Gov. Quinn Open To Great Lakes-Mississippi Split