A commercial angler working on the St. Croix River at Prescott recently captured a silver carp. The non-native fish is infamous for its tendency to leap out of the water when disturbed by the sound of boat motors.
Several bighead carp, another non-native invasive species with the potential to seriously harm the St. Croix, have been caught as far upstream as Bayport. Those fish do not jump out of the water. Both species feed on plankton, potentially competing for food with native mussels, minnows, and other adult fish, including paddlefish.
Silver carp can jump up to 10 feet out of the water, often flying into boats and injuring boaters and water skiers.
The DNR noted that silver carp have previously been captured nearby in the Mississippi River, and it’s possible the fish just moved into final reach of the St. Croix temporarily.
“The location where the carp were captured is a well-known over-wintering area for several species of fish,” DNR invasive fish coordinator Nick Frohnauer said. “At this time, it is hard to predict if these individuals would have moved further upstream the St. Croix River, or back into the Mississippi River when water temperatures warm up in the spring.”
There is little scientific information on how to detect reproducing populations of invasive carp. In Minnesota’s Invasive Carp Action Plan, managers said any capture of juvenile carp would be considered sustainably reproducing — such fish have not yet been caught in the St. Croix or the upper Mississippi, and the closest that reproduction is believed to be happening is in the Mississippi River between Iowa and Illinois.
The announcement reinforces calls for aggressive action to keep carp from infesting the St. Croix River. Because there is nowhere on the lower St. Croix that would work for a carp barrier or deterrent, efforts are focused downstream of the confluence with the Mississippi River.
Recently, the the Minnesota Aquatic Invasive Species Research Center at the University of Minnesota has installed sound deterrents at Lock and Dam No. 8 downstream of La Cross, Wis. The state’s action plan calls for exploration of other systems, including adjusted gate operation, installation of deterrent measures in locks, and evaluating high-water bypasses to reduce the ability of Invasive Carp getting by the structures.