Brule Headwaters.jpg

U.S. Army Corps of Engineers map showing the Brule Headwaters area of northern Wisconsin. The red line marks the divide between the Great Lakes and Mississippi River basins.

(Courtesy | USACE)

BAYPORT, Minn. -- Invasive species specialists say there's little chance of Asian carp reaching Lake Superior via waterways in Minnesota and Wisconsin despite the invader's continued push up Mississippi River tributaries.

On June 2, the Minnesota Department of Natural Resources announced the capture of five bighead carp in the St. Croix River near Stillwater, about 7 miles farther north than the last place the invasive fish has been detected in the river, part of which marks the Wisconsin-Minnesota boundary line.

The discovery highlights the species' quiet push into other areas along the basin divide between the Great Lakes and Mississippi River watersheds besides Chicago, where attention on the carp's approach to Lake Michigan has been focused.

To reach Lake Superior via the St. Croix River, the fish would need to hurdle a 20-foot hydroelectric dam and make its way through a northern Wisconsin wetlands complex that marks the headwaters of the Brule and St. Croix rivers, say officials.

"While there may be water that could flow between the two basins, it's a very interstitial connection," said Bob Wakeman, water resources management specialist for the Wisconsin Department of Natural Resources.

In the Great Lakes and Mississippi River Interbasin Study, the U.S. Army Corps of Engineers included the Brule Headwaters as one of 18 other aquatic routes the fish might use to reach the Great Lakes other than through Chicago area waterways.

The Army Corps lists the Brule Headwaters as a "medium probability" pathway for transfer of aquatic nuisance species (ANS) between the two basins.

Bottom line, said Wakeman, is the water that connects the two basins at the Brule Headwaters is not something that would support natural carp transfer.

"For them to move without human assistance is very unlikely, if not impossible."

Officials are more concerned with fish hitching a ride to the Great Lakes in a recreational boat's bilge water, or through the purchase of live bait from other carp-infested regions.

Wakeman said the DNR stations officers at Lake Superior boat launches to talk to anglers and inspect boating equipment.

"If anglers do catch a fish they suspect is something unusual, don't throw it back," he said. "Take it into a DNR office to have it identified."

That's how the recent bighead carp were discovered in the St. Croix, he said.

According to the Minnesota DNR, there is no evidence bighead or silver carp are reproducing in Minnesota's part of the Mississippi or St. Croix rivers, although bighead and silver carp populations are established elsewhere in the Mississippi River.

There's currently no evidence of Asian carp in Michigan waters, according to the Michigan DNR, but the agency similarly relies on anglers to be its eyes and ears.

The carp are considered an ecological threat to native ecosystems and an economic threat to boating and sport fishing industries in waters where they are established.

Bighead, black, grass, silver and large-scale silver carp are the five species of Asian carp that are illegal to possess, transport life or stock in Michigan. Bighead and silver carp are considered to be the species of primary concern.

In May, the U.S. Geological Survey said that if Asian carp were to reach Lake Erie through the Ohio River basin, green and blue-green algae in the lake would provide an abundant food source for the fish population to thrive on.

Garret Ellison covers business, government and environment for MLive/The Grand Rapids Press. Email him at gellison@mlive.com or follow on Twitter & Instagram