In latest discovery, DNR finds four bighead carp in the Wisconsin River near Prairie du Sac

The five adult bighead carp found below the Prairie Du Sac Dam on the Wisconsin River this fall are a stark reminder that Asian carp are not just a threat to the Great Lakes but also to Wisconsin’s thousands of inland lakes and rivers.

The good news is that the pathway up the Wisconsin River basin is blocked by the massive dam on the river about 30 miles northwest of Madison. The dam, opened in 1914, was built to generate hydroelectric power, but a century later it is also proving to be a valuable invasive species barrier.

“I don’t know the exact (height) of it, but it is certainly more than any fish is going to be able to jump over,” said Bob Wakeman, aquatic invasive species coordinator for the state Department of Natural Resources. “And the velocity of water coming through the gates is certainly more than they can swim through.”

Earlier this week the DNR announced that routine surveys for sturgeon below the dam yielded four adult bighead carp. A fifth dead bighead was found on a nearby river bank. Similar bighead finds have happened almost every year since the fish were first found below the dam in 2011.

Wakeman said the fish will be sent off to a lab where their ear bones can be analyzed in a manner that will allow biologists a peek into their life history. That is because fish ear bones carry chemical signatures specific to the waters they previously swam in.

“We can get an awful lot of information from a dead Asian carp,” Wakeman said. “So if people do catch them, they should bring them into a DNR office. … The more we know, the better off we are.”

Four species of Asian carp — bighead, silver, black and grass — were imported to the U.S. in the 1960s and 1970s by the federal government and Southern fish farmers to be used as natural weed and algae control agents. The bighead and silver species, remarkably efficient filter feeders that can decimate plankton populations and overwhelm native North American species by starving them out, escaped soon thereafter and have been migrating up the Mississippi River basin ever since.

In some stretches of river in the Mississippi basin the fish have been estimated to account for up to 90% of the fish “biomass” — for every 10 pounds of fish swimming in the river, nine are Asian carp. The only thing holding the fish back from swimming into Lake Michigan is an electric barrier system on the Chicago Sanitary and Ship Canal, a human-made channel linking the previously separated Great Lakes basin and Mississippi River basin.

Earlier this year, silver carp was found beyond those barriers, about seven miles from the shore of Lake Michigan. Silver carp are considered a particular menace because of their penchant for leaping out of the water when spooked by the whir of a boat motor. This poses a serious threat to recreational boaters and water skiers.

The threat of a Great Lakes invasion has gathered the most political attention, but the fish would also have unfettered access into Wisconsin’s interior lakes and rivers were it not for dams on the Wisconsin and St. Croix rivers.

Wakeman said there is no evidence of silver and bighead carp reproducing in Wisconsin waters below those dams, and said that may be due to the relatively high-quality habitat of Wisconsin water compared with the algae-rich stretches of polluted rivers farther south. This means Wisconsin waters are more hospitable to healthy numbers of native predator fish, and those fish likely play a role in controlling numbers of Asian carp.

“One of the things we can do to prevent ourselves from getting sick is to eat well, sleep well and wash our hands,” he said. “It’s the same with invasive species. If you have a healthy ecosystem, you’re going to be able to resist invasions better.”

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That doesn’t mean there is no risk. Like maintaining good personal hygiene, Wakeman said it is important that anglers keep their bait buckets clean and follow state regulations prohibiting the transfer of bait fish from one water body to another. He noted that native juvenile gizzard shad look remarkably similar to juvenile carp.

So while dams can block the species’ natural migration, an ignorant angler could destroy those barriers with one illegal bait bucket dump.

“People need to be aware of the bait laws,” he said.