About 30 more invasive carp were pulled out of the Mississippi River this week, suggesting that the destructive fish may have established a foothold in Minnesota.
The silver and grass carp were caught in shallow backwaters of the river's Pool 8, near La Crosse, Wis., just above the southernmost lock and dam in Minnesota.
More than 50 of the carp were caught in the same pool last spring, and another 14 were caught earlier this year.
Despite the high numbers, however, it's still unknown if enough invasive carp are above the dam to successfully breed or establish permanent populations in the state, said Ben Larson, invasive carp field lead for the Minnesota Department of Natural Resources (DNR).
All the fish caught were adults, he said.
"I don't think it indicates they're spawning," Larson said. "They're mass spawners that need a lot of fish together to successfully pull it off. That's why we came here to do this and why we're so excited about taking 29 of them away."
For the first time in Minnesota, state and federal agencies worked together over the course of the week to systematically hunt the carp.
The fish have been working their way north up the Mississippi River since the 1970s, when fish farms and sewage treatment managers in the American South brought them over from Asia to clean algae.
Some of the carp can grow to more than 100 pounds and upend native ecosystems by eating up to 20% of their body weight daily. Silver carp, which grow to about 20 pounds, gather in schools and leap out of the water en masse when they're scared, injuring boaters and water skiers.
An increasing number of the fish have been caught each spring in Minnesota.
Blocking river access
The Minnesota and Wisconsin DNRs, the U.S. Fish and Wildlife Service and the U.S. Geological Survey (USGS) worked together to try a relatively new method of fighting carp.
It was developed by the USGS, which spent time learning from commercial fishermen in China who herd the carp into smaller and smaller areas of shallow lakes by directing the fish with nets and noise.
On the Mississippi, the agencies set up block nets to create a series of cells. Boats with electric fishing equipment and underwater speakers then herded the carp into smaller and smaller cells, until they were all pulled up with a commercial seine fishing net.
While carp flee the noise, native fish tend to hide from it, keeping them safe from the herding operation, said Duane Chapman, supervisory fish biologist for the USGS, who helped implement the program.
The method has been used over the past several years in waters in Iowa and Kentucky already overrun with thousands of the invasive carp. This was the first time it was tried in a pool with relatively low numbers of the fish.
"Other than poison, we don't have many tools to keep the number of these fish down below their critical mass," Chapman said. "So this gives us a way to detect them early, and to capture them when they're still at low abundance."
It's unclear exactly how many carp are needed for the fish to start spawning.
"They won't try to spawn until there are a certain amount of pheromones in the water," Chapman said.
"If we can keep the population down, there's a good chance at prohibiting the spawn."
Minnesota biologists long have feared that the carp will move up the Mississippi to the Minnesota River, where conditions are near perfect for them to thrive and breed.
The last and best chance at keeping the carp out of the Minnesota would be to make a stand at a lock and dam near Lake Pepin, said Peter Sorenson, a University of Minnesota invasive species researcher.
Funds urged for study
While every dam on the Mississippi creates a barrier, some are more effective than others, Sorenson said. The dam gates that create Pool 8, for instance, are often left open because of high water, allowing the fish to swim freely upriver.
The gates at Lock and Dam 5 near Lake Pepin, however, are almost never left open. That makes it much harder for carp to cross because they must follow boats and barges during the brief periods the lock is open.
Sorenson is seeking $400,000 from the state's Environmental and Natural Resources Trust Fund to study installing noise, light and air bubble fixtures at the lock to scare carp away when barges are going through. The system has proved to be effective in lab settings.
The fortified barriers, coupled with targeted fishing operations to keep carp numbers low, could be enough to keep carp away from the Minnesota River for decades, Sorenson said.
Greg Stanley • 612-673-4882