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Killing more carp: can this destroyer of waterfowl habitat be eradicated?

MALHEUR NATIONAL WILDLIFE refuge in Oregon was once a thriving marsh that attracted ducks by the tens of thousands. It's now an ecological desert of open, murky water that supports little aquatic life. Except carp. Less than 10 percent of the refuge's suitable wetland habitat remains and waterfowl production on the refuge is down even more. Over the last 70 years, managers have tried to remove the troublesome intruder. As many as 7.2 million pounds of common carp have devastated the refuge's three lakes.

"I'm a hard core duck hunter and I wouldn't hunt here," says refuge deputy project leader Jeff Mackay. "We have very few birds in the winter. On a scale of one to 10, I'd say the carp impact has been a 10. The entire ecology of the lakes has been destroyed."

Common carp inhabit 49 states and leave an equally dramatic trail of destruction in shallow water everywhere. A study conducted on two Illinois lakes found that carp reduced aquatic vegetation by nearly 90 percent and the number of waterfowl that visited Hennepin and Hopper lakes showed an equal decline. Minnesota DNR waterfowl staff specialist Steve Cordts says hundreds of lakes have a significant carp problem in his state. Many of them no longer attract ducks. Or duck hunters.

Non-native, highly adaptable and tolerant of poor water quality, a single common carp can lay a million eggs at a time. They were brought to the U.S. around 1850 from Europe and Asia as a high-quality, farmable food source and a perfect substitute for dwindling populations of native fish. Not surprisingly, carp made their way into waters all over the country, but wild carp had a decidedly different flavor than farm-raised carp and many of the native fish species. Nobody ate them. As they spread to more waters, sportsmen looked upon them with disdain, not only because they were a poor food source, but for what they were doing to the waters they inhabited. The fish root up the bottom in search of invertebrates and vegetation, stirring up sediment as they go. That sediment blocks sunlight, a necessary ingredient for submerged aquatic vegetation.

As the grass disappears, so do the ducks that feed on it and the bugs that live in it. It doesn't take a scientist to see that, but plenty of them have examined the impact of carp on waterfowl, including Dr. Przemek Bajer, a research assistant professor at the University of Minnesota. He led the study that followed the return of carp to Hennepin and Hopper lakes after an eradication effort failed. The lakes were drawn down and then treated with rotenone, a non-selective piscicide (fish poison). A barrier was also erected between the lakes and the Illinois River.

"Water clarity improved, grass returned and lots of ducks returned. Then the carp somehow got back into the lake and the water quality declined, the aquatic vegetation disappeared and so did the ducks," he recalls.

It's a pattern that's been repeated wherever carp removal efforts have been attempted and failed. The carp are removed or reduced, grass coverage spikes, ducks flock to the grass, carp return and everything spirals downward.

"It's shocking how well they can move from one system to another. They'll even swim through drain tiles buried underground to get from one lake to another," says Cordts. "Many of our lakes are connected by sloughs or ditches during periods of high water, so eradicating them from one lake is often only temporary. It takes between just five and 10 years for carp to rebuild their numbers enough to have a detrimental impact."

Despite ongoing control efforts dating back to the 1950s, carp keep rebounding on Malheur. Their numbers are down due to a lengthy drought, but Mackay says abundant water from last winter will likely result in a carp boom in the near future.

"We will probably never get rid of them. We've tried a lot of techniques, but so far nothing has worked. We are considering building a series of levees, but that idea is controversial," he says.

Eradication efforts can work in smaller, closed systems. Bajer worked on several projects in Minnesota where the lake was lowered and the surviving fish poisoned. It's a lengthy and sometimes expensive process, but in the right situation, it can be effective.

"We've learned a lot about common carp and efforts to control them, but there are still a lot of things we don't know," says Bajer.

Even less is known about the long-term impact of America's newest carp problem. Bighead and silver carp are best-known for their tendency to leap from the water when spooked. Search "jumping fish" or "flying carp" on YouTube and you'll be greeted with countless videos of massive schools of carp jumping as a boat runs across the water.

The only good news, says U.S. Geological Survey fishery biologist Dr. Matt Neilson, is that silver and bighead carp are filter feeders that primarily eat plankton. They don't root on the bottom or eat aquatic vegetation or invertebrates as much, which means they likely aren't having as direct an impact on ducks.

"They are mostly found in river environments, although they can do well in lakes," says Neilson. "The fact they eat plankton means they probably aren't having much impact on waterfowl habitat."

He says researchers are working to develop a selective piscicide that only affects larger filter feeders like silver and bighead carp.

"It's still very early, but it does show some promise," he says.

Scientists are also developing a toxin aimed at common carp, according to Bajer. It is delivered via a pellet that native fish won't eat. Other control methods have been fine-tuned, as well.

"We can actually train carp to come to a specific area just by feeding them. Once we get a large number to congregate, we can net them and remove them from the system. We also are using implanted GPS transmitters to locate fish in the winter. They gather in large schools in some waters, which means if we can find one, we likely found a large number," says Bajer. "We are probably never going to completely eradicate them, but we are getting better at controlling them."

He added that if their numbers are kept low, common carp can coexist with waterfowl. The threshold seems to be around 25 pounds per acre.

"I've seen it around 450 pounds per acre, which seems to be their maximum carrying capacity," adds Bajer.

Native fish like bluegills and perch are also somewhat effective at keeping common carp numbers in check in some waters. A single bluegill can eat thousands of carp eggs or fry. However, shallow lakes don't support many native fish because oxygen levels sink in the winter. Managers have found that aerating lakes helps native fish survive, which means carp numbers are held at a manageable level. Aerators cost money, though, and with common carp in so many lakes, it's impossible to treat every one. In other words, carp are here to stay.


David Hart


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Author:Hart, David
Date:Sep 29, 2017
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