Fish Cannons, Koi Herpes and Other Tools to Combat Invasive Carp

June 04, 2019

Why is someone loading a fish into a tube?

That’s Whooshh. It’s a high-tech fish removal system, something like a cross between a potato gun and a pneumatic tube at a drive-in bank.

And that fish is a common carp, one the oldest and most invasive fish on the planet.

Cyprinus carpio, the common carp, is a close relative of the goldfish, native to Eurasia. It has been farmed throughout Europe for about 2,000 years. In the 1880s, the federal government brought over more than 300 carp to the United States from Germany, at the request of recent European settlers. They bred the fish in ponds in Washington, loaded their offspring onto rail cars, sent them across the country, sometimes dumping them directly into lakes and rivers.

Around 1910, it was clear this was a mistake. Common carp are tolerant of poor water quality. They travel miles to spawning grounds. And they can live up to 50 years, especially without the limits of pathogens, predators or weather from their native ranges.

Where they invaded, lakes turned brown and muddy. Plants and ducks disappeared. They displaced other fishes and dominated.

They’ve thrived in the nutrient-rich waters of the Midwest, where they can re-engineer entire ecosystems. When they forage for bugs and seeds on lake bottoms, they stir up sediment, uproot plants, change water chemistry, promote algal blooms and leave little food for waterfowl.

So for the past decade, researchers at the Minnesota Aquatic Invasive Species Research Center have been searching for the most effective ways to remove common carp from lakes, ponds and marshes where they do the most damage. Last summer, the team implemented a comprehensive common carp control campaign, including year-round strategies that take advantage of the fish’s life cycle. There is no one-size-fits-all solution, they’ve found, and some approaches work better at certain times and places.

“In some it’s very easy to control, and in some, it’s very, very challenging,” said Przemek Bajer, a common carp researcher leading the Whooshh team. “Once you see the life cycle, it becomes much, much easier.”

Learning from failures and successes, the team hopes wildlife managers elsewhere may develop removal strategies fine tuned for their own invasive species, ecosystems and geography. Here are some tactics they’ve been testing.

During the spring spawning season, tens of thousands of adult carp migrate miles through lakes and streams to marshes where they were born. If conditions are right, a single carp can produce 2,000 babies a season. But sometimes natural predators gobble up their eggs and larvae.

The team has done many experiments to better understand this natural removal method. In one, they dangled carp eggs attached to green yarn in different lakes. In waters dominated by voracious bluegill, the eggs were gone the next day. But with no bluegills, they survived.

Dumping bluegills in carp nesting grounds is impractical. So the researchers are looking at ways to help more bluegills survive winter conditions that deplete oxygen in their waters. That way, they might be alive and ready to dine when carps show up to spawn during the spring thaw.

The team is also testing which barriers might keep surviving juveniles from dispersing from their nurseries, and block adults from spawning sites in the first place. Electric guidance systems are promising.

These deployable underwater fences produce small electric fields that guide the fish to traps. When tested last April, they helped remove more than a third of the 15,000 carps in one lake.

From late summer to the start of autumn, actively feeding common carp can also be made to fall for a classic bait and switch. Researchers train carp to swim to large nets baited with food on lake bottoms. As long as some fish find it, the rest follow. All are then scooped up.

In November and December, shallow lakes start freezing. The carp gather in large groups beneath thin ice. Then, they’re at the mercy of what researchers call “the Judas technique.”

Because common carp play follow the leader, researchers can catch and tag a few fish in the fall, then follow their signal to locate the rest in winter. The scientists can swoop up most fish at once with special nets. This requires careful strategy, because after a few escapes, the fish learn to avoid the nets.

This summer the team is testing a bait and switch/Judas combination, tagging and training many carp to find food placed by sensors for gathering behavioral intelligence. This can help them target the carp that lead others in later experiments or removals.

They’re also experimenting with toxins and pathogens.

For decades, a poison called rotenone has been used to control carp, but it basically wipes out all fish. As an alternative, the team is looking to another toxin, Antimycin A, to poison corn pellets, which only carp will eat in their waters. They’ll train the carp to eat clean corn, then swap in poisoned niblets.

This has worked in test ponds and laboratories. But as long as fish gather around food, a net is cheaper, nontoxic and just as effective.

There’s also naturally-occurring Koi Herpes Virus (KHV) that appears to only affect common carp and their ornamental variants, koi. Infected fish shed KHV into the environment, where it spreads by close contact. It damages gills and can lead to suffocation, killing up to 90 percent of a population.

Nicholas Phelps, a veterinary pathologist studying KHV, is studying how and when the virus spreads, and hopes to better understand whether the disease could be used safely to control the common carp.

“I understand this sounds like a scary idea, releasing viruses to kill fish,” Dr. Phelps said. But with cautious and incremental testing, he thinks that, “at some point, perhaps it’s not a scary idea anymore.”

In the meantime there’s Whooshh, the fish cannon. It was originally developed to help spawning salmon traverse interruptions in their migratory path, but these researchers hope it will cut back on laboriously netting large groups of trapped carp.

They’ve been trying to cajole the fish into independently swimming one-by-one into the Whooshh, but common carp are smart, stubborn, cautious and sometimes coy. Unlike salmon, they “don’t want to swim into the carp vacuum,” Dr. Bajer said. Right now, it’s just a transport system.

After netting the fish, they are loaded into the Whooshh, one-by-one, to be slurped down and chucked up into a container. They’re then euthanized and made into compost or feed for bears and wolves at wildlife centers.

Dr. Bajer wonders if the fish will ever feel comfortable entering a fish vacuum voluntarily: “It might be possible, we just haven’t cracked it yet.”