Herpes kills thousands of carp in Storm Lake

The koi herpes virus is killing a massive number of young carp in Storm Lake, where they are washing ashore and rotting by the thousands, according to the Iowa Department of Natural Resources.

It’s the first time the virus — officially known as cyprinid herpesvirus 3 — has been detected in Iowa. It is not known to infect humans.

“It’ll probably continue on here for a little while,” Ben Wallace, a DNR fisheries biologist, said of the ongoing fish kill in northwest Iowa. “We can still see schools of stressed-out carp near shore, and all of them are exhibiting signs of the disease.”

The virus was first identified about three decades ago and affects common and koi carp, according to research published by the National Library of Medicine. It has caused substantial financial losses in areas where the fish are raised to eat or for display.

That’s less of a concern in Iowa, where the common carp are most often a nuisance. They can muddy waters when they forage for food and can also reduce the populations of more-desirable game fish.

The disease is very contagious and deadly. It attacks the fish’s gills and can produce lesions on their bodies. Wallace said it is killing mostly young carp in Storm Lake that are up to 2 years old and between 8 and 12 inches in length. Parts of the shoreline are thick with carp carcasses.

“It smells like dead, rotting fish,” he said. “The silver lining is they’re all younger fish, so they’re not very big. It’s those large individuals that take a long time to decompose.”

It’s unclear how the virus arrived at Storm Lake. Wallace said outbreaks have previously been identified in Minnesota and Wisconsin.

Wallace received a report about a week ago that carp were dying en masse at Storm Lake. He captured one of the infected fish and shipped it to the Minnesota Department of Natural Resources, which confirmed the disease on Monday.

The virus doesn’t appear to be affecting other fish species at the lake, so the DNR is taking no action other than monitoring the situation, Wallace said. The disease is unlikely to eliminate the entire carp population there.

“It really hits the population hard but not to the point that it wipes them out,” he said. “They’ll bounce back from it.”

That’s why researchers and others derided a plan by the Australian government — initially dubbed “carp-aggedon” — to deliberately infect carp with the virus in the country’s longest river to kill them. The plan was scrapped in 2020 after research found that the virus has limited long-term effects on carp populations.

Wallace said several people have recently suggested using the infected fish in Storm Lake for similar biological attacks on carp in other Iowa waters. That’s not likely to happen, he said, because it won’t be effective and is ethically dubious.

Future outbreaks at Storm Lake that result in mass carp deaths are possible, he said. The outbreaks are usually brought on by environmental stresses, such as extreme heat.

The smelly predicament at Storm Lake is being compounded by a recent algae bloom that is unrelated to the dead carp, Wallace said. DNR tests of the lake water last week did not reveal detectable amounts of algae toxins that are harmful to people. Test results for this week have not yet been revealed on the DNR’s beach monitoring website.