Dead carp washed up on shore on Pomme de Terre Lake in Grant County this spring. Isaiah Tolo, a graduate assistant with the Minnesota Aquatic Invasive Species Research Center, estimated that around 4,000 carp died in the lake due to a virus exclusive to the common carp called Carp Edema Virus. Photo courtesy of Isaiah Tolo

ALEXANDRIA, Minn. — Finding ways to control the common carp in Minnesota lakes has been an issue that researchers have looked at for years. Now, another virus specific to the common carp is killing some of the invasive fish, including in lakes within the Pomme de Terre River system in western Minnesota.

Isaiah Tolo is a graduate assistant from the University of Minnesota Aquatic Invasive Species Research Center funded by the Legislative‐Citizen Commission on Minnesota Resources.

Tolo collected tissue samples from stressed carp from Pomme de Terre Lake in Grant County this spring. The fish were confirmed to have the Carp Edema Virus (CEV) Disease. Carp have been affected by the disease in Lake Christina in Douglas County, along with local waters of Pomme de Terre, Ten Mile and Barrett Lakes.

“On Pomme de Terre, I estimated that there were around 4,000 fish that died in that mortality event,” Tolo said. “That might be a third (of the carp population), or depending on what the density of that population is, it could be a significant fraction of what’s in that lake.”

Researchers are looking at how viruses specific to the common carp could be used against them as a biocontrol method. Carp are notorious bottom feeders that uproot aquatic vegetation. As carp biomass goes up, vegetation goes down and lakes become turbid, reducing the ability for native species to thrive in lake systems.

“The reason I’m studying these viruses is in order to learn more about their ecology and what to expect from them, confirm they don’t impact other natives, and then to look at other ways they can be used as biocontrol tools,” Tolo said. “So ways we can actually instigate natural outbreaks of these viruses to cause declines in ways that we want.”

What is CEV?

Koi Herpes Virus (KHV) is another virus exclusive to the common carp that was first detected in mass mortality events between 1996 and 1998 in Germany, the United Kingdom and Israel. Since then, KHV has been detected in 33 countries, including the U.S.

Tolo and Dr. Nicholas Phelps at the Minnesota Aquatic Invasive Species Research Center have dedicated work to exploring whether pathogens such as KHV could be used to control the common carp without any harm being done to native species.

Carp Edema Virus has more recently shown up on their radar in Minnesota, and it too is being looked at as a potential biocontrol method. CEV has been confirmed in 15 Minnesota lakes, with the first confirmation coming from Cottonwood Lake in Cottonwood County in 2017. It is generally associated with outbreaks in water temperatures of about 48-65 degrees.

CEV can cause high mortality rates in wild carp and cultured varieties, including koi. It does not infect humans. Like Koi Herpes Virus, there is no treatment and other game-fish species have never been shown to be infected with the virus.

“I would say the jury is still out on whether we’ll say it has no negative effects whatsoever,” Tolo said. “Just having a bunch of fish die in a lake can cause balloons of algae or bacteria that could be detrimental to other fish, but as far as the virus actually infecting any of them, I would say it seems really unlikely.”

Isaiah Tolo (pictured) is using his research to determine if viruses specific to the common carp can be used as a bio-control method in the attempt to control the invasive fish in Minnesota waters. Photo courtesy of Isaiah Tolo

Carp infected with CEV generally display lethargic behavior, often congregating around the water surface as they become unresponsive. Carp can show swelling of the body, sunken eyes, and ulcers around the mouth and fins.

“The target tissue is their gills, which is kind of like if we had an infection in our lungs,” Tolo said. “It just makes it hard for them to exchange oxygen…If you were to look at it with a microscope, you’d see their gill tissue is really puffy and there’s a lot more mucus that makes it difficult to breathe.”

It can cause serious mortality events like the one on Pomme de Terre Lake, but the virus is not always fatal to the fish. Tolo said the assumption had been that CEV might not be as pathogenic as other viruses such as Koi Herpes Virus.

“What we’re seeing in Minnesota is definitely starting to call that into question,” Tolo said. “Worldwide, this is becoming sort of an issue that people are realizing that Carp Edema Virus might be a little bit more of a significant pathogen than we thought.”

How does it spread?

So far, Tolo said CEV has only been found in adult carp.

That could be because adult fish are the only ones taking part in mating. Carp’s immune systems are already stressed as they come into close contact with each other during spring spawning. That might allow the virus to spread rapidly and lead to more large fatality events.

“The other possibility is that it does affect recruitment, the younger fish, and we just don’t see them because they’re smaller,” Tolo said. “No one reports a die off of young fish, especially carp, and they may not be available for sampling for a very long time.”

A virus spreading from lake to lake is sometimes attributed to bird species moving fish to different bodies of water. But in a situation like this on the Pomme de Terre River system, Tolo said it is likely that the spread comes from carp moving naturally through the system.

Isaiah Tolo nets a carp at the top of the water during sampling work on Pomme de Terre Lake in Grant County this spring. Photo courtesy of Isaiah Tolo

What’s its impact and what’s next?

To what extent can viruses like KHV and CEV actually knock down a carp population?

That’s the question that researchers are still trying to answer. Tolo pointed to some research done on the upper Mississippi River basin where a decline in the overall carp population has been documented.

“They’ve kind of speculated that maybe that’s due to some of these viruses,” Tolo said. “Which I think is completely possible and may be happening, especially if we have season mortality on an annual basis. I would think that could cause decline in these populations.”

That year-after-year infection is what it’s likely going to take to see population-level impacts. Depending on the lake, carp populations can reach levels in the hundreds of thousands, so a single die-off event will likely not be the trick in controlling the problem.

Tolo said he will continue to survey populations he knows have been infected with CEV. Does the virus stick around? How many fish is it infecting? Does it have impacts on different age classes of fish? Those are all questions future research will try to answer in the quest to see if these viruses can be used as tools in the future.

“I think (the viruses) represent a really nice foothold that we may be discovering in this serious problem of common carp invasion,” Tolo said. “If we can get support from the DNR and support from the public, then this is something we might be able to leverage in order to actually see some improvement in water quality in these infested lakes.”