EDITOR'S NOTE: This story is in response to a reader-submitted question through Open Source, a platform where readers can submit questions to the staff.
FREDERICKTOWN – When Flora Marlatt went to kayak on Knox Lake last Saturday, she noticed something was wrong.
There were dead carp – everywhere. She saw “more than a dozen” in the cove of the lake, near her Shipley Road home. When she ventured out into the lake’s deeper middle, she witnessed more carnage.
“They were everywhere,” she told Knox Pages in an email.
The fishermen she spoke with on the lake that day all noticed the same thing: an unusual amount of dead carp, all around 10 to 15 inches in length, floating near the lake’s north end.
Marlatt shared this information with Austin Levering, the Knox County wildlife officer, who was patrolling the lake that day. She also reached out to Knox Pages to find answers.
We spoke with Levering and Marty Lundquist, the Ohio Department of Natural Resources Division of Wildlife fisheries biologist, this week to answer her question. Here’s what we found.
Process of elimination
According to Levering and Lundquist, this was not an act of malice or human negligence. It was simply nature running its course.
Carp have long been plagued by a deadly viral disease that only rears its head under certain environmental conditions. It’s called “Spring Viremia of Carp,” or “SVC.” It made its way to North America in 2002 after originating in Europe and the Middle East.
The disease is contagious, and it “can cause significant mortality of common carp,” Levering said. It has become increasingly common in Ohio over the past decade, Lundquist added, as outbreaks have occurred in Deer Creek, the Olentangy River, and Hoover Reservoir.
Levering, who has been Knox County’s wildlife officer since 2018, said this is the first time he’s seen an outbreak in Knox Lake.
“Nowhere else in Knox County have I seen it,” he added.
After receiving numerous phone calls and complaints about the dead carp in Knox Lake last weekend, Levering began to investigate. He initially wondered if it was the result of bowfishing, where the fish are speared and sometimes thrown back, but he soon realized that was not the case. There were simply too many carcasses.
“I was getting complaints of 40 to 60 (dead carp)," Levering recalled. "And my thought was, ‘Something is definitely off here.'"
While inspecting the dead carp on the lake’s north end, he realized most were the same size – 10 to 15 inches long. That struck Levering as odd.
During a typical pollution situation, where chemical or manure runoff has poisoned a large number of fish, “you’d see dead fish within all different species and class sizes,” Levering noted.
“But it was just carp and they were all about the same length and size.”
Stumped, Levering reached out to Lundquist, who has worked with the Division of Wildlife for 26 years. He knew right away what the problem was: SVC.
It all lined up. The fish Levering found exhibited many of the standard SVC symptoms: darkened bodies (with pale gills), enlarged eyes, hemorrhaging in the tail gills. Many of the carcasses were found in shallow areas of the lake, which is where researchers say infected carp will gather when suffering from an infection.
Levering concluded this was the cause of death, marking Knox Lake’s first official SVC outbreak in recent history. He could not estimate Friday how many carp have died in the lake from the disease, but he called reports from fishermen of 60 or more fatalities “pretty accurate.”
The path forward
Unfortunately, Levering said, little can be done to prevent the spread of this disease among the carp in Knox Lake – or to prevent future outbreaks.
SVC is largely uncontrollable in lakes and rivers. It is carried in clinically ill fish and asymptomatic carriers, and it is shed in feces and urine, as well as the gill and skin mucus of infected fish.
The virus often enters through the gills, and after a one-to-two-week incubation period, infected carp begin experiencing symptoms. It spreads quickly to the liver, kidney, spleen and alimentary tract.
The mortality rate is high, especially in young fish. According to research done by Iowa State University’s College of Veterinary Medicine, up to 70 percent of infected carp one year old or younger die from this disease.
Lundquist noted that fish immune systems are particularly vulnerable during the spring and early summer months, as they are coming out of spawning.
“Their immune systems are low and they’re weakened…” he said. “So a virus can affect them quite a bit.”
The disease’s ability to spread depends largely on water temperature. The optimum temperature for viral replication is between 68 and 72 degrees Fahrenheit, Levering said. These conditions are impossible to control in an outdoor setting.
“We’re kind of at the mercy of the weather, really,” Levering said.
It’s unclear how the disease reached Knox Lake, but research suggests it won’t be leaving soon. Once SVC establishes itself in a pond, the Iowa State study concluded, “it can be very difficult to eradicate unless all forms of aquatic life at the site are destroyed.”
In wild bodies of water like lakes, where temperature regulation is not an option, there appears to be no way of stopping the spread of the virus. A vaccine has not yet been invented, according to a 2019 analysis by the World Organization for Animal Health.
While Levering lamented the loss of wildlife in Knox Lake, he noted that carp are “not an integral part” of the lake’s ecosystem. Knox Lake – and all of Ohio’s lakes – have large numbers of carp, he said, and they are not a prominent food source for predatory fish.
Lundquist agreed the lake’s food chain would not suffer serious disruption from the loss. Shad are plentiful in Knox Lake, and those are typically the primary food source for predatory fish like largemouth bass.
When it comes to the lake’s fishing scene, Levering said people do target carp because “they’re fun to catch.” Their natural strength provides a challenge for anyone with lighter tackle. But people rarely take carp home and cook them for dinner.
“Some people refer to them as a junk fish,” Levering said. “They’re not designated as a sport fish or a game fish, by our definition.”
Knox Lake is “highly sought-after for its largemouth bass and flathead catfish,” Levering added, and those species will remain relatively unaffected by the outbreak.
Having been around Ohio’s fish population for decades, Lundquist did not seem phased by the recent situation at Knox Lake.
“It’s just kind of a natural thing that it just kind of goes through,” he said of the lake. “I don’t know what percentage of the fish it kills, but it doesn’t kill them all. And it’s just sort of a nuisance, more than anything, because you’re gonna have dead fish, you’re gonna have smell. After a week or so, they sink and that’s the end of it.”
Research indicates SVC is not harmful to humans in any way. Still, Levering recommends practicing proper hygiene when consuming fish from any body of water.