Grass carp studies divide expert opinions

American researchers have been tagging and releasing both sterile (triploid) and fertile (diploid) grass carp to study them. A female diploid grass carp can contain thousands of eggs, and they are known to be reproducing in the Great Lakes basin. photo by Ryan Hagerty/USFWS

Part II of a series on grass carp in the Great Lakes and the threat they pose to Lake Huron’s wetland ecosystems

GREAT LAKES—It is a seemingly never-ending debate. Fisheries managers know they have to eradicate grass carp as soon as possible, but some contend they do not have enough knowledge to be very effective in those efforts. Call it catch-22, chicken-and-egg or otherwise, an ideological impasse is dividing efforts on both sides of the border.

“We are asking Ohio and Michigan to rethink their study/management strategies and instead implement eradication methods immediately to target and remove all grass carp possible,” writes Mary Muter of the Georgian Bay Great Lakes Foundation in a press release published on July 25. The foundation advocates for preserving the sensitive ecology of Georgian Bay and Lake Huron.

Ms. Muter is on the side that believes conducting any more study efforts is simply wasting time that could be better spent on eradication. Nick Mandrak at the University of Toronto agrees.

“We know what the risk is, and we know what we should be doing with invasions that are found early. The current actions are going against, basically, conventional wisdom for managing invasions,” he says.

“This is what’s been frustrating some of us. They’ve been reproducing since 2012, and I and other colleagues have been strongly suggesting that the first thing we should be doing is eradication and catching them while the population is still low,” says Mr. Mandrak.

Detailed photo of an adult grass carp’s head. Compared to the established common carp, grass carp have no barbels on the sides of their mouths and have a short
dorsal fin along their back. A common carp’s dorsal fin stretches nearly to its tail.
photo by Ryan Hagerty/USFWS

The need to study

The lower populations of grass carp when those decisions had been made meant the fish had been harder to catch. It was not for want of trying, either. Michigan conducted its first major planned action to attempt to catch grass carp in 2014. Seth Herbst, aquatic invasive species coordinator at the Michigan DNR, says immediate eradication was the primary goal at that time.

“We allocated a lot of effort and had 10 agencies and 70 people participating in a week-long event in Monroe, Michigan. We only captured two individuals,” says Mr. Herbst. “One of the lessons we learned was that we had to be more efficient and effective.”

That week-long endeavour had cost the Michigan DNR about $140,000, so seeing an outcome of two fish captured had been less than ideal. Mr. Herbst says the effort essentially used up Michigan’s annual budget for managing grass carp—the state has been setting aside $5 million annually to control all its invasive species since the 2014-2015 fiscal year budget.

Ohio and Michigan DNRs had agreed that, in order to have more targeted impacts, they should embark on a tagging study to track fish movements around the Lake Erie basin. In 2014, they had begun to catch, tag and release some adult fish to explore their behaviour patterns, with a goal of having 50 tagged individuals in the water.

That study has proven contentious for several reasons.

Releasing diploids

First, problems have arisen in the types of fish that have been caught and released. As mentioned in the first part of this series, there are two kinds of grass carp: diploid and triploid. Diploid are regular, naturally occurring fish that are capable of reproducing. Triploid fish are raised in a lab that gives them a treatment during their early development. That treatment causes them to keep an extra, third set of chromosomes which means they are unable to pass along a functional set of genes to their offspring. Triploid fish are effectively sterile.

The tagging study has not tagged and released triploid fish exclusively. In fact, triploids make up an incredibly small portion of the fish that have received transmitters – Mr. Herbst says the agencies only have tagging data for two triploid individuals. His estimates place the total number of fish in the study at 59, meaning only five percent of studied individuals have been triploid. The rest could theoretically be reproducing after the researchers had released them back into the watershed.

Two reasons have surfaced to justify tagging triploid individuals. Mr. Herbst says using diploid fish, since they are of the greatest concern, will provide the most accurate results.

“We don’t know if sterile fish behave the same ways, or know if they have the same movement patterns (as fertile fish),” says Mr. Herbst.

The concern his agency had presented is if triploid fish cannot reproduce, they may not visit the same spawning locations at the same times as regular, diploid fish.

Mr. Mandrak disagrees. “They have hundreds of thousands of eggs each,” he says. “There was an argument that triploid fish may act differently from diploid fish, so the infertile fish might not run upstream to spawn. But even early tagging data suggested that they do move upstream like the others,” he says.

Mr. Herbst says the two triploid individuals have not been able to provide researchers with enough conclusive information, but concedes that “those two fish seem to be moving up rivers similar to what a fertile fish would be doing.” He also says Mr. Mandrak has not been directly involved with these particular tagging studies.

The other reason for using triploid individuals is that currently-used methods of determining whether a fish is fertile or sterile cannot be completed fast enough. Mr. Herbst says grass carp will not survive if they are out of the water for extended periods of time, so tagging and releasing must be done immediately—ideally, within one hour.

“If you don’t do it quickly, the fish becomes stressed and will end up dying,” he says.

That seems to be a reasonable claim. Of the original planned 50 fish, half have stopped transmitting data, leading researchers to believe they have died. Nine more have since been added to offset the losses.

By the current standard technique, determining whether a fish is fertile or sterile (referred to as ploidy testing) can take days to weeks to receive results, because DNA samples must be shipped to external labs for analysis. For organizations with their own testing facilities, such as Fisheries and Oceans Canada (DFO), these tests can be completed in under two hours.

But there are other, faster options.

New methods

In 2015, then-graduate student Katherine Krynak partnered with professor Ronald Oldfield at Case Western Reserve University in Cleveland, Ohio to determine a new way of determining ploidy. It involves observing the nuclear shape of a blood sample under a microscope. It can only be used on live individuals.

Mr. Oldfield says the American Fisheries Society has hosted presentations about the method and it is now a widely-known technique. Because it does not require much specialized equipment, it can be used in the field, such as before fish are tagged and released back into the water. This would enable agencies doing fish telemetry studies to only tag triploid individuals and kill their diploid catch.

“Our method was designed for speed. If you want the information in a hurry (which you would if you were a fish and wildlife enforcement officer), then you want to do the analysis right when you obtain the fish, and those fish are alive,” says Mr. Oldfield.

In a 15-person blind test, participants were able to determine ploidy correctly 98.7 percent of the time, which would mean every person studied five samples each for a total of 75 samples, and one person misidentified one slide. In that case, a triploid fish was mistakenly identified as diploid meaning, in actuality, the presumed fertile fish was sterile.

The U.S. Fish and Wildlife Service (USFWS) does not use this newer, faster method to conduct ploidy testing on farmed grass carp. It instead opts for the established flow cytometry method that uses lasers to test up to 10,000 cells in a couple minutes. However, studies have shown the blood smear technique to be effective, and Mr. Oldfield encourages organizations around the Great Lakes to use it for their field testing.

Ultimately, even, if researchers resolve to only tag triploid fish, they may face great difficulties in finding those individuals in the first place. A study between 2014 and 2016 caught 60 grass carp in Lake Erie whose ploidies could be determined. As Mr. Herbst says, the split between diploid and triploid individuals is “a little discouraging.”

“Roughly 80 percent of fish in Lake Erie were diploid,” says Mr. Herbst. The official number is a little higher, at 86.7 percent. Of the 60 fish, 52 were confirmed as diploid, seven were triploid and one had both diploid and triploid cells. Those numbers confirm that enough individuals already exist in Lake Erie to support a self-sustaining population.

A fishy fate

Capturing 60 individuals over two years may not seem like a high number, but Lake Erie is a large, complex body of water. Therein lies another problem with the tagging study: The fate of a tagged fish is often not known with certainty.

“They say they think the grass carp they inserted transmitters into have died, but in reality, they don’t know,” says Ms. Muter. “They may have lost them, they may have swum into Lake Huron; they don’t know.”

Mr. Herbst acknowledges he is aware of one tagged fish that has swam up from Lake Erie to Lake Huron. He also says if a fish has not been detected for 60 or more days, or if it appears to be sitting in one location for an extended period of time, researchers presume it to be dead. Those are still guesses, albeit based on realistic scenarios. However, equipment failure is also a risk. The transmitters are battery-powered, with an estimated life of six to seven years.

Repeated efforts

Mr. Mandrak is critical of the studies because, he claims, they do not add enough to the existing body of knowledge about grass carp in the Great Lakes. In 2012, the United States Geological Survey (USGS) had published a grass carp spawning model that identified the Sandusky River as a viable spawning ground. That model has turned out to be fairly accurate based on the tagging studies.

Becky Cudmore manages DFO’s Asian Carp Program, and she agrees with Mr. Mandrak.

“While we find the telemetry data interesting, it’s basically confirming what we already knew. So that’s where it stands,” says Ms. Cudmore.

Mr. Mandrak says larger eradication efforts should have been started sooner.

“To me, there was sufficient information to start trying to track down and kill grass carp in the Sandusky,” says Mr. Mandrak. “I’ve worked in the Great Lakes basin for over 30 years. Grass carp is an issue that has stuck with me that is very troubling because we know … what we should be doing with invasions that are found early. The current actions are going against conventional wisdom for managing invasions,” he says.

In defense of studies

Rich Carter, Ohio DNR’s executive administrator for fish management and research, says the studies have provided excellent insights and enabled better work that would have otherwise been impossible.

“Eradicating a fish, in this case grass carp, from a 6.25-million-acre system which is Lake Erie is not a simple process,” says Mr. Carter. “Closing knowledge gaps to understand seasonal movements, spawning locations and life history characteristics is essential if we are to be successful in removing the infestation of grass carp.”

Mr. Herbst says this work is a necessary evil.

“Is it a risk? Absolutely, we understand that it’s a risk, but we think it’s a risk worth taking right now in making us more effective in how we respond,” says Mr. Herbst.

He notes that, although the spawning models have proven to be accurate, their accuracy had not been known at the time. He says the tagging study has now validated that model, growing the body of knowledge about these fish.

“A combination of the work that we’ve done since 2012, the egg findings that USGS and the University of Toledo have been doing since 2015 along with the telemetry project are all starting to help us narrow down locations and timing of where we can be most effective with our targeted efforts,” says Mr. Herbst.

Whether it is because of more knowledge, better tools or simply higher fish populations, one thing is evident: Agencies are catching more fish now than in previous years.

Compared with the week-long 2014 Michigan operation that had netted two fish, Ohio DNR has since led two planned actions in the Sandusky and Maumee Rivers, one in August 2017 and the second in June 2018. The first captured eight grass carp, and this year’s effort resulted in 31.

Mr. Carter estimates efforts at the Ohio DNR are currently set at about 65 percent eradication and 35 percent research. Mr. Herbst estimates Michigan’s efforts to be roughly 15 to 20 percent research. Both have said their agencies will place a greater focus on eradication next year.

This is the second part of a series on grass carp in the Great Lakes. If you would like to learn more about Asian carp and the work being done to manage and understand them, visit or See our website at for further coverage.