While it’s too early to tell if researchers have turned a corner in their ongoing efforts to remove invasive grass carp from the Maumee and Sandusky rivers, they’re encouraged that they aren’t yet finding as many of the nuisance fish as they have in past years.
So far, only 85 grass carp have been caught. Annual catch numbers range from about 150 to 200 grass carp.
The fish are caught by five strike teams assembled by the University of Toledo, with help from the Michigan Department of Natural Resources, the University of Buffalo, the Ohio Department of Natural Resources, the U.S. Fish and Wildlife Service, and U.S. Geological Survey.
They put in hundreds of hours a year scouring area waterways and physically removing as many grass carp as they can.
But Christine Mayer, a UT Lake Erie Center aquatic ecology professor involved in planning and executing the catches, said she wouldn’t put too much stock in reduced numbers yet.
She and Robert Mapes, a UT Lake Erie Center field research crew coordinator, said it’s too early to tell if they’ve turned a corner.
“Every year is different,” Ms. Mayer said. “It would be encouraging and happy to say that with the same amount of effort if we caught fewer fish. We just won't be able to make a conclusion after one year, though. One year is not a trend.”
There’s a reason for some optimism in the sense that about 70 percent of the grass carp captured each year are taken during the spring spawning season, Mr. Mapes told journalists at Ohio State University’s Stone Laboratory during an annual two-day workshop for science writers.
Having caught only 85 fish by now could mean that the year’s total will be fewer than those in years past.
But if it ends up being fewer than 150 fish, is that the beginning of a positive trend?
“That’s what we don’t know,” Mr. Mapes said.
As Ms. Mayer points out, August is usually pretty slow for catches because of its warmth. While the crews seem to be finding fewer grass carp this year, she notes that only two years ago, in 2020, crews caught a surprisingly large number of fish in October. The operation typically goes into early November before knocking off until the following spring.
Equally as important is what wasn’t found: grass carp eggs.
This year, crews expanded their search for them. They went into the Cuyahoga, Huron and Grand rivers flowing into Lake Erie; the St. Joseph River flowing into Lake Michigan, and the Tittabawassee River flowing into Lake Huron, Mr. Mapes said.
None were found, which appears to mean that the only rivers the invasive fish are spawning now are the Maumee and the Sandusky, which could make confinement and control efforts easier to manage, he said.
Or, as Ms. Mayer added with a word of caution, that could be anecdotal.
“It doesn't mean it's not happening,” she said. “But we've looked, and we're not finding them.”
Grass carp is the only one of the four so-called “Asian carp” now reproducing in the Great Lakes. Researchers prefer to call them invasive carp now, and efforts continue to keep the other three — the silver, bighead, and black — invasive carp out of the lakes. All four impact habitat and food supplies for native fish.
Grass carp were brought into the region to control aquatic vegetation. But the original ones were sterile and, thus, didn’t reproduce. In 2012, grass carp capable of reproducing, called diploids, were found in the Sandusky River, Mr. Mapes said.
Larvae from those fish were found in the Maumee River in 2018.
“They can eat their body weight in plants daily,” Mr. Mapes said, explaining how grass carp often grow to become 35 to 55 pounds. “They eat plants and damage wetlands. Unchecked, the grass carp population could really explode.”
The fish are caught with specially designed nets. Strike teams also, at times, use a brief, electrical current into confined parts of the rivers to stun fish and scoop up grass carp that rise to the surface.
“Getting rid of a species once they are here is really, really difficult,” Mr. Mapes said. “What we have is a rare, unique opportunity now to really hit these while their populations are low.”
He said this year’s potential for a lower catch number is encouraging.
“The big thing is that, hopefully, they aren’t increasing,” Mr. Mapes said. “I think this is cause for cautious optimism.”
First Published September 5, 2022, 7:00 p.m.
