BUFFALO -- Minnows swim near the bottom of most food webs. Fish managers count on them to feed trout or walleye. Anglers dangle them from lines as bait.

But in stock ponds in northeast Wyoming, some researchers are putting their faith in fathead minnows as top dog.

Every fathead minnow Canadian graduate student Ryan Watchorn catches in his traps or sees in his ponds he records. He notes when wooden pallets he places in the ponds are covered with minnow eggs, and how many mosquito larvae he sees.

Fathead minnow survival in those ponds could ultimately mean survival for Wyoming’s sage grouse. When minnows eat mosquito larvae, they minimize the chance of adult mosquitoes hatching and infecting sage grouse with the West Nile virus. Watchorn is trying to find out if and where it works, how many minnows a pond needs and just how effective of a long-term solution it could be.

The research

Fathead minnows eat mosquito larvae. That’s not new. Researchers have proven it in laboratory settings, and a few landowners have stocked minnows in their ponds.

What no one really knows is how much mosquito larvae minnows eat, and how well the tiny fish survive real-world conditions.

The Northeast Wyoming Sage Grouse working group, a coalition of people interested in the future of sage grouse, first brought up the idea of using minnows to fight the West Nile virus. The group sponsored a professor from Montana State University to explore the possibility of minnows eating mosquito larvae, and results in a lab were positive, said Tom Maechtle, president of Bighorn Environmental Consultants in Sheridan and a member of the sage grouse working group.

Years later, the group, along with Maechtle’s environmental firm, decided it wanted a full-blown field experiment.

“It’s easy to throw fish in a pond, but there’s never been a quantitative analysis,” he said.

Watchorn’s professor, Brad Fedy, has an extensive background in sage grouse research in the Intermountain West, and decided to take on the project at the University of Waterloo in Canada.

The researchers chose fathead minnows for several reasons.

First, they are native to most Wyoming drainages, said Bud Stewart, energy development biologist with the Wyoming Game and Fish Department.

They can also survive at high temperatures and low oxygen levels, a key in small stock ponds, Stewart said.

Most important, they eat mosquito larvae. Biologists know of two other fish species that effectively control mosquito populations: the plains killifish and the mosquito fish. The plains killifish isn’t native to the Powder River drainage in northeast Wyoming. The mosquito fish isn’t native anywhere in the state, Stewart said.

Watchorn spent two months this summer in the field and will spend another couple of months in 2014. He chose 15 ponds near Sheridan and Buffalo. Most ponds in which he stocked minnows are similar in size to other ponds without minnows. Each evening he set wire cages to trap minnows to monitor survival and carbon dioxide traps for adult mosquitoes.

Each morning he checked the minnow traps, collected the adult mosquitoes and sampled the water for mosquito larvae.

He won’t have conclusive results until after next summer, but preliminary data show that the minnows are surviving in most of the dankest of stock ponds, he said.

The implications

The state's first recorded West Nile virus sage grouse death was in 2002 in northeast Wyoming. By 2003, dozens more had died, Maechtle said.

The Culex tarsalis mosquito species carries and transmits the disease, also infecting people and horses. It does, at times, cause flu-like symptoms in people, and can kill anyone with a compromised immune system.

But for sage grouse, it’s always deadly, Maechtle said.

Sage grouse spend their time during summer near watering holes in the open prairie looking for insects and flowering plants. Those stagnant, still ponds are the same places where the Culex mosquito species breeds and lives, he said.

“Their risk is really accelerated,” Maechtle said. “Next to removing all of the sage brush, which would ruin food and get rid of habitat, it’s the most dangerous thing they face.

The disease thrives in mosquitoes during hot, dry years.

Some landowners already control mosquito populations with larvicides. They’re efficient, but costly and time consuming, Maechtle said.

The chemical is either delivered by an airplane, which is expensive, or time-release briquettes, which have to be placed around every pond’s perimeter. The doses also only work for one mosquito egg cycle. This means landowners need to reapply up to three times per summer.

Buffalo rancher Steve Adami read about minnows as a biological control several years ago. Spreading larvicide is a full day of work. It just wasn’t practical, he said. Now, his ponds are some of those Watchorn is using.

Even if the minnows freeze during the winter, it would be cheaper and easier to stock them each spring than spread larvicide.

“This might be the one thing we can do to make a difference with sage grouse and change the way things are going,” he said.

Next summer, Watchorn will find out if any minnows lived through winter, and if so, in what conditions.

At the end of the study, he hopes to be able to tell landowners what types of ponds minnows will work in and how many to stock.

“This would save money and time and be better for the environment,” Watchorn said.

Reach Open Spaces reporter Christine Peterson at 307-746-3121 or christine.peterson@trib.com. Follow her on Twitter @PetersonOutside.



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