Montana’s Buffalo Trout

Why some of the West's biggest rainbows grown on prairies where bison herds once roamed. By Bruce Auchly

This story is featured in Montana Outdoors
May–June 2002

East of Great Falls lies a prairie pond, home to big rainbow trout. As water bodies go, it’s small—less than 30 acres. Just a dammed-up coulee. Yet the weed beds grow thick, the aquatic insects swarm, and the trout look like footballs with fins. The exact location is not important. (And did you really think I was going to tell you where it is?) The pond could be almost anywhere in Montana east of the Rockies. Due to a combination of rich soil and dry, warm weather, much of this region is capable of growing trout weighing 3 pounds, 5 pounds, and even larger.

“ It’s simple,” says Steve Leathe, Fish, Wildlife and Parks (FWP) regional fisheries manager in Great Falls. “Just add water.”

And, of course, trout.

Pick a coulee. Fill with water. Add fish. As long as there’s periodic trout stockings and water deep enough so the fish don’t die from lack of oxygen in winter, the path to a trophy fishery is clear.
What follows, then, is a fish story. Not how to build a pond, or where to find one. Rather, why and how fish get to be so big on Montana’s high plains.

Locked in the soil
The origin of Montana’s large prairie trout actually begins in the sky, says retired University of Montana geology professor Dave Alt. “In a wet climate, the rain washes down through the soil and rinses all the fertilizer out,” says Alt, co-author of Roadside Geology of Montana. “But here you’ve got a very dry climate, which means the nutrients for plant growth are locked in the soil.”

Hundreds of millions of years ago, this region of the North American continent was covered with a vast shallow sea. The bottom of that ocean, like most water bodies, became thick with layers of sediment composed of sand and mud. Eventually, the sea dried up, and the sediment turned to sedimentary rock, which trapped nutrients such as potassium, calcium, magnesium, and phosphate.

Over time, says Alt, weather and climate broke down the rock into soil. Then, about 75 million years ago, the rise of the Rockies left that soil in a dry rain shadow, preventing the nutrients from leaching out. Nowadays, the addition of just a little water will cause almost anything to grow in central and eastern Montana.

“ There is a wealth of fertilizer and nutrients out there ready to go into any plant,” Alt says.
Many people traveling across eastern Montana consider it a wasteland due to the lack of lush vegetation. They’re mistaken, says Alt.

“ The bison once ate nutritious grass here and grew fat on it,” he explains. “And Montana wheat has a very high protein content, which is why it commands a premium price.”

Regardless of how prairie ponds are created—whether by people or by nature—the combination of water, soil, and summer sun provides the perfect growing conditions for aquatic vegetation. Stocked rainbow trout grow fat on the wide variety of insects attracted to the plants, such as algae, which are magnets for scuds.

Scuds are tiny freshwater shrimp. An exceptionally large one measures about one-half inch long. But they are filet mignon to trout. In addition to scuds, a healthy prairie pond ecosystem comes loaded with damselflies, leeches, and water boatmen.

A mess of minnows
As simple as it is to grow big trout, however, it’s equally simple to mess up a trout pond. Several years ago on a warm, late May day, I accompanied a group of sixth-graders on a school outing. Their teacher had received permission to visit and fish a local farmer’s trout pond. As the kids fished, I asked the farmer about the minnows I’d noticed swimming in the shallows. He said he put them in to feed the trout.

It’s a common practice but usually a big mistake.

A pond is like a pasture. And every pasture—even one irrigated and warmed by Montana’s hot summer sun—has a limit as to what it can produce. A pasture with too many cattle gets overgrazed fast. Similarly, a trout pond can have too many mouths and not enough food. A pond—even one thick with productive weed beds—can produce only so many insects. And rainbows need insects.

Unlike brown trout, which switch to eating minnows when they get big, most Montana rainbow trout stick to an insect diet no matter how large they grow. Because minnows also eat insects, stocking them in a trout pond usually results in too many fish “overgrazing” the food. The trout and minnows vie for the biggest, most nutritious bugs, such as scuds and water boatmen. Eventually, like cows in a slicked-off pasture, the trout can’t find enough food to grow big and maintain their weight.

The final indignity to trout is their inability to reproduce in a pond. “Trout need a good flowing stream with gravel beds for spawning,” Leathe says. That’s something ponds lack. Other fish not only out-compete trout for food, but they also can spawn in ponds.

Most minnows that people catch and transport are actually various sucker species, which replicate well in a pond ecosystem. Yellow perch are also a favorite of those who stock illegally, usually in a futile attempt to start a new fishery. The problem is that when perch strip a pond of food, they don’t stop reproducing. The result becomes skinny trout and stunted perch.

Illegal perch
For a tale of a trout pond ruined, look to Buffalo Wallow Reservoir, a remote northern Fergus County water body about 20 miles from the pavement. In the late 1980s, someone illegally planted yellow perch in this eight-acre pond, which FWP had been successfully stocking with trout since 1972. When FWP fisheries crews netted Buffalo Wallow in 1986, they found no yellow perch. Their fist hint of a problem came two years later with the netting of three perch, all over 11 inches long.

“ The rainbow were still doing reasonably well,” says Anne Tews, FWP fisheries biologist in Lewistown. “But things went downhill from there.”

Over the next six years, the perch population skyrocketed. In 1994 Tews set three fish traps and caught an estimated 15,000 perch, all 2 to 7 inches long. It was a textbook case of an overpopulation of stunted fish, Tews says. In September that year, she was forced to “reclaim” the pond, using a substance that suffocates fish but doesn’t injure plants or birds.

“ I walked around the pond a few days afterwards and estimated there were 100,000 dead yellow perch,” Tews says. “Only five were bigger than 12 inches.” She later restocked the pond with trout, which today thrive without competition from perch.

For trout pond success stories, consider those in the Blackfeet Reservation. Spreading from the mountains on the eastern edge of Glacier National Park, the reservation contains dozens of prairie ponds and lakes renowned for big rainbows. Anglers from across the state and around the world travel to the reservation each year.

“ You kinda get addicted to catching those big fish,” says Great Falls angler Roland “Spider” Lencioni, who has spent the last 20 years fishing the reservation in a float tube using a beefy fly rod and weighted fly line to get weighted nymphs to fish holding along the bottom of lakes. Fly-angling artistry it’s not, but Lencioni’s technique does catch fish—some up to 10 pounds.

Ira Newbreast, director of the Blackfeet Reservation Fish and Game Department, says the reservation’s trout are big for one reason: “It’s food production—all those scuds, those freshwater shrimp. We also have damselflies and leeches, but basically it’s the scuds.”

The reservation stocks about two dozen lakes each year. The rainbow trout, which average 5 inches long when planted, grow a remarkable 7 to 8 inches each year.“ That’s all due to the food production,” Newbreast says.

That phenomenal trout growth rate reels in close to 4,000 non-tribal anglers every year, making the fishery a huge moneymaker for Newbreast’s agency. “It’s really the cornerstone of our department’s budget,” he says.

Buffalo once roamed throughout the northern and eastern Montana prairies, feeding on the rich grass growing there. Today the great herbivores are gone, but another prairie resident is appearing in the region, making use of the abundant nutrients locked in the dry soil.
Just add water, stock trout, and keep out the minnows and perch. The result: big, beefy rainbows.
“ It’s not hard to do,” Newbreast says."Bear bullet

Bruce Auchly is an FWP information officer in Great Falls