Rainbow trout experiment 'promising' for Poudre

By Cherry Sokoloski
North Forty News

Preliminary results of a new Poudre River study look promising for the future of rainbow trout. It's probably too soon for fishermen to jump up and down in their waders, but folks at the Colorado Division of Wildlife are excited.

Biologists with the DOW have been working for several years to develop strains of rainbows that are resistant to whirling disease. The new strains have been stocked in several Colorado rivers. However, even with this built-in resistance, the 'bows have not done well in the Poudre River.

In fact, the DOW stocked 667,500 rainbow trout in the Poudre during the last 20 years, and none survived to breed and establish new wild populations.

One theory is that brown trout, which took over the river when whirling disease eliminated all the rainbows, have out-competed the stocked 'bows and made it difficult to reestablish this popular fighting fish.

This summer and fall, the DOW conducted an experiment to test the theory. Doctoral student Eric Fetherman got his feet wet in fisheries research by leading the project.

The experiment

In August, the DOW and volunteers removed 1,400 brown trout, or 90 percent of the estimated population, from a 0.6-mile section of the Poudre River downstream of the Poudre Unit hatchery, then moved the browns to a different location. At the same time, the agency planted thousands of rainbows in the section where browns were removed and in a control section where browns were not removed.

Special antennae at either end of each section have tracked movement of fish by reading tags implanted in the stocked rainbows. The relocated browns were also tagged, and Fetherman discovered that some of them made it back to the removal area.

The DOW planted 4,000 rainbows using strains that are resistant to whirling disease, which wiped out wild rainbows in the Poudre in the 1990s. Half the 'bows were Hofer-Harrison crosses, and the rest were Hofer-Colorado River Rainbow hybrids.

In late October, Fetherman and others did a fish count in both the removal area and the control area, to check on populations of browns and rainbows. The results were significant.

"In the short term, the removal was successful," a smiling Fetherman said. In the control area, where browns were not removed, only 503 rainbows remained – 26 percent of the planted fish.

In the removal area, however, the DOW counted 1,185 rainbows, or 60 percent of those planted.

This initial success suggests that, as hoped, stocked rainbows can succeed when competition from browns is removed. Fetherman thinks it's likely that removal of the browns gave the rainbows a toehold in the territory before competing browns moved back in.

And indeed, browns from the neighboring stretches of river did eventually find their way into the removal area.

By late October, the numbers of browns in the removal and control areas were close to the same: 678 browns in the removal area, and 744 in the control area.

The study has also produced data about the surviving rainbows. Rainbows in both sections showed good growth and weight gain, Fetherman said. But, the Hofer-Colorado River Rainbows had a better survival rate than the Hofer-Harrisons in both areas.

In addition, Fetherman found evidence that browns do prey upon the rainbows. Nine browns that were captured in October had rainbow tags in their stomachs.

DOW senior aquatic biologist Ken Kehmeier said this experiment shows that 6- to 7-inch rainbows can "stay in there really well" if competition from brown trout is eliminated. "They dig in, and they're going to make that home."

Next steps

Kehmeier noted that this initial success is only a beginning. "We will let Mother Nature take her course," he said.

First, the young rainbow must survive this winter. Next, the DOW will watch to see if they grow to adulthood and reproduce. If both of these steps are successful, the agency will determine whether the rainbow offspring stay in the same area or "pioneer" into new stretches of the river.

The hope, Kehmeier said, is that "if we open up a niche on a small stretch of the river, the rainbow will expand from there, upstream and downstream."

If the longer-term experiment is successful, the DOW will likely try the brown relocation approach in other, smaller streams in Colorado.

Fetherman will continue to monitor the two sections of river for the next two years. He'll determine whether the rainbows stocked this year will be able to grow, reproduce and thrive once again in the Poudre River.

If this long-term lease works out, brown trout will have to share the space once again with their old neighbors, the rainbows.