Maybe the problem is brown trout more than the winter?
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."