Rainbow trout experiment on Poudre River shows promise

Today’s teenagers have not had thrill of catching wild rainbow trout in the Cache La Poudre River.

That’s because whirling disease virtually wiped out the wild rainbow population in the river two decades ago. The disease causes deformities in young fish, or fry, making them “whirl” in the water. Infected fry rarely survive their first year of life.

However, researchers with the Colorado Division of Wildlife (now the Colorado Division of Parks and Wildlife) are trying hard to reestablish wild rainbows in the Poudre, since ‘bows are a favorite sport fish for anglers.

[media-credit name=”Nick Shannon” align=”alignnone” width=”300″][/media-credit]
Researchers from the Colorado Division of Parks and Wildlife conducted a rainbow trout survival survey in August on the Poudre River.

One new experiment is showing promise. A good number of rainbows planted in the river last year survived the winter, and that’s a positive start.

The new approach involved removing brown trout from a section of river and stocking whirling disease-resistant rainbows in that section. As a control, a second stretch of river was also stocked with rainbows, but the browns were not removed. The idea is that young, 6- to 7-inch rainbows will likely have a higher survival rate if they’re not competing with mature browns for food and territory.

Browns were removed from the river upstream of Rustic in August 2010, and 4,000 disease-resistant rainbows were planted in that area and the control area. Since then, CDPW researcher Eric Fetherman and his crew have conducted two fish counts to measure how the rainbows are doing.

The first count last October showed better rainbow survival in the removal area than the control area, an indication that rainbows did better without competition from browns.

Fetherman conducted another count in April to measure winter survival rates.

The April results show that rainbows did survive the winter, but their numbers declined in both sections, as researchers expected. In a somewhat surprising result, more of them survived in the control area — with browns present — than in the stretch where browns had been removed. Rainbow numbers declined by just 7 percent in the control area and by 28 percent in the removal area.

However, rainbows still comprised a larger percentage of the total fish population in the removal section compared to the control section (42 percent versus 30 percent).

The brown trout population remained constant in both sections.

Fetherman theorizes that the reason for greater rainbow mortality in the removal area has to do with the nature of the river along that stretch. It’s a steeper section of the canyon, with fewer holes for fish to occupy and more ice in the winter. That creates more competition for habitat during the winter months.

It’s not certain yet how many rainbows died and how many merely moved out of the study section. Researchers will find that out when they do another fish count later this month.

Ultimately, for the experiment to be successful, the planted rainbows must start reproducing. That could happen as early as 2012, Fetherman said — if the fish survive that long.

Finding new rainbow fry in the Poudre River would be exciting indeed. Then, researchers would watch to see if they survive to one year of age — something that hasn’t happened in the Poudre for 20 years.

There are a lot of “ifs” involved in the process, but researchers are hopeful for a successful return of the rainbows. Before whirling disease wiped out the wild ‘bows, the ratio of rainbows to browns in the Poudre was about 60/40. Currently, browns make up 90 to 95 percent of fish in the river.

The CDPW goal is to restore the 60/40 ratio to the river. If that happens, the river’s ecosystem will be more diverse — and a lot more fishermen will be happy.

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