Summer experiment could help reestablish rainbows

By Cherry Sokoloski
North Forty News

"J ust trying to keep my customers satisfied." That line from an old Simon and Garfunkel tune could be the motto of the Colorado Division of Wildlife.

To please their customers, the anglers of Colorado, the DOW has been working hard to reestablish healthy populations of rainbows in the Cache la Poudre River.

"People are demanding them," said George Schisler, aquatic wildlife researcher stationed at Parvin Lake.

Offspring of wild rainbows have all but disappeared from the Poudre River, since whirling disease got a foothold in Colorado. For the past several years, Schisler has been involved in developing strains of rainbows that are resistant to the disease.

With rainbows gone, the population of brown trout has increased to fill the vacuum. They now make up 90 to 95 percent of fish in the river.

This summer the DOW will conduct a study to see what role competition from browns is playing in the effort to reestablish rainbows in the Poudre.

The DOW has tried many stocking strategies in the past to get rainbows back in the river. In fact, according to aquatic biologist Kurt Davies, the agency has stocked 667,500 'bows in the river during the last 20 years.

But the current river residents, the browns, are loath to give up their feeding spots to new populations of rainbows. When small rainbows are introduced, the browns are out-competing for both habitat and food.

The DOW decided that if it were ever going to have rainbows in the Poudre again, it would have to open up a niche for them to occupy. According to Schisler, the agency wants to give the rainbows "a little bit of a toehold" so they can get established without so much competition from browns.

Why not just leave the Poudre River as a monoculture, with brown trout only? Ken Kehmeier, senior aquatic biologist for the DOW's Northeast Region, explained that it's a matter of customer satisfaction. Browns, he said, are not as desirable to fishermen, because they're harder to catch and also provide a different type of action for the angler.

Before whirling disease, Schisler said, the ratio of rainbows to browns in the Poudre was about 60/40. The ultimate DOW goal is to restore that ratio to the river.

New project

A brown trout relocation project will begin this August. The agency will remove brown trout from a one-mile stretch of the river and move them to a lower part of the river. They will then stock Hofer-cross rainbows in their place. Hofers are highly resistant to whirling disease.

As an experimental control, the agency will stock rainbows in another one-mile stretch without relocating the browns. The removal area will be above Rustic, about one and one-half miles below the DOW hatchery. The control area will be below Rustic in the Indian Meadows area.

Eric Fetherman, a doctoral student in Colorado State University's Department of Fish, Wildlife and Conservation Biology, is in charge of the project. He predicts that the survival rate – and staying power – for rainbows will be greater in the stretch of river where browns have been removed. He believes more rainbows will move out of the control area because browns are present.

Here's how the experiment will work. First, workers will electro-shock all fish (expected to be browns) in the removal area. The browns will be moved downstream below the narrows. Then, 4,000 Hofer-cross rainbows will be stocked, half in the brown removal area and the other half in the control area that will still hold brown trout. The stockers will be half Hofer-Harrison crosses and half Hofer-Colorado River Rainbow crosses.

All stocked rainbows, plus browns on either side of the removal area, will be marked with PIT tags, or passive integrated transponders. The devices use radio frequencies to transmit location.

At both ends of the one-mile study sections, antennae will be installed on the riverbanks. The antennae can read the tags when the fish pass by.

"It's like reading a barcode," explained Kehmeier. With the tags and the antennae, researchers will be able to track movement in and out of the experimental areas.

The study will run for three months, until late October. At that time, the DOW will do a population estimate for both rainbows and browns in the two study areas. Researchers will be able to determine how many rainbows stayed in each study area and how fast browns moved back into the removal area.

Starting next year, the DOW will do annual population studies on the two sections of river to determine longer-term viability of the stocked rainbows. By the end of the second year, Kehmeier noted, rainbows should be large enough to reproduce, so the DOW will assess whether reproduction in the wild is occurring.

"The entire idea," said Schisler, "is to have less reliance on stocking and more on natural reproduction."

The local study could have statewide implications, Davies said, since the majority of streams in Colorado were affected by whirling disease.