Prey Base, Trout and the Colorado River

Colorado Parks and Wildlife Aquatic biologist Jon Ewert gave a presentation on the health of the Colorado River to the BOCC this month.
Colorado Parks and Wildlife Aquatic biologist Jon Ewert gave a presentation on the health of the Colorado River to the BOCC this month.

Reduced invertebrate and sculpin population after Construction of Windy Gap affecting Colorado River health

by Tara Walker

Colorado Parks and Wildlife aquatic biologist Jon Ewert gave a presentation on the health of the Colorado River to the Board of County Commissioners this month. He gave a summary of past and recent studies that indicate the invertebrate populations and mottled sculpin populations in the Colorado River are significantly reduced since the construction of Windy Gap Reservoir.

Ewert explained how the prey base has impacted the trout populations. Ewert explained he kept hearing stories from anglers about how the fishing has changed so much over the years, “When I started talking to people who have fished the river their entire lives, it was very common to hear from people that fishing in the Colorado River isn’t what it used to be when they were younger, and that the past few decades, they don’t see the stoneflies that they used to see in the Middle Park section of the Colorado River. I heard that consistently from multiple sources.”

Giant Stoneflies and mottled sculpin are two important food sources for fish in the Colorado River. Stoneflies live for years as larvae when most invertebrates have a one-year life cycle.

Ewert said, “Giant stoneflies are a flagship organism for the history of the Colorado River for the past few decades.” He went on to explain mottled sculpin are a native fish in our area that is an excellent food source for trout. They are the most common fish in the Fraser River and sometimes outnumber trout by 10 to 1.

By looking at past studies completed by Dr. Bob Erickson in 1980 and 1981 before building Windy Gap reservoir, research biologists were able to repeat Erickson’s invertebrate studies to compare results and analyze changes. Ewert, “When we found out the Erickson study existed, it was like finding the holy grail. Here was the most detailed bug study on the Colorado River before Windy gap was built.”

In the summers of 2009 and 2010, CPW biologists repeated the study as closely as possible so they could compare the results to the 1980 and 1981 invertebrate results. Ewert smiled as he told the BOCC, “We even used the same nets. We tracked Erickson down, he was retired and still had his nets in his garage and in great condition.”

The net used in the 1980-81 Erickson study and the 2009-10 invertebrate study.
The net used in the 1980-81
Erickson study and the 2009-10
invertebrate study.

When results of 2009 study were compared to the 1980 Erickson study, Ewert explained that many species are now absent or significantly reduced. For example, in 1980, the densest Giant Stonefly population was 2 miles downstream from where Windy Gap dam is today. The 2009 results show that this species of stonefly is basically nonexistent downstream from windy gap. The 2009 study found that 54% of mayflies are gone, 40% of stonefly species are gone and 62% of caddis species are gone. This pattern also emerges when analyzing the mottled sculpin population. Ewert, “We saw this distribution pattern around the county. Upstream of reservoirs (Williams Fork is an excellent example), sculpin are common and dense. Downstream you can’t find a single one. When you have the most common fish above a lake and it is absent below a lake, that is a big ecological shift in a short geographic distance and something is going on there. Wolford is the same story.”

The changes in the invertebrate populations and the change in the sculpin population is biologically significant and a summarized report is available on the Colorado Parks and Wildlife website.

Ewert explained Colorado Parks and Wildlife continues to monitor year to year fish populations. They use that information to inform management decisions regarding stocking, special regulations and commercial use. They also use the information to assess the overall ecological health of a body of water. Eric Fetherman, Matt Kondratieff, Dan Kowalski and Eric Richer are research biologists for Colorado Parks and Wildlife while Ewert is a management biologist.

CPW began collecting fish population information on the Kemp-Breeze State Wildlife Area near Parshall in 1981. In 1987, whirling disease struck the rainbow trout population and by mid-1990’s, stocking of rainbow trout was needed for the first time. Prior to whirling disease, rainbow trout dominated the Colorado River with the trout population averaging 80% rainbow trout and 20% brown trout. Current populations in Colorado are 10% rainbow trout and 90% brown trout.

New strains of rainbow trout have been developed and introduced to the Colorado River that are resistant to whirling disease. Ewert explained that they are hoping the rainbow trout numbers will continue to go up over time. However, there is still a concern the invertebrates and sculpin are affecting the production of quality trout.

The density of quality fish that are 14 inches or larger has also significantly decreased. Gold Medal Waters are defined as having at least twelve 14” or larger trout per acre. Only 322 miles of Colorado’s 9000 miles of trout streams, and three lakes, carry the “Gold Medal” signature.

Ewert went over some statistics relating to the Parshall Sunset area of the Colorado River and explained his concerns, “We are flirting with slipping below gold standards when in 2012 we only have 17 fish per acre that are over 14 inches. Good years now are 50 fish per acre and those used to be the bad years.

There has been downshift in the ability of the river to maintain fish over 14 inches per acre, and we believe it has to do with long-term degradation in the prey base. We believe it is a result of the construction of Windy Gap Reservoir and the effects it has had.”

Currently, Colorado Parks and Wildlife do not stock brown trout as they are self-sustaining. Ewert emphasizes the need to see an improvement in the prey base so we can see more well-fed wild rainbow trout. The Hofer Trout that are resistant to whirling disease are helping the rainbow trout populations. Ewert explained that the downward trend has stabilized and they are hopeful for the future.