Protecting Endangered Fish from Predation

In 1998, the Upper Colorado River Endangered Fish Recovery Program was developed to protect four species of endangered fish that reside in the Colorado River: the humpback chub, bonytail chub, Colorado pikeminnow and the razorback sucker. This program is a unique partnership between federal, state and private entities with the goal of creating natural, self-sustaining populations and the eventual “delisting” of these four fish species. 

One of the contributing factors to the low population numbers of these now protected fish was predation from non-native fish species. Smallmouth bass, walleye and northern pike were some of the biggest threats to the survivability of the four endangered fish species in the Colorado River.  

Rifle Gap Reservoir provides irrigation water to local farms and ranches and any unused water flows into the Colorado River. Smallmouth bass, walleye and northern pike are plentiful in the reservoir and highly sought after by anglers. But, these predatory fish used to regularly escape the reservoir and end up in the Colorado River. Actions had to be taken to comply with the Recovery Program, and protect the endangered fish downstream.

In-stream Fish Screen

In 2013, an in-stream fish screen was installed in Rifle Creek just below the Rifle Gap Reservoir dam. All water released from the reservoir passes through this section. The screen is designed to prevent anything larger than one millimeter from passing through and escaping downstream, including all fish and their eggs. The fish, eggs and other debris are trapped in “live wells” within the fish screen.   

Park staff and volunteers check the fish screen daily to make sure it is functioning correctly. Any debris, typically plants or algae, is removed from the screen to make sure water can flow through. The screen is scrubbed regularly to prevent mineral deposits from forming and interfering with water flow. The live wells are regularly checked for fish and any fish found are netted, measured and recorded. Any rainbow trout or brown trout are released downstream, but all other fish species are removed so they do not make it into the Colorado River. Since 2013, park staff and volunteers have spent over 1,000 hours cleaning the fish screen and making sure it is functioning properly.  

Annually, sections upstream and downstream of the fish screen are electroshocked by Colorado Parks and Wildlife staff to survey which fish species are present in these locations. Prior to the fish screen being installed, all species of fish from the reservoir, including walleye, northern pike and smallmouth bass were found within the creek. Since the screen was installed, thousands of fish have been surveyed. All but two fish surveyed were brown trout or rainbow trout. In 2013, one northern pike was surveyed but it was likely in the creek before the screen was installed. The only other fish species found was a bluegill in 2017. The fish screen has proven to be very effective at keeping predatory fish in Rifle Gap Reservoir from entering the Colorado River and preying on any of the endangered fish found there.

A lot of hard work goes into making sure the fish screen is operating as intended. This hard work is justified when looking at the data that shows how effective the fish screen has been. On November 17, 2021, the status of the humpback chub was reduced from “endangered” to “threatened.” I like to think that our efforts have contributed to this change in designation. All of this was done while maintaining a fishery in Rifle Gap Reservoir that anglers want, a fishery that provides opportunities to fish for walleye, northern pike and smallmouth bass.

This article was written by By Brian Palcer. Brian is a Park Manager, Rifle State Park Complex.