Date of Award

5-1995

Degree Type

Thesis

Degree Name

Departmental Honors

Department

Wildland Resources

First Advisor

Wade W. Brinski

Abstract

Effects of fish introductions are relatively well studied in lentic habitats, and apparently range from extremely disruptive to notably benign (Kruegger and May 1991). Though researched less completely, fish introductions may also alter native faunas in lotic habitats. Ptychocheilus lucius, commonly known as the Colorado squawfish (C. squawfish), is a fish species endemic to the Colorado river system. The population is currently experiencing dramatically reduced recruitment successes relative to historical rates. Introduced fishes such as channel catfish, smallmouth bass, and green sunfish are prevalent in the Upper Colorado River. The introduced fish presence is increasingly suspected by researchers as a factor limiting C. squawfish recruitment (Minckley 1991). Direct predation, particularly on Young-Of-the-Year (YOY) and juvenile (2-5 year) squawfish in nursery habitat areas is potentially the most severe factor limiting C. squawfish recruitment. Results of cage experiments (Schaugaard, unpub. data) suggest that at least smallmouth bass, small channel catfish and green sunfish consume C. squawfish. Given the domination (both in terms of densities and biomass) of the foodweb by introduced fishes, I hypothesize that the overall energy balance in the upper basin system is dramatically altered relative to historic states. Most energy in the system, at all levels, now appears to flow through introduced fish.

As a first step toward testing the predator-impact hypothesis, a foodweb model, developed from three years of data, is presented. These data estimate previously unknown quantitative assessments of the Green River fish community, including prey body length/predator gape size relationships, predator stomach contents, and relative densities of introduced predators in a 60 mile flatwater reach of the Green River which has been identified as key nursery habitat for YOY C. squawfish (Schmidt pers. comm.). Finally, I develop a native fish predation risk model, based on these data, to determine to the extent that direct predation alone could contribute to the population trends currently exhibited by C. squawfish.

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