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t ~ ~ IDENTIFICATION OF HABITAT REQUIREMENTS AND LIMITING FACTORS FOR COLORADO SQUAWFISH AND HUMPBACK CHUBS Charles M. Haynes, Robert T. Muth, and Gary T. Skiba As a consequence of documented declines in numbers and ranges, the Colorado squawfish and humpback chub (Gila cypha) have been listed as endangered by both the federal government and the State of Colorado. Suspected causes of decline have been discussed by Haynes anal Muth (1982), Holden and Wick (1982), Valdez and Clemmer (1982), Haynes et al. (in press), and others. Since 1977, the Colorado Division of Wildlife has been investigating the distribution, ecology, and status of these rare fishes via systematic sampling in selected reaches of the Colorado, Gunnison, White, and Yampa rivers. Aspects of study have been conducted cooperatively with the U.S. Fish and Wildlife Service (Colorado River Fisheries Project). Nongame Research (Colorado Division of Wildlife, Fort Collins) investigations have been directed toward evaluating reproductive success for both species via collection of young-of-the-year (YOY) and juveniles, evaluating distribution and timing of spawning in selected reaches of the Colorado and Yampa rivers, and identifying habitat electivity for early life-history stages. P. N. OBJECTIVES The overall objective of this investigation is to identify the physical and biotic factors which limit the distribution and reproduction of Colorado squawfish and humpback chubs in Colorado. Similarly, this project is designed to develop field and laboratory methods for evaluating squawfish and humpback chub habitat, and reproductive success which may be used by management personnel for future habitat enhancement and/or reintroduction programs in accordance with overall recovery efforts. Additionally, information derived from this and other state and federal studies may be used to evaluate the impacts of several upper Colorado River water development projects and enhance the probability of meaningful mitigation where such development projects are believed to have deleterious effects upon rare fish populations. Aspects of this investigation relating to the humpback chub will be incorporated into a doctoral dissertation in 1984-85 (R. T. Muth, Dep. Fish. and Wildl. Biol., Colo. State Univ.). SEGMENT OBJECTIVES 1. Identify, measure, and analyze habitat parameters which limit the distribution and abundance of Colorado squawfish (Ptvchochielus lucius) and humpback chubs (Gila c ha) in the upper Colorado River. 2. Identify macro- and microhabitat features (e.g., flow, temperature, substrate, depth) which are associated with presence - absence of young-of-the-year (YOY) squawfish, humpback chubs, and other associated species.