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Orth and Maughan 1982). The SI curves are integrated with streamflow data in the Physical Habitat Simulation (PHABSIM) models (Stalnaker 1978; Milhous et al. 1981; Bovee 1982) to compute weighted usable area of fish habitat at specified flows. Weighted usable area is usually computed for a given fish species by life stage and for a given reach of stream. The availability and accuracy of micro- habitat SI curves often limits the applica- tion of IFIM. Collection of appropriate and sufficient microhabitat data can be expen- sive and time-consuming, particularly when dealing with endangered species in turbid, swift riverine systems like the Col- orado River. The measurement of precise microhabitat and hydraulic conditions in swift, deep, and turbid streams may not be economically or technologically feasible (Larimore and Garrels 1985). Some data may be available from past studies conducted primarily for other purposes, but these must first be thoroughly evaluated. Also, micro- habitat data may be available for some life stages but not others, and it may be nec- essary to develop SI curves without the aid of specific data. This paper describes the development of 51 curves for the endangered humpback chub (Gila cypha) in the Upper Colorado River Basin. It is part of a larger project (Valdez et al. 1987) to assimilate and eval- uate previously-collected microhabitat data on the endangered fishes of the Upper Col- orado River Basin, and to develop appro- priate SI curves. This information was col- lected for various purposes over a period of 20 years by many investigators using a variety of sample gears. Four categories of SI curves or criteria have been identified (Armour et al. 1984; Bovee 1986), based on the source, quantity and quality of information available. Cat- egory I criteria are based on information other than field observations, such as life history studies in the literature, profes- sional experience or judgment. Category II criteria are based on frequency analysis of microhabitat conditions utilized by differ- ent life stages of the species. These criteria are termed "utilization functions" because they depict the conditions used during the observations. Utilization functions may not always depict the conditions preferred by the species because such conditions may I~ 32 not be present. Category III criteria attempt to correct this bias by factoring out the lim- ited habitat availability. Category IV cri- teria are conditioned curves developed for specific conditions such as cover or season. The development of SI curves for the endangered humpback chub in the Upper Colorado River Basin came primarily from category I and II criteria. Some field ob- servations were available for most life stages (category II), but little or no data were available for others (category I). Hab- itat suitability index curves for humpback chub were previously developed by Hol- den (1977) and Valdez and Nilson (1982). Data from these past curve development projects were incorporated into the present effort. Although a population of humpback chub occurs in the Little Colorado River of the lower basin, data from this population were not used in this project. The humpback chub is one of four en- demic mainstem fishes of the Colorado River Basin threatened with extinction. The others are the Colorado squaw fish (Ptych- ocheilus lucius), bony tail (Gila elegans), and razorback sucker (Xyrauchen texanus). All but the razorback sucker, currently a can- didate species, are listed as endangered un- der the Endangered Species Act of 1973 (16 U.S.c. 1531-1543, as amended). The pres- ervation and recovery of these endangered fishes has prompted a need to accurately assess the flow needs of their various life stages. This has shown to be a formidable challenge for fishery biologists in the Up- per Colorado River Basin because of the scattered occurrence of the species, high water turbidity, and severe turbulence and swiftness of some occupied habitats. In 1984, the U.s. Fish and Wildlife Ser- vice (FWS) chose to implement IFIM to as- sess the flow needs of these endangered species. Due to their endangered status and to the importance of water use and water management to the people of the Upper Colorado River Basin, an integrated sys- temwide process was initiated to quantify the effect of various water management scenarios upon the quality and quantity of fish habitat. One aspect of this process was the development of SI curves for the Col- orado squawfish, humpback chub and ra- zorback sucker. It was determined that there was insufficient information on the bony tail to develop SI curves for that Rivers . Volume 1, Number 1 January 1990 species. Although SI oped for all three spel with only the humpt for the other two spec in subsequent papers. overall curve develop] (1) assimilate a datal In 1986, the FWS co Inc. to coordinate thl curves for the endangl per Colorado River ] proach of the ~roject database of prevIOusl} itat data and through shops, attain a consen: on developing accural curves. Six species eXI the FWS to participate long workshops. The into five tasks: (1) data workshop 1, (3) preli velopment, (4) worksl' refinement. Task 1: Database Assi Most data used for ceived from the FWS , had been previously agency. Some inform, this database was also as compatible files. T the period 1964-198t lected by 17 differeI gators for various pur ganize the data a evaluation and select: study narrative was d nearly 100 investigati objectives, methodolo dates and gear types. j was developed for eat senting the number 0 measurements for inc ated with water deptl and habitat for each li narratives and summ to each species expert to provide familiarity Task 2: Workshop 1 The purpose of WOJ uate the data, de term I R. A. Valdez et al.