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STOCKING PLAN FOR RAZORBACK SUCKER IN THE UPPER COLORADO AND GUNNISON RIVERS I. INTRODUCTION The razorback sucker (Xyrauchen texanus) is one of several native fishes that were formerly common and widespread throughout warm-water reaches of the Colorado River Basin, primarily in the mainstem and large tributaries from Wyoming to Mexico. Razorback sucker numbers have declined appreciably since human alteration of the aquatic environment began in the basin near the turn of this century. The decline of the razorback sucker is attributed to changes in physical and biotic factors leading to very low recruitment because of high mortality during the larval and juvenile life stages. Because of its precarious position, the U. S. Fish and Wildlife Service (USFWS) listed the razorback sucker as an endangered species (effective 22 November 1991; Federal Register, Vol. 56, No. 205, 23 October 1991) under authority of the Endangered Species Act of 1973 (USFWS 1973). This stocking plan provides the rationale to re-introduce razorback sucker in both the Upper Colorado River (Rifle, Colorado downstream to its confluence with the Green River: river miles 241-0) and Gunnison River (river miles 68- 0.7) with the intent of re-establishing a population between 1995 and 2000. The short-term goal of this initial stocking effort is to establish an adult population of 10 adults per river mile in floodplain reaches of the two rivers. The long range or ultimate goal is to establish self-sustaining populations in both the Colorado and Gunnison rivers. The target number does not include fish that may occupy off-channel habitats (e.g., gravel-pit ponds). The target for a razorback sucker population in the Upper Colorado and Gunnison rivers will be modified when the Biology Committee's Interim Management Objectives (IMOs) are developed. The IMOs will provide some biological parameters that describe a self-sustaining razorback sucker population. The stocking plan will be reviewed periodically by the Biology Committee. It will be revised as necessary, based on IMOs and results of the proposed stocking and monitoring experiments. II. BACKGROUND Most investigators (e.g., Tyus 1987 et al.; Osmundson and Kaeding 1989; Bestgen 1990) consider the razorback sucker to be much more rare than the Colorado squawfish (Ptychocheilus lucius). Total numbers of razorback sucker collected from the Upper Colorado River have declined dramatically since 1976. Fifty-two razorback sucker were collected during 1976-1981 from Lake Powell to Rifle, Colorado; 35 razorback sucker were captured during 1982-1985; and seven fish were collected from 1986-1989 (Burdick 1992). The last adult razorback sucker in the Gunnison River was captured in 1981 (Holden et al. 1981). Since 1988, only ten adult razorback sucker have been captured from riverine environments in the Upper Colorado River-three in 1988, three in 1992, three in 1993, and one in 1995. About 125 adult razorback sucker have been collected recently from lentic habitats in man-made gravel-pit ponds. The