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On 1 July 1974, two months after the water tem- perature had reached 18 C and after the fish had been injected with hormones, eggs were found attached to rocks and gravel at the head of the raceway, where water percolated through the gravel. (D. Toney, Hatchery Supervisor, personal communi- cation: Toney 1974). In addition to providing potential spawning locations, the high water levels of the Yampa River in early summer provide habitat and food. As the Yampa recedes in late summer, habitat and food are greatly reduced and the fish apparently return to the Green River where conditions are more favorable. A few squawfish may remain in the deeper pools of the Yampa throughout the win- ter: although our attempts to capture them in the late fall and early spring were unsuccessful, local residents reported observing them in winter in the deeper ice-covered pools. Squawfish are piscivorous, being a top carni- vore in the Upper Colorado River. Vanicek (1967) found that squawfish as short as 50 mm (total length) had eaten fish. As the squawfish grow, fish increase in importance as food, comprising substantially the entire diet of squawfish longer than 200 mm. The fish are opportunists, however, and also eat other food items such as frogs and mice. Squawfish reproduction has declined steadily since the 1960's. Vanicek (1967) determined that year classes were strong in 1959, 1961, 1963, 1964, and possibly 1966, but weak in 1962 and 1965. He collected 275 young of the year in 1964, 42 in 1965, and 560 in 1966. Holden (1973) found young of the year in Desolation Canyon in 1971 and in the Green River at Canyonlands National Park in 1970 and 1971. He considered juvenile squawfish to be abundant at Echo Park in 1968, but found very few in 1969 and none in 1970. We collected young squawfish (about 75 mm TL) in the Green River at Jensen, Utah in 1975 where condi- tiuns still a?pear to be suitable for successful reproduction. The decline in successful repro- duction can be correlated with the impoundment at Flaming Gorge and concurrent changes in com- position of the fish fauna. Conditions in the river which favor exotic species may be detri- mental to the native species. . We determined that sexual maturity for squaw- f2sh occurs when the fish reach a minimum'total length of 420 mm, at about age VII. Squawfish once attained much greater size than they do to- day. It is not known whether this greater size was due to longevity or whether conditions in the past favored faster growth. The larger fish in the past may have had a greater reproductive potential than the smaller fish that are present today. Perhaps lower fecundity today, coupled with high mortality of young squawfish, is result- ing in fewer offspring in this species. These factors may have yet other implications for the reproductive potential that are not fully under- stood. Humpback sucker - The range of this unique fish has been drastically reduced in the Colorado River system (Fig. 3). The species has been completely extirpated from the Gila River system of the lower Colorado River basin, where it was once abundant (Minckley 1973). In 1949, a commercial fisherman in Sahuaro Lake (Salt River, Arizona) had a spawn- ing season catch of more than six tons of this species (Hubbs and Miller 1953). However, not one specimen was found when Sahuaro Lake was drained in 1966 (Minckley and Deacon 1968). Humpback suck- ers were also relatively common in Colorade River reservoirs during the 1940's and 1950's (Dill 1944: Wallis 1950: Douglas 1952: Jonez and Sumner 1954). This species is still found in Lakes Mojave and Mead, but is declining in numbers and appears to be approaching extinction below Lake Mojave (Minckley 1973). Although the humpback sucker is more widespread in the upper basin, it has been considered to be rare by all investigators (Vanicek, Kramer and Franklin 1970: Miller 1972: Holden and Stalnaker 1975a, 1975b). During the present study (1974-76), humpback suckers were considered to be relatively common, but not numerous, at two locations in the upper basin: at the mouth of the Yampa River dur- ing early spring and late fall, and in a flooded gravel pit (Walter Walker Wildlife Area) that is connected to the Colorado River near Grand Junc- tion, Colorado, during all seasons. Humpback suckers are captured primarily in quiet water areas except during the spring, when they congre- gate in swift water over gravel bars for spawning. Evidence of successful reproduction has been lacking in recent years (Vanicek, Kramer and Franklin 1970: Minckley 1973: Holden and Stalnaker 1975a, 1975b). In the spring of 1975, ripe male and female humpback suckers were collected over a gravel bar in the Yampa River about 400 m upstream from its mouth. One of these females (tagged at Island Park) had traveled 21 kID upstream to Echo Park (Fig. 1) in 3 weeks. Another spawning bar was located about 2.5 kID upstream at Box Elder Park (Fig. 1) on the Yampa River. This is the farthest of the (The ~ FIGURE 3. Historical distribution (all circles) and present distribution (solid circles only) humpback sucker in the Colorado River basin based on published reports and recent collections. qJr'tion mark, indicate nncertainty in present fi,h d',tribution.' 609