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Yampa River, are common in this portion of the Green; the red shiner (Notropis lutrensis) is rare in the Yampa, but is abundant in the lower reaches of the Green River in the Monum~nt and downstream. We collected two juvenile squawfish in the Green River at Jensen, Utah, in October 1975 and the Squawfish Recovery Team captured several below Jensen in May 1976. Thus, conditions appear suita- ble for at least limited reproductive success downstream from the Monument, but not upstream. The species composition appears to reflect the differences in the habitats that were described above. CURRENT STATUS OF ENDANGERED AND THREATENED ENDEMIC FISH OF THE UPPER COLORADO RIVER SYSTEM The decline of the four endemic fish species of the upper Colorado River may be attributable to various factors. The following outline lists some of the possible causes contributing to the decline of these species. I. Stream Alteration A. Dams 1. Physical obstructions to movement and migration. 2. Flow Patterns - more stable on annual basis, less stable on a daily basis. a. Loss of the scouring effect from peak flows. b. Loss of flows that may elicit spawning migration or be- havior. c. Effect on other habitat char- acteristics such as changes in substrate, river morpholo- gy, and riparian vegetation. 3. Temperatures - cooler in summer, warmer in winter. 4. Turbidity - reduced downstream. 5. Reservoirs a. Not suitable for successful reproduction of large-river fish. b. Favor other species. B. Irrigation 1. Increased leaching of the soil resulting in increased salinity through return flows. 2. Changes in water quality due to the addition of fertilizers, pesticides, and other materials. C. Dewatering (transmountain diversion, evaporation from reservoirs, irriga-, tion, industry, municipal water use, etc.) . \ 1. 2. Reduces flow (volume and velo- city) Concentrates dissolved solids, increasing salinity. D. Channelization 1. Eliminates slow-mdving back- waters and eddies. 2. Alters species composition. 3. Substrate becomes unstable, ad- versely affecting invertebrates that may serve as a food supply. E. Unstable Banks (caused by livestock grazing, roads, and other activities of man) alter river morphology. II. Competition and predation are increased by introduction of non-native fish spe- cies. III. Pollution and Eutrophication A. Industrial, agricultural, and munici- pal wastes that may be toxic may increase. Water quality may change --in par- ticular salinity and sedimentation may increase. B. IV. Other Factors A. Parasites may be introduced by exotic fish species. B. Food organisms may change in species composition and abundance with changes in stream habitat. C. Fishing pressure may be too heavy on large, adult Colorado squawfish and the bony tail and humpback chubs, which are very vulnerable to sport fishing. Colorado squawfish - Whereas this species was once abundant throughout the Colorado River sys- tem, with a range extending from Wyoming to the Gulf of California, it is now rare and restricted to parts of the upper basin (Fig. 2). A few squawfish may exist below Lake Powell, but the exist~nce of self-sustaining populations is doubt- ful. The occurrence of this species in the San Juan River is also uncertain. It is no longer found in the now clear, cooler waters of the Green River above the confluence with the Yampa River, but adults were collected in the lower Yampa in 1968-71 and 1974-75 during July and Aug- ust after water temperatures reached 20-21 C (Holden 1973, McAda and Seetha1er 1975). Most captured squawfish have been males; no ripe fe- males have been collected. However, several squawfish believed to be spent females were col- lected in the Yampa between Cross Mountain and the confluence of the Little Snake River, and a fisherman reported to us of catching a ripe fe- male in early August 1975 at this same location. These suspected females were captured in deeper water than most of the males, which tended to congregate in shallow eddies near the bank. It has been assumed that the movement of squaw- fish into the Yampa River during the summer is a spawning migration. It is also commonly assumed that this migration is triggered by some seasonal factor or factors such as temperature, turbidity~ flow, photoperiod, or water chemistry. . Spawning has not been observed ~n the ~urbid lriver waters. At Willow Beach Nat~onal F~sh Hatchery, Arizona, spawning of c~ptive Colorado squawfish occurred but was not d~rectly observed. 607