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1022 COPEIA, 1986, NO. 4 t` ~~~ ,'~~r •. ~, ~ eel ~ ~'~ ~ ~ +t~,~,,' s~.- ~. ~~ ~~ ~ '{ r .r.r •~ r _ ~ - t - or hybrids of these species. The Bonytail was collected in a trammel net that also captured humpback chub and roundtail chub, some of which were in reproductive condition as re- vealed bythe occurrence of expressable ova and milt. The small number of Bonytail remaining increases the likelihood that they will inter- breed with these more abundant Gila (Hubbs, 1955). Recent artificial hybridization experi- ments showed gene exchange among these Gila is possible (Hamman, 1981). Presumed Bony- tail xhumpback chub intergrades have been reported from Lake Powell-a Colorado River reservoir about 230 km downstream from our study area-and from the Green River (Holden and Stalnaker, 1970). The decline of Bonytail and that of other na- tive southwestern fishes has been attributed to the alteration of discharge and temperature re- gimes downstream from dams and diversions, conversion of riverine ecosystems to lacustrine ones in the reservoirs upstream, introduction of non-native fishes and altered water quality (Miller, 1961; Minckley and Deacon, 1968). However, aside from the obvious detrimental effect on Bonytail of severe dewatering, only that of unseasonably cold, hypolimnetic waters from dams has been demonstrated. Vanicek and Kramer (1969) studied Bonytail in the Green River near its confluence with the Yampa River after the 1962 closure of Flaming Gorge Dam, upstream on the Green. Because age-0 Gila were absent, Vanicek and Kramer concluded that Bonytail (as well as roundtail chub) did not re- produce in the reach between the dam and the Yampa in 1964 and 1966-years when tem- peratures were sharply reduced by large, hy- polimnetic discharges from the dam (Vanicek et al., 1970). Subsequent work by Hamman (1982) and Marsh (1985) showed survival of Bonytail embryos is appreciably reduced by low water temperatures such as those evident below the dam during the Vanicek and Kramer study (and continuing today). Our observations on the Colorado River in- dicate the effects of altered temperature re- gimes alone do not account for the decline of Bonytail in the upper basin. Unlike the Green, the Colorado upstream from its confluence with the Green has no large, mainstream dams. Al- thoughthe Colorado has dams on its tributaries, recent analyses of U.S. Geological Survey rec- ords showed dam operation has not reduced Colorado River temperatures in reaches that Fig. 1. Bonytail captured from the Black Rocks area of the Colorado River, Mesa County, Colorado, July 17, 1984. The background pattern is a 1 cm grid.