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48 GREAT BASIN NATURALIST MEMOIRS that reproductive success falls off dramatical- ly. Essentially, the same relationships exist if mean flows from January to June, inclusive, are compared. This examination suggests that reproductive SUccess of woundfin (and round. tail chub) in their only remaining habitat is extremely poor when mean winter and spring flows fall to about 100 cfs. The drought of 1977, resulting in some of the lowest flows on record in the Virgin Riv. er, has permitted a significant insight into the habitat requirements of the endangered na- tive fishes of the river. It is apparent that current utilization practices of the water re- sources permit survival of the native fishes in about 29 percent of their remaining potential habitat. Intermittent flows coupled with higher summer temperatures throughout the remainder of the potential range (Schumann ~~ 80 ,..;: 70 60 50 40 .-' ~- No.3 1m 1978, Lockhart 1979) make it unreliable as a fish habitat. Within the remaining 29 percent of the potential habitat, reproduction OCcurs during years of normal flow, but is extremely poor to absent during years of low flow. This circumstance suggests that at present the fishes are living in a habitat which has ex- tremely little potential for further devel- opment or alteration without adverse im- pacts on the endangered species present. Continued monitoring of reproductive suc- cess and population structure under varying conditions of stream flow will permit refine- ment of flow requirements. It is apparent that the roundtail chub is in an even more precarious position than is the woundfin and that both species require higher flows in spring and winter than they do in summer. Obviously, problems associated with the eff( oft. ac eVl ide Vir thr fac ed thf ca1 rar ha nil je, ml Cr pll o upper river o lower river :3 2 2 FALL SPRING FALL WINTER SPRING FALL WINTER '73 '77 '77 '77 '78 '78 '78 Fig. 6. Comparison of mean size of woundfin in the upper and lower mainstream Virgin River 1973, 1977, 1978.