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1979 THE ENDANGERED SPECIES: A SYMPOSIUM woundfin fry at various locations along the Virgin River during summer 1977. Collec- tions were made at one- to two-week inter- vals until fry were taken at each location in- dicated. The uppermost location, indicated .by 23 July in Figure 8 was actually below the Hurricane Diversion but above LaVerkin Creek. It is apparent that hatching occurred earlier in the lower river than in the upper river. Furthermore, in the lower river hatch- ing appears to have been delayed by about two weeks at the lowermost station where habitat modification is most obvious. The earlier appearance of young woundfin in the Arizona segment of the lower river was followed by relatively good survival in 1977 (Fig. 7). By contrast, the later appear- ance of young woundfin in the upper river was followed by very poor survival in 1977 (Fig. 3). With higher flows in 1978, both up- stream and downstream populations of woundfin showed good reproduction, and by fall 1978 the mean size was nearly identical in the two populations (Fig. 6). Comparisons of hydrographs of Virgin Riv- er flows for 1973, 1977, and 1978 show that the major differences in flow occurred during winter and spring. Summer flows suggest.a relatively greater degree of similarity for all three years (Vaughn Hansen Associates 1977). If winter and spring flows significantly in- fluence reproductive success of the endan- gered fishes of the Virgin River, the effect should be discemable in the population struc- ture during the following fall. Figure 9 pres- ents data comparing mean size of woundfin in the fall in both the upstream and down- stream populations against mean flows of the river during the spring. Of particular signifi- cance is the fact that when stream flow is low, mean size is high and vice versa. Inter- estingly, Figure 9 also suggests that when mean spring flows are above 700 cfs, repro- ductive success may be slightly poorer than when mean spring flows are between 400 and 600 cfs. Data are not available for times when mean spring flows fall between 100 and 400 cfs, but at about 100 cfs it is clear - 9/28/78 (N=102) 0 11/25/77 (N=3) 25 z 20 0 ti ~ ::> 15 a.. 0 a.. - 0 10 ~ 0 5 30 60 90 120 o 47 Fig. 5. Length frequency of roundtail chub in Virgin River above the narrows. The o's indicate size of the only three individuals taken in extensive sampling on 25 November 1977. 150 180 SIZE 210 240 270 300