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86 TYUS AND HAINES 48 48 Y . 46.247 - 0.576X Y . 50.296 - 0.140X r . -0.565 r . -0.810 44 44 0 0 40 40 36 36 32 32 28 28 40 60 80 100 120 140 160 180 200 220 20 40 60 80 100 120 140 160 Y . 4.730 - 0.024X 3 Y . 4281 - 0.039X r . -0.859 r . -0.797 E 0 >'! "- iJ'i ~ <!J. -1 S -2 -1 -,J 40 60 80 100 120 140 160 180 200 220 20 40 60 80 100 120 140 160 STREAMFlOW (m3/sec) STREAMFlOW (m3/sec) FIGVRE 3. - Regressions of Colorado squawfish mean total length (Y) on mean August-September streamflow, and 10g..(fish/lOO m2) (Y) and streamflow in the lower (left) and the upper (right) Green River, 1979-1985 and 1987-1988. Flows for the lower Green River were measured at the town of Green River, Utah; flows for the upper Green River were measured at Jensen, Utah. River within 3 to 15 d after hatching, and who presumed that they were present in a 6-km river reach for only about 1-2 d. We are aware of no age-O or age-I juveniles captured in autumn above the mouth of the Yampa River in the last 10 years. Some age-O fish present in our upper Green Riv- er samples could be the result of spawning below the Yampa River. However, we believe that the vast majority, ifnot all, ofage-O Colorado squaw- fish found in the upper Green River are the result of downstream drift from the Yampa River. Ra- diotracking of adults on spawning migrations has not confirmed otheT spawning locations in the up- per Green River (Wick et al. 1983; Tyus 1985, in press). Drift ofIarvae into the Green River is well documented (Haynes et al. 1984; Nesler et al. 1988), and summer and autumn concentrations ofage-O Colorado squawfish downstream from the Yampa River spawning area suggested down- stream movements. Finally, substrates in most of the upper Green River (from Jensen to Desolation Canyon) are fine silts and sand rather than cobbles and boulders that exist in the known spawning areas. We presumed that spawning would not oc- cur on such substrates. Although our data are not as extensive for the lower Green River, we presume that a similar pat- tern of larval drift occurs there. Years of radio- tracking migrating adults and collecting ripe fish there have indicated that RK 225-257 is a major spawning area, and no other such area has been confirmed downstream. Larvae were caught in drift nets 8'km downstream (at RK 217) of the spawn- ing area, and seining results show summer and autumn concentrations of postlarval Colorado squawfish downstream of the spawning area. Thus, data collected at the Green River site support our conclusions drawn from the Yampa River Tegard- ing larval movement, and it is possible that larvae drifting out of the Yampa River reach nursery hi'tats in the lower Green River. .ewereconcemed about the valid~ of using. ~ ... c ery':.~~~UAt~onsf07estI~irnr~ Colorado squawfish spawning dates from total length of fish, particularly for the larger postlar- vae, a concern also expressed by Nesler et al. (1988). Average temperatures in the Green River at mean larval hatching dates ranged from 20 to 250C, but the July-August backwater tempera- tures at fish capture averaged 25-280C. Thus, backwater temperatures w( 260C temperature range in . were raised, and we expect grow faster. However, otoh collected in 1987 (mean, 3: 45 mm) indicated that oto about 13 d older than age~ ratory fish of the same si, State University, personal could be the result of slov fish, inadequate sample siz ing. However, growth eq should be further validat< wild fish of different sizes 1: applied to management of Habitat Use The timing of spawnin~ is critical to larval survi, during the period ofdeclin temperatures after spring poral adaptation (midsun ated with declining flows; reproductive biology of It whereby the length of exp< to cold temperatures of sp reduced. It is perhaps criti attained during the remai od, and that the larvae aI able feeding areas before depleted. Backwaters use nursery habitats were cr creasing summer flows af melt). Growth and Survival We expected greater grc lower Green River beca warmer than in the upper r that age-O fish in the upp' same average annual size in the lower area. We a5 ronmental factors (e.g., food abundance) may in the Green River, at least i temperatures alone. One a difference in available I records indicated that the! backwaters per kilometel lower Green River. Back' River may yield a higher squawfish than in the up 2) and greater numbers r growth. Reduced growth