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... ~_ .. \_ L\ ~ \ (\ e ':J tJ.\). ~~\N~\r \ . ... (.1 b {1d- BIOLOGY OF YOUNG COLORADO SQUA WFISH 81 to shore. Backwaters were defined as shallow ephemeral embayments adjacent to the main river channel with no measurable water velocity. Maximum depth of each backwater was mea- sured and surface temperature was recorded at that point. The dominant substrate was classified as silt (< 1 mm in diameter), sand (1-3 mm), gravel (3-75 mm), cobble (76-305 mm), and boulder (> 305 mm). In 1988, backwaters were classified as clear (bottom visible throughout), turbid (bot- tom not visible), and partly turbid (clear at ter- minal end, turbid at mouth). Backwaters were also sampled in spring (March 3D-May 10) 1984, and each spring in 1986-1989 to evaluate overwinter survival. Spring sampling began soon after river ice began to break up, and this was usually the first part of April. Spring sampling was confined to river reaches known to contain Colorado squawfish (i.e., F, E and B, A) and done with the same procedures used in the previous autumn. Data analysis. - Larval hatching dates were es- timated by back-calculation from total length at capture by use of equations developed by Haynes et al. (1984) and modified by Nesler et al. (1988). For Y = number of days after hatching, Y= -76.71 + 17.49 TL - 1.056 TU + 0.0221 TV (for TL < 22 mm); Y = -26.64 + 2.78 TL (for 22 < TL < 47 mm). Equations were derived from hatchery-reared progeny of parents collected from the upper Col- orado River basin (Hamman 1981). Colorado squawfish fry were raised at Willow Beach Na- tional Fish Hatchery (Arizona) for 107 d at tem- peratures of 2l-260C. We reported annual hatch- ing dates as means and as ranges that included 90% of fish-hatching dates each year (i.e., dates read from a cumulative frequency distribution at 5% and 95%). Age-O fish captured above Gray Canyon (RK 256) were judged to have hatched in the Yampa River and fish captured below RK 256 presumably hatched in the Yampa and Green riv- ers. Hatching dates of larval Colorado squawfish were compared with streamflow!> by use of U.S. Geological Survey water resources data from gag- ing stations at Deerlodge, Colorado, and the town of Green River, Utah (Figure 1). Water temper- atures during the hatching period in the Green River were obtained from the Green River gaging station. No temperature records were available for the Yampa River spawning area. An evaluation of relative overwinter survival of age-O fish was calculated by dividing the seine catch (fish/lOO m2) of young Colorado squawfish in riv- er reaches sampled in the autumn of 1983, 1985, 1987, and 1989 into the catch in those reaches the following spring. In each evaluation, the upper Green River strata (E and F) were evaluated sep- arately from the lower strata (A and B), so that two comparisons were made for each year. Differences in mean total length of Colorado squawfish between the lower and upper Green River and between autumn and spring samples were tested with independent t-tests, with unequal variances. Standard regression techniques were used to determine the relationships between fish abundance (logefish/lOO m2), size (total length), and late-summer (August and September) stream- flow. Independent t-tests and G-tests of indepen- dence (Sokal and Rohlf 1981) were used to eval- uate the relationship between environmental conditions (backwater temperatures, depths, and sizes; substrate; turbidity) and the presence (at least one fish captured) or absence of Colorado squaw- fish. Results Distribution Altogether, 11,379 age-O Colorado squawfish were captured in the lower Yampa and Green riv- er study area: 601 in drift nets in June and July; 3,079 in seines in July and August; 6,459 in seines in September and October; and 1,240 in seines in April (Table 1). No Colorado squawfish larvae were found in the Green River above its confluence with the Yampa River (RK 552-584) or in the lower 225 km of the White River. Larvae were present from RK 29 on the Yampa River downstream to the lower boundary of our study area at RK 35 on the Green River. When all years were combined, the average July-Aug4st seine catch showed two areas of post- larval abundance that were about 130 km down- stream of the spawning areas: one downstream of the Yampa River, but upstream of Gray Canyon (i.e., near RK 448), the other downstream of the Gray Canyon spawning area (i.e., near RK 112; Figure 2). Within-year seining also demonstrated this pattern, but catches varied among years in both the upper and lower Green River strata (Ta- ble 1). Hatching of Colorado squawfish was associated with declining water levels and rising water tem- peratures after the peak spring runoff, and lasted 2-6 weeks (Table 2). Fish captured in the Yampa and upper Green rivers hatched about 54.0 d