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Table 2. Dates of egg deposition, collections of ripe fISh, presence of radio-tagged fish on spawning grounds, and spawn- ingperiod, Colorado squawfish, Yampa River, 1981-88. Egg Ripe Radio-tagged Spawning periodc Year deposition3 fish fishb Total Optimum 1981 19 June-IOJuly 1-10 July 20 June-20 July 19 June-20 July 23 June-13 July 1982 10-18 July 10 July-7 August 4 July-8 Augustd 4 July-8 August 8 July-l August 1983 20 July-5 August 14 July-18 August 12 JuLy-8 August 12 July-18 August 15 July-l0 August 1984 19 July-13 August 18 July-14 August 10 July-17 August 10 July-17 August 16 July-15 August 1985 27 J une-13 July 27 June-25 July 21 June-12 July 21 June-25 July 25 June-17 July 1986 27 June-5 August 5-30 July N/A 27 June-5 August 1 July-2 August 1987 11 June-16 July 3-30 June 9-30 June 3 June-16 July 8 June-5 July 1988 27 June-19 July 29 June-6 July 20 June-24 July 20 June-24 July 25 June-16 July a Dates obtained f~m back-calculations of latval age using equations in Nesler et al. (1988). b Dates represent first and last appearance of radio-tagged fish on spawning grounds. C Total includes all indication of spawning activity; optimum is the average of dates for egg deposition, collections of ripe fish, and presence of migrating radio-tagged fish on spawning grounds. d Data provided by Wick et at (1982). spawning (e.g., presence of migrating radio-tagged fish on spawning grounds, collections of ripe fish, or calculated dates of larval emergence in spawning reach) and lasted 4-5 weeks. Optimum spawning period was the time of greatest spawning activity, calculated by averaging the dates when radio-tagged fish and ripe fish were present in the spawning reach and back-calculated dates of egg deposition. From 1981 to 1988, spawning requirements of Colorado squawfish were evaluated in the Yampa River spawning reach. The length of the estimated optimal spawning period - about 26 days - was similar for all years (Table 3). Spawning generally occurred earlier in lower-water years -1981, 1987, and 1988- and later in high-flow years-1983 and 1984 (Table 3). Water temperature and discharge varied between years during the optimum spawning period (Table 3). Water temperatures ranged from 14.5'Cto 27SC for all years. Average minimum temperature was 19"C and average maximum temperature was 24'C. During optimum spawning period, mean discharge ranged from 25.27 m3/s (1981) to 108.25 m3/s (1982). Vanicek and Kramer (1969) first suggested that discharge and temperature influenced spawning in Table 3. River conditions during optimum spawning period, Colorado squawjish, Yampa River, 1981-88. Water temperature (oC)d Water Period of DischargeC Minimum Maximum Year veara optimum spawnb Mean Range (mean) (mean) Range 1981 Low 23 June-13 JuLy 25.27 12.37-37.55 19.3 24.8 18.0-25.5 1982 Average 8 July-l August 108.25 68.77-176.31 19.5 23.3 16.5-27.5 1983 High 15 July-to August 86.17 41.1>-141.22 21.0 24.3 18.0-27.0 1984 High 16 July-15 August 71.74 29.43-131.60 20.3 23.8 20.0-24.0 1985 High 25 June-17 July 64.02 28.58-135.84 17.8 22.8 14.5-25.5 1986 High 1 July-2 August 69.82 26.41-145.46 19.5 22.5 18.5-23.0 1987 Low 8 June-5 July 58.69 23.97-128.20 17.9 22.5 16.5-24.5 1988 Average 25 June-16 July 49.14 15.73-104.43 19.5 23.0 18.0-25.0 a Designation of low-, average-, and high-water years based on average annual discharge for 1922-87: low, <43.15 m3js; average. 43.15- 77.83 m3js; high, > 77.83 m3js (M. Butler, personal communication). b Derived from back calculations of larval age, and contact with radio.tagged adults and collection of ripe ftsh on spawning ground. c USGS flow records (1981 sum ofYampa River near Maybell. Colorado, and Little Snake River near Lily, Colorado; 1982-88 Yampa River at Deerlodge Park, Colorado). Data are daily discharges for optimum spawning period. d USGS flow records (1981 Yampa River near Maybell, Colorado, 1982 Yampa River at Deerlodge Park, Colorado) and hand-held thermometers (1983-88, minimum (mean) and maximum (mean) values were calculated from early morning and afternoon temperatures taken on the spawning grounds because a continuous temperature recorder was not present). 10