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Spawning Periods Estimates of spawning periods have been made through the aging of larval .' Colorado squawfish collected in the vicinity and downstream of spawning areas, the observation of radio telemetered fish and the capture of ripe females. The condition of male ripeness does not appear to be nearly as definitive because males have been found to expell milt ovet' a much longer period prior to and following the estimated spawning period. Studies of spawning period have been primarly keyed to the behavior of radiotelemetered adult fish in the Green and Yampa Rivers and less so in the Colorado River because of tracking difficulties. Generally, in the Green River system, there has been a concurrent initiation of migration and subsequent congregation of several radiotelemetered fish in the same local as the spawning season approaches. At that point extensive sampling for adults was carried out in those areas with the objective of capturing a number of other adult fish with physiological characteristics of spawning. Coincident with this are larval sampling activities intended to capture very young larvae for back calculating date of spawning. Back calculations were CaDputed using the methods developed by Haynes and Muth (1984). Observations by CRFP and CDOW workers have been generally caDpatable and suggest a spawning period of 20-30 days (Table 2). Except for the low-wa ter year of 1981 spawning appears to have commenced in the Yampa and Green River by 7 July and concluded by 28 August. Spawning probably occurred as early as 19 June and was con- cluded by l7 July during 1981. During the high water years of 1983 and 84 spawning appears to have been delayed 15 to 25 days in the Yampa and Green Rivers. Estimates of spawning dates in the Colorado rely primarily upon larval fish collections made periodically during the summer throughout the study area. Spawning apparently occurs 5-10 days earlier in the lower than in the upper reaches. Spawning appears to have been initiated between 5-26 July and concluded by 10 August to 20 September (Table 2). Spawning was delayed 5-10 days during the high water years of 1983-84 and extended 30-35 days later. 17