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Young-of-the Year (YOY) Colorado squawfish Larger Age-O Colorado squawfish young (fingerlings) referred to as YOY fish when collected in the fall, are very rare in the Yampa River, but larvae are abundant soon after spawning (Haynes and Muth 1984). This, and the fact that the Green River below the Yampa has large numbers of YOY fish but apparently no major spawning area, has led to the development of a downstream transport and nursery hypothesis for larvae hatched in the Yampa River (Miller et al. 1982). This drift theory is now generally supported by other workers (Haynes et al. 1984, Wick et al. 1983) and suggests that the Yampa River reproduction may be the source of at least the upper Green River population of Colorado squawfish. Since the young Colorado squawfish apparently drift out of the Yampa system and into the more productive nursery habitats of the Green, some of these fish must, as they mature, also move back into the Yampa and repopulate it. Therefore, it is important to consider the survival of YOY fish in the upper Green River as of supreme importance in maintaining the Yampa River Colorado squawfish population. A fall YOY survey was conducted in the Green River in 1983 using the same sampling design as in previous years. Results from the 1979-82 seasons (Figure 12) indicated that the Green River is good nursery habitat for Age-Q Colorado squawfish, with two peaks of abundance occurring in the Ouray and Labyrinth Canyon areas (Figure 7). YOY surveys and other sampling in 1983 indicated poor survivorship of YOY Colorado squawfish (Figure 13). The cause of this year class failure in the Green River is unknown; however, it is probably related to the increased discharge of cold water from Flaming Gorge Dam. Temperature measurements taken' at the confluence of the. Yampa and Green rivers during the period of abundance for Colorado squawfish larvae, indicated about a 10 C difference in temperature between the Yampa River (21.5 C) and the Green River (12 C) in Echo Park. Records provided by the Bureau of Reclamation and our collections indicated that in 1979, 1980 and 1981, the temperature differences between the Yampa and Green River water, before mixing at Echo Park in August, were: 0 C (8-9-79),1.5 C (8-8-80), and 1.5C (8-5-81) respectively. These were years when relative abundance of Colorado squawfish in the Green River was high. On August 4, 1983, however, a temperature differential of 10 C (16.2 F) was recorded at this location. Figure 14 provides temperature records for Flaming Gorge from 1979-83, and it can be seen that the temperature in 1983, apparently a very poor year for YOY Colorado squawfish, was also the lowest released in July and August from 1979 to 83. 28