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2 the sample sites, and catch rates were below 0.5 fish/10 m2. The cause of the apparent year class failure is unknown, however, it could have been related to the increased discharge of cold water from Flaming Gorge Dam that year. Temperature differences of 10 C were recorded between the Yampa (21 C) and the Green (12 C) Rivers in late August when larvae were present. Larvae drifting from the Yampa into the Green River could have been shocked by the colder waters. In addition, warm backwater nursery habitat was virtually nonexistant in the Green River because of the high flow (Archer et al., 1984). Data on cold shock effects on other fish indicate that shocks of 5-10 C are not usually lethal in laboratory experiments (see reviews by Nickum 1966, Coutant 1977} Elliott 1981). However, shocked fish may exhibit behavioral changes (Larimore and Duever 1968, Peterson and Anderson 1969, Hocutt 1973, Otto and Rice 1974), loss of equilibrium (Chittenden 1972), reduced ability to avoid predators (Coutant et al., 1974;-Wanjala and Tash 1973), and hematological changes (Stanley and Colby 1971, Maetz 1972, Houston 1973). The summation of these sublethal effects, especially loss of equilibrium, would mean ecological death to a wild fish. Squawfish seem to be as resistant as other species to cold shock (Hammon 1985). While rearing squawfish fry at Willow Beach National Fish Hatchery, temperatures were routinely reduced 6 C in 30 minutes without effect, and once were accidentally reduced 14 C in 10 minutes. In the latter case, only a change in feeding behavior was noted (Hammon 1981). The temperature preference of juvenile and adult squawfish was 21.9 C and 21.5 C respectively, which is similar to other eurythermal species (Bulkley et al., 1981). However, no definitive data is available on larval tolerance of sudden temperature decreases. Consequently, a laboratory