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t ' <br />18 <br />1984). Temperature differences were 10 C in 1983, the year of suspected <br />year class failure, and 6-7 C in 1984. Laboratory data partially support <br />the hypothesis that low temperatures could have caused year-class failure <br />because the 10 C decrease in 5 minutes caused reduced activity, but not <br />loss of equilibrium. Whether this in turn led to increased mortality from <br />other causes is unknown but likely. On the other hand, a larvae floating <br />into the Green River may not have experienced such an abrupt temperature <br />change if it remained in the Yampa plume which gradually mixed with colder <br />Green River waters. Squawfish larvae can withstand a 15 C decrease if it <br />occurs at a rate of 3 C/hr. <br />CONCLUSION <br />The conclusion that a 15 C shock in 5 minutes is lethal and a 10 C <br />shock in 5 minutes causes behavioral changes was reached both years, <br />however, the validity of the data is compromised by problems with the 1984 <br />experiment, and by high mortality in all treatments including controls. <br />Never-the-less these results agree with most literature that shows that <br />larval fish of some species can tolerate 8-15 C cold shocks, and with <br />visual observations by fish culturists raising squawfish. Additionally, <br />the squawfish behavioral changes and effects of age were also typical of <br />other fish. Unfortunately, the bioassay results were equivocal concerning <br />the 1983 river temperature and year-class observations because the exact <br />shock conditions are unknown. The temperature differences found between <br />the Yampa and Green Rivers in 1984 were probably harmless; those in 1983 <br />(Miller et al. 1984) could have caused behavioral changes that increased <br />larval mortality if the shock was abrupt.