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<br />Effects of Cold Shock on Colorado Squawfish LarvaeI <br /> <br />Sampling larval fishes in search for rare and endangered species has been <br />an important aspect of the field work that has been conducted over the past few <br />years by State and Federal agencies in the Upper Colorado and Green rivers. <br />After the initial problems of gear type, sampling methods,'and identification <br />were solved, the larval fish data helped us understand more about the population <br />status and biology of rare fish, such as the Colorado squawfish. We now have <br />information on larval distribution, habitat use, spawning times and locations, <br />nursery areas, and yearly abundance. For example, the appearance of larval <br />Colorado squawfish in drift nets, coupled with radio tracking results from adult <br />fish, were used to locate spawning sites on the Yampa River, just upstream from <br />the confluence with the Green River. <br />It was the larval data below the Yampa spawning site that first raised a <br />question about the changes in water quality at the confluence, and prompted our <br />study of the effects of cold shock on larval survival. Spawning occurs in late <br />July In the Yampa, and the larvae appear 10-20 days later downstream and make up <br />as much as 10% of all larval fish sampled in the lower reaches of the Yampa. <br />However, once in the Green, the larvae become scare (Miller et al. 1984). The <br />water temperature of the Green can be colder than that of the Yampa because of <br />releases from Flaming Gorge Dam. Previous studies at the Utah Cooperative Unit <br />showed that Green River temperatures below the dam were below the preference and <br />avoidance temperatures of juvenile rare fish (Bulkley and Pimentel 1983), but we <br />had no information on the tolerance of squawfish larvae to sudden decreases in <br />water temperature. The possibility of-adverse effects existed because cold <br />1This report is the text of a paper presented at the Annual meeting of the <br />Western Division of the American Fisheries Society held at Snowmass, Colorado, <br />8-11 July 1985. <br />a226PrR.crb Page 2