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B -;z c;: , \:, ,:.:~ .; .1~t :J / /7<>ivf f fl:)I''''P/r-,(V ! !:)'I~O _ I I f I I i r J f :.' " r r; f r l t ' j' JUNE 15, 1988 EFFECTS OF COLD SHOCK ON COLORADO SQUAWFISH LARVAE CHARLES R. BERRY, JR. ABSTRACT-Larvae oC Colorado squawCish (Ptychocheilus Lucius) acclimated to 220C were subjected to water temperature decreases oC 5, 10, or 150C within 5 min or within 240 to 300 min. Most mortality due 10 shock occurred in the Cirst 48 h. The 150C shock in 5 min caused signiCicant mortality oC 14-day-old larvae but did not aHect 40-day-olds, In addition to causing (his direct mortality, shocks oC 10 or 150C in 5 min caused behavioral changes in l4-day-olds that could result in indirect mortality of wild Cish. The Colorado squawfish (Ptychocheilus lucius) is endemic to the Colorado River basin and historically was widely distributed in the Colorado River and its major tributaries. After water developments on the Colorado River system (Haynes et al., 1984), squawfish populations declined; the species is now protected by the Endangered Species Act of 1973. One of several possible causes for this decline is lowered water temperature below large dams (Miller, 1961; Marsh, 1985). Water temperature from 20 to 260C is best for Colorado squawfish embryo survival (Marsh, 1985), yearling growth (Black and Bulkley, 1985a) and adult spawning (Haynes et aI., 1984). The final temperature preferendum of yearlings is 250C, and larvae are usually collected where water temperatures range from 19 to 260C (Haynes et al., 1984). Adult and yearling squawfish can regulate somewhat the effect of water temperature change by avoiding \ extremes, but passively drifting larval stages cannot. Therefore, larvae drifting into any cold water plume could be adversely affected. Such a situation could occur at the confluence of the Yampa and Green rivers. Colorado squaw fish spawn in the lower Yampa River, and larvae drift into the Green River, which is influenced by discharges from Flaming Gorge Dam (Haynes et al., 1984; Tyus and McAda, 1984). No information on the effects of rapidly decreasing water temperatures on Colorado squaw fish was available, so I conducted a laboratory study to determine larval tolerance to various cold shocks. t, ~ . '. i ~ . . ' ~ r I I i , I I."" , Li'. ~. ,.\ , ,", j" ." ",. , . f " I I MATERtALS AND METHODS-Cold shock experiments were conducted on 14-day-old (total length 9.0 :t 0.3 mm) squawCish larvae in 1984 and 1985 and on 40-day-old Cish (total length 24.4 :t 0.4 mm) in 1984. Larvae Cound in the lower Yampa River in July 1981 by Haynes et al. (1984) ranged in total length Crom 9 to 13 mm and in estimated age Crom 8 to 26 days old; hence, the 14-day- old, 9-mm-long larvae used in this study Cairly represented wild Cish that might drift into the Green River. Adults that produced larvae Cor my study were spawned, and eggs incubated, at the Dexter (New Mexico) National Fish Hatchery. One-day-old sac Cry were air-shipped to Logan, Utah, where they were held at the Utah Water Resources Research Laboratory at 220 in 74-1, f1ow- through aquaria. Fish were Ced nauplii oC brine shrimp (Artemia). ad lib throughout the study. Fish were counted and moved to bioassay aquaria by siphoning. Bioassays were in 3.7-1 plastic aquaria receiving a continuous 0.25-l/min flow oC water (hardness 174 mg/l; pH 7.1; dissolved oxygen 7.9 to 10.0 mg/l). Photoperiod was 14L:IOD. Two aquaria with 100 larvae each were used Cor each treatment including controls in 1984, and three aquaria with 50 larvae each in 1985. t. I. I I I 0,' I. . I , i !