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<br />Environmental Biology of Fishes 46: 197-209, 1996,
<br />@ 1996 Kluwer Academic Publishers, Printed in the Netherlands.
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<br />Growth, survival, and starvation resistance of Colorado squawfish larvae
<br />
<br />Kevin R. Bestgen
<br />Larval Fish Laboratory, Department of Fishery and Wildlife Biology, Colorado State University, Fort Collins,
<br />1 CO 80523, Us.A.
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<br />Received 17.11.1994
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<br />Accepted 14.8.1995
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<br />Key words: Ptychocheilus lucius, Cyprinidae, Endangered, Colorado River, Temperature, Food abundance,
<br />Regulated river, Fish larvae, Critical period, Point of no return
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<br />Synopsis
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<br />Growth and survival of Colorado squawfish, Ptychocheilus lucius, larvae under fluctuating 18, 22, and 260 C
<br />(50 C diet fluctuations) and constant 18, 22, 260 C, and 300 C temperature conditions and ration size corre-
<br />sponding to 12.5, 28, 64, 142, 320 brine shrimp nauplii fish-I day-I was determined from laboratory experiments.
<br />Growth was optimal at 310 C and high at temperatures of 260 C to 300 C, at the highest food abundance.
<br />Lowest growth was under lowest food rations and highest temperatures. Growth of Colorado squawfish lar-
<br />vae declined substantially at temperatures < 220 C. Neither growth nor survival was significantly different
<br />between fluctuating or constant regimes. Survival of Colorado squawfish larvae was highest (95 %) at 26.20 C
<br />and 235 nauplii fish-1 day-I and high at temperatures of 20 to 300 C with food abundance> 180 nauplii fish-I
<br />day-I. Survival was lowest when food abundance was low and temperature was high. Highest mortality oc-
<br />curred more than 20 days after experiments began and mortalities occurred sooner in higher than lower
<br />temperatures. Colorado squawfish larvae denied food for 5, 10, or 15 d after first feeding could have begun
<br />(6 d), had survival greater than 87% which was equivalent to continuously fed controls. Survival of fish denied
<br />food for 17.5 d after feeding could have begun declined from 84% before feeding to 57% after feeding. Point
<br />of no return was estimated between 17.5 and 20 d. Colorado squawfish have relatively high starvation resist-
<br />ance. Low, stable flows that simulate natural hydrographs may enhance growth, survival, and recruitment of
<br />early life stages of Colorado squawfish by increasing water temperature and food abundance in regulated
<br />rivers of the Colorado River basin.
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<br />Introduction
<br />
<br />The Colorado squawfish, Ptychocheilus lucius, is a
<br />large, predaceous cyprinid endemic to the Colorado
<br />River Basin. Although formerly widespread and
<br />abundant, populations are now limited to the San
<br />Juan, Colorado, and Green rivers and their major
<br />tributaries in the upper Colorado River basin (Pla-
<br />tania et al.1991, Tyus 1991a). The species is listed as
<br />endangered by the U.S. Fish and Wildlife Service.
<br />Decline of Colorado squawfish has been at~:~ollted
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<br />partly to nonnative fishes that compete with or prey
<br />on early life stages (Miller 1961, Williams et al.1989,
<br />Carlson & Muth 1989). Mainstream dams con-
<br />structed throughout the Colorado River Basin
<br />since 1913 have also negatively affected distribution
<br />and abundance of Colorado squawfish (Vanicek et
<br />al. 1970, Holden 1979, Tyus & Haines 1991). Low
<br />summer water temperatures caused by hypolimnet-
<br />ic releases from these dams may curtail reproduc-
<br />tion of Colorado squawfish and reduce growth, sur-
<br />vival, and recruitment of early life stages (Vanicek
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