GROWTH, SURVIVAL, AND STARVATION RESISTANCE OF COLORADO SQUAWFISH LARVAE

Growth and survival of Colorado squawfish, Ptychocheilus lucius, larva e under fluctuating 18, 22, and 26 degrees C (5 degrees C diel fluctua tions) and constant 18, 22, 26 degrees C, and 30 degrees C temperature conditions and ration size corresponding to 12.5, 28, 64,142, 320 bri ne shrimp nauplii fish(-1) day(-1) was determined from laboratory expe riments. Growth was optimal at 31 degrees C and high at temperatures o f 26 degrees C to 30 degrees C, at the highest food abundance. Lowest growth was under lowest food rations and highest temperatures. Growth of Colorado squawfish larvae declined substantially at temperatures < 22 degrees C. Neither growth nor survival was significantly different between fluctuating or constant regimes. Survival of Colorado squawfis h larvae was highest (95%) at 26.2 degrees C and 235 nauplii fish(-1) day(-1) and high at temperatures of 20 to 30 degrees C with food abund ance > 180 nauplii fish(-1) day(-1). Survival was lowest when food abu ndance was low and temperature was high. Highest mortality occurred mo re than 20 days after experiments began and mortalities occurred soone r in higher than lower temperatures. Colorado squawfish larvae denied food for 5, 10, or 15 d after first feeding could have begun (6 d), ha d survival greater than 87% which was equivalent to continuously fed c ontrols. Survival of fish denied food for 17.5 d after feeding could h ave begun declined from 84% before feeding to 57% after feeding. Point of no return was estimated between 17.5 and 20 d. Colorado squawfish have relatively high starvation resistance. Low, stable flows that sim ulate natural hydrographs may enhance growth, survival, and recruitmen t of early life stages of Colorado squawfish by increasing water tempe rature and food abundance in regulated rivers of the Colorado River ba sin.