Juvenile and adult hardhead Mylopharodon conocephalus oxygen consumption rates: effects of temperature and swimming velocity

California’s populations of hardhead Mylopharodon conocephalus , a species of special concern, have declined, possibly due to dam construction with consequent temperature and water-velocity changes, and the introduction of non-native species. Environmental temperature effects on this large (to 60 cm SL) cyprinid, and its swimming abilities, are not well known. To address these deficiencies and to assist conservation efforts, we measured resting and swimming metabolic rates of adult and juvenile hardhead acclimated to four temperatures (11, 16, 21, or 25 °C). Resting metabolic rates (RMR, mg O 2 kg −0.79 h −1 ) generally increased with acclimation temperature, in adults and juveniles, with low to moderate thermal sensitivity (Q 10 range: 1.33–2.04). Swimming metabolic rates, in Brett-style respirometers, of adults ranged from 209 to 1342 mg O 2 kg −1 h −1 at velocities from 30 to 90 cm s −1 , and juveniles ranged from 393 to 769 mg O 2 kg −1 h −1 from 10 to 50 cm s −1 . Adults were lethargic at 11 °C and juveniles frequently refused to swim at 11 and 16 °C, but all fish swam well at 21 and 25 °C. These results suggest that hardhead are well-suited for sustained aerobic activity over a range of flow velocities, at moderate temperatures (ca. 16 to 21 °C). However, juveniles, emerging in spring, may not be able to perform in cold water and/or high flow velocities, providing a caution to dam managers and regulators to avoid spring and summer operations whereby juveniles experience conditions outside of those occurring in unregulated rivers.