Resting routine metabolism was monitored for the tahoe sucker, (Catostomus tahoensis), in acrylic static respirometers while exposed to naturally-occurring diel temperature fluctuations (8-18?C) and to constant temperatures (3, 8, 13 and 18? C). Oxygen consumption rates increased exponentially with increasing temperature according to: (for constant temperature) log Y = 1.130 + .060T; (for fluctuating temperature) log Y = 1.392 + .051 T, where Y = oxygen consumption rate (mg 02 kg-1 body mass h-1 and T = temperature (?C). Comparison of these regressions indicates metabolic increases for fish exposed to fluctuating temperatures, compared with those in constant temperature regimes. Metabolic rate was also monitored in Brett-type respirometers where suckers were subjected to water velocities equivalent to one body length sec-1 in a fluctuating temperature environment. Oxygen consumption rates of these fish increased with increasing temperatures according to: log Y = 1.378 + .046T. Physiological effects of fluctuating temperatures on metabolism and growth of stream fishes should be considered in forestry and fisheries management decisions.
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