The Energetics of the Tahoe Sucker, Catostomus tahoensis, in Cyclic and Constant Temperatures

DOCTORATE DISSERTATION: Recent hypotheses suggest that aquatic organisms will exhibit faster growth rates in diel temperature cycles as compared with organisms that remain at constant temperatures. Attendant assumptions are lower metabolic rates and increased assimilation efficiency in cyclic as opposed to constant temperatures. The growth and metabolic rate fo the Tahoe sucker, Catostomus tahoensis, were evaluated in diel temperature cycles which approximate natural temperature regimes occurring between late spring and early autumn. Three experiments were conducted in cyclic temperatures and constant temperatures equivalent to the arithmetic mean of the temperature cycles. The Tahoe sucker grew faster in a reservoir than in two tributary streams. Growth rates were evaluated both by comparison of back-calculated lengths using pectoral fin rays and by comparison of standard lengths at capture of age V suckers. Four possible factors were hypothesized to explain the observed growth differential: 1. genetic differences, 2. extra energy costs of swimming in flowing systems, 3. food availability, and 4. temperature regimes. Stampede Reservoir was formed in 1969 and the parental stock of suckers was originally present in Sagehen Creek and the Little Truckee River suggesting genetic differences are not a factor. The Tahoe sucker selects areas of low flow rate obviating significant energy expenditures to maintain station. Digestive tract fullness and energy content of the diets were not significantly different in the streams of the reservoir, which indicated food availability was not a factor influencing the growth differential. Stream temperatures flucturated 12 C daily, whereas reservoir temperatures remained nearly constant daily and varied little throughout July and August. Mean temperatures where the suckers reside in both streams and reservoir were nearly identical during the summer sampling period. Growth season length or increased maintenance metabolic costs for stream residents are likely temperature-related phenomena explaining observed growth rate differentials. Resting routine metabolic rates of the Tahoe sucker were measured using static, plexiglass respirometers in cycling and constant temperatures over the thermal range of 8-20C. Q10 values approximating three for both cyclic and constant temperature regimes indicate that the Tahoe sucker in a cyclic temperature regime are significantly higher than rates of suckers acclimated to a constant temperature equivalent to the arithmetic mean of the cycle. These higher energetic costs for routine metabolism may explain reduced Tahoe sucker growth rates in streams compared with reservoirs. Growth rates, gross growth efficiencies and maintenance rations of the Tahoe sucker were determined in cyclic and constant temperatures over a thermal range of 4-23 C. Growth rates and gross growth efficiencies were determined in three temperature pairs (low, 8 C constant, 4-12 C cycle; intermediate, 13 C constant, 8-18 C cycle; and high, 18 C constant, 13-23 C cycle) and at three ration levels (starvation, restricted and repletion). Growth rates and gross growth efficiencies were not significantly different at any temperature pair-ration condition, except at low temperatures and starvation ration. Maintenance rations increased from approximately 1% initial wet weight/d at low temperatures 1.7% at high temperatures and 2.0% at intermediate temperatures. Brought together, the results of the three experiments question the universality of previous assumptions regarding growth and metabolism of fishes exposed to sinusoidal temperature fluctuations.