Effect of Temperature on Growth and Survival of Bull Trout, with Application of an Improved Method for Determining Thermal Tolerance in Fishes

Abstract

Elevated temperature is considered an important factor in the decline of the threatened bull trout Salvelinus confluentus, but the thermal requirements of this species have not been defined. We used the acclimated chronic exposure (ACE) method to assess the upper thermal limits and growth optima of bull trout fed daily to satiation over test temperatures ranging from 8°C to 28°C during 60-d trials. Survival of age-0 bull trout was at least 98% at 8, 10, 12, 14, 16, and 18°C, but 0% at 22, 24, 26, and 28°C after 60 d. The predicted ultimate upper incipient lethal temperature for these trout was 20.9°C. Peak growth, as estimated by regression analysis, occurred at 13.2°C (95% confidence interval, 10.9–15.4°C). Feed consumption declined significantly (P < 0.001) at temperatures greater than 16°C, and fish held at temperatures of 22°C and above did not feed. Feed, lipid, and protein efficiencies were similar at 8–18°C but declined significantly (P < 0.001) at 20°C. Our results corroborate field investigations suggesting that bull trout have among the lowest upper thermal limits and growth optima of North American salmonids. The slower acclimation times and long-term duration of the ACE method resulted in a more realistic measure of thermal tolerance in natural situations than would have been obtained with traditional methods and afforded sufficient time for sublethal differences in growth rate, feed consumption, and feed efficiency to become apparent.