Influence of heating rate on measurement of time to thermal maximum in trout

Thermal tolerance in fish is commonly assessed by exposing experimental animals to conditions of controlled temperature increase and by measuring the temperature and time period at which they reach loss of equilibrium (LE). To better understand how the rate of temperature increase might influence the sensitivity of a thermal tolerance test, we conducted a series of trials on juvenile rainbow trout Oncorhynchus mykiss, brook trout Salvelinus fontinalis and brown trout Salino trutta in a common experimental tank exposed to water heated at rates of 2, 4, 8 or 24 degreesC/day. Analysis of variance of rank-transformed data for time to LE indicated that at 2, 4 and 8 degreesC/day, differences between species were all significant. The brook trout were the first to reach LE, then rainbow trout and lastly the brown trout, although the differences between species tended to be less dramatic with an increase in heating rate. In two trials conducted at 24 degreesC/day, the brown trout were again the most tolerant. In one of these trials, the rainbow trout and brook trout were not significantly different, and in the other, the rainbow trout reached LE significantly sooner than the brook trout. Regression analyses indicated no consistent effect of fish size (length and weight) nor condition factor on time to LE within the ranges tested. Of the four heating rates, the slowest (2 degreesC/day) appeared to provide the greatest sensitivity for detecting differences in thermal maximum. We have adopted 2 degreesC/day as the heating rate for continued research on relative thermal tolerance in trout, in the perspective that the test might prove useful in efforts to selectively breed for increased tolerance among trout strains for commercial culture. (C) 2004 Elsevier B.V. All rights reserved.