Evaluation of the thermal tolerances of different strains of rainbow trout Oncorhynchus mykiss by measuring the effective time required for loss of equilibrium at an approximate upper lethal temperature

The thermal tolerance phenotypes of different strains of rainbow trout Oncorhynchus mykiss was investigated in this study. The time at which a fish, acclimated to 17 °C, was no longer able to maintain its equilibrium after reaching the test temperature of 28 °C was recorded as its effective time (ET) in the zone of thermal resistance and used as a parameter of its thermal tolerance. The ET was compared among two strains of rainbow trout, thermally selected and Nikko, and two types of their hybrid progenies (thermally selected male × normal female and thermally selected female × normal male), using 14–15 individuals of each group in tests. Individuals of the thermally selected strain with smaller bodies had a longer ET than those with lager bodies. Five groups of F2 juveniles produced from a full-sibling cross of F1 progenies (Nikko strain females × thermally selected strain males), each containing 40 or 41 individuals, had an ET of 39.3–46.4 min, which was not significantly correlated with body size. Although its ET was influenced by body size, the thermally selected strain showed good thermal tolerance at all body sizes. Moreover, the coefficient of variation of ET was 32–39%, suggesting there was a wide diversity of ET values. Because samples subjected to ET experiments can recover and survive, ET is a promising, convenient indicator to use when examining thermal tolerance phenotypes of large numbers of individuals, which can subsequently provide live individuals with inherited thermal tolerance for use in selective breeding.