• demand feeding;
  • high temperature breeding;
  • Oncorhynchus mykiss;
  • rainbow trout;
  • thermal tolerance

ABSTRACT:  Feeding activity was examined at high temperatures by using a demand feeder for thermally selected and normal rainbow trout Oncorhynchus mykiss strains. When the water temperature was raised in experiment 1 from 17.5 to 25.7°C at 0.3°C/day in 21 days, the average daily food consumption rate in the thermally selected strain rose to 7.1%, which was significantly higher than that of the normal strain (4.1%, P < 0.05). The corresponding rate was also significantly higher in the thermally selected (0.8%) than in the normal strain (0.2%, P < 0.05) of fish in experiment 2 where water temperature was raised to 24.1°C in 0.5°C/day increments. When water temperature was raised rapidly in experiment 3 from 16.7 to 21.7°C in one day and gradually to 24.4°C in 28 days at 0.1°C/day, the average daily food consumption rates were 1.0 and 0.1% for the thermally selected and normal strains, respectively, with significant difference (P < 0.01). These results suggest that the thermally selected strain has acquired thermal tolerance as a result of artificial selection.