Simulated Thermal Tempering versus Sudden Temperature Change and Short-Term Survival of Fingerling Rainbow Trout

Abstract

We examined the effects of thermal shock on poststocking survival of fingerling rainbow trout Oncorhynchus mykiss (mean total length, 99 mm; mean weight, 10 g), some of which were thermally tempered at stocking and others not. In one experiment we assessed survival when fish experienced increases in water temperature from 8°C to 24°C at two tempering rates (4°C/h and 8°C/h) and by an immediate transfer. In a second experiment we assessed survival when water temperatures were cooled to 0.3°C for 2 h and fish were subsequently transferred into 8°C and 24°C water without tempering. Both experiments were conducted with controls at 8°C that were subjected to the same handling procedures without any changes in water temperature. Fingerling rainbow trout were found to be very tolerant of all treatments, with few or no mortalities during the treatments or during the 2 weeks following treatment. Our experiments suggest that rapid changes to high but sublethal temperatures will not directly cause high mortalities of fingerling rainbow trout stocked into waters for sport fisheries, but short-term behavior anomalies or other stressors may affect poststocking survival in the wild.