An unusually high upper thermal acclimation potential for rainbow trout

Thermal acclimation, a compensatory physiological response, is central to species survival especially during the current era of global warming. By providing the most comprehensive assessment to date for the cardiorespiratory phenotype of rainbow trout (Oncorhynchus mykiss) at six acclimation temperatures from 15°C to 25°C, we tested the hypothesis that, compared with other strains of rainbow trout, an Australian H-strain of rainbow trout has been selectively inbred to have an unusually high and broad thermal acclimation potential. Using a field setting at the breeding hatchery in Western Australia, thermal performance curves were generated for a warm-adapted H-strain by measuring growth, feed conversion efficiency, specific dynamic action, whole-animal oxygen uptake (ṀO₂) during normoxia and hypoxia, the critical maximum temperature and the electrocardiographic response to acute warming. Appreciable growth and aerobic capacity were possible up to 23°C. However, growth fell off drastically at 25°C in concert with increases in the time required to digest a meal, its total oxygen cost and its peak ṀO₂. The upper thermal tipping points for appetite and food conversion efficiency corresponded with a decrease in the ability to increase heart rate during warming and an increase in the cost to digest a meal. Also, comparison of upper thermal tipping points provides compelling evidence that limitations to increasing heart rate during acute warming occurred well below the critical thermal maximum (CT_max) and that the faltering ability of the heart to deliver oxygen at different acclimation temperatures is not reliably predicted by CT_max for the H-strain of rainbow trout. We, therefore, reasoned the remarkably high thermal acclimation potential revealed here for the Australian H-strain of rainbow trout reflected the existing genetic variation within the founder Californian population, which was then subjected to selective inbreeding in association with severe heat challenges. This is an encouraging discovery for those with conservation concerns for rainbow trout and other fish species. Indeed, those trying to predict the impact of global warming should more fully consider the possibility that the standing intra-specific genetic variation within a fish species could provide a high thermal acclimation potential, similar to that shown here for rainbow trout.

Influence of preimaginal constant and alternating temperatures on growth rate and longevity of adults of five genotypes in Tribolium castaneum

The longevity of adults of five genotypes of Tribolium castaneum differing in their body weights was measured at a single constant temperature, 35 degrees C, after they had developed at three constant temperatures, 25, 30 or 35 degrees C, and one alternating temperature 25/35 degrees C (mean = 30 degree C). Two genotypes had been naturally selected for heavy and light body weights, two had been artificially selected for extreme pupal weights and one was the pygmy mutant. The main results are as follows. (1) There is a negative correlation between growth rate and imaginal longevity for four of the five genotypes, when the variations in growth rate are due to the influence of constant developmental temperatures. (2) The genotype has a marked effect on mean longevity and on the slope of the regression of longevity on growth rate. (3) Growth rate is larger and longevity is longer when the larvae are raised at alternating 25/35 degrees C than when they are raised at a constant 30 degrees C. These results seem to confirm the developmental theory of ageing.