Hematological adaptations in diploid and triploid Salvelinus fontinalis and diploid Oncorhynchus mykiss (Salmonidae, Teleostei) in response to long-term exposure to elevated temperature

Abstract

Hematology is a simple but reliable method to determine the influence of environmental stressors on the physiology and performance of teleost fish. Understanding the detailed impacts of elevated temperature on the hematology of fish can help us to evaluate fish responses to climate change. Therefore, cellular and biochemical changes in peripheral blood were investigated in diploid (2n) and triploid (3n) brook trout and in 2n rainbow trout exposed to 20 °C for 32 days in comparison to fish acclimated to 9 °C. Additionally, survival and growth rates and the relative mRNA expression of heat-shock proteins were recorded. All brook trout and rainbow trout survived exposure to 20 °C. At 20 °C, growth was decreased in 2n rainbow trout, increased in 2n brook trout and similar to growth at 9 °C in 3n brook trout. The erythrocyte cell volume, nucleus volume, cell surface, and nucleocytoplasmatic ratio were significantly lower at 20 °C than at 9 °C in all investigated species/ploidy levels. In contrast, the erythrocyte surface-to-volume ratio was not affected by temperature. In 2n and 3n brook trout, erythrocyte concentration and blood hemoglobin content were increased at 20 °C; in 2n rainbow trout, there were no differences in comparison to 9 °C. Exposure of 2n and 3n brook trout and 2n rainbow trout to 20 °C had no negative effect on cellular and molecular immune components in blood. Serum diagnostic enzymes and metabolites indicated neither organ inflammation nor disease nor disturbance in energy metabolism or nutrition status in fish exposed to 20 °C. In addition, the mRNA expression of Hsp70 and Hsp90 relative to the 28S ribosomal protein S32 did not differ between 9 °C and 20 °C.