Chronic Social Stress Impairs the Thermal Tolerance of Rainbow Trout (Oncorhynchus Mykiss)

• Main Author: Bard, Brittany
• Other Authors: Gilmour, Kathleen
• Format: Others
• Language: en
• Published: Université d'Ottawa / University of Ottawa 2020
• Subjects: Thermal tolerance; Social stress; Cardiac; CTmax; Trout; Cortisol
• Online Access: http://hdl.handle.net/10393/40734; http://dx.doi.org/10.20381/ruor-24962

Juvenile rainbow trout (Oncorhynchus mykiss) held in pairs form dominance hierarchies, with subordinate individuals experiencing chronic social stress, as evidenced by prolonged elevation of the stress hormone cortisol. Prior work revealed that the thermal tolerance (measured as critical thermal maximum, CTmax) of subordinate fish was reduced, but the cause of this impairment was unknown. Here we tested the hypothesis that reduced thermal tolerance in subordinate trout is caused by prolonged elevation of circulating cortisol levels, affecting cardiac structure and function. In support of this hypothesis, subordinate trout that were allowed to recover from social stress for 48 h, a period sufficient to return cortisol to normal baseline levels, no longer showed a reduced CTmax. Furthermore, treatment of subordinates with cortisol to maintain elevated cortisol levels during the period of recovery from social stress prevented thermal tolerance from recovering. The possibility that prolonged elevation of cortisol levels induces cardiac remodelling in subordinate trout was explored by assessing heart histology and cardiac remodelling markers, and monitoring heart rate (fH). Picrosirius red staining revealed lower collagen levels in the ventricles of subordinate relative to dominant trout, although this difference was not accompanied by changes in collagen type I transcript abundances or protein levels, or by changes in markers of collagen turnover. Transcript abundances of markers of cardiac remodelling and ventricle mass were not significantly altered by chronic social stress. Heart rate in subordinates during social interactions was comparable to that in dominant fish. However, differences in fH responses of subordinate versus dominant fish were detected during acute warming. Specifically, peak heart rates tended to be observed at lower temperatures in subordinate fish relative to dominant. Thus, high baseline cortisol levels in subordinate trout result in lowered thermal tolerance, and chronic social stress has only minor effects on cardiac structure and function.