It has been showed that juvenile rainbow trout (Oncorhynchus mykiss) held in pairs form dominance hierarchies in which subordinate individuals experience chronic social stress accompanied by lower thermal tolerance.

In a recent study published in Journal of Experimental Biology, scientists at the University of Ottawa, Carleton University and Aarhus University explored the hypothesis that chronic elevation of circulating cortisol levels reduces thermal tolerance in subordinate trout. This was done in a series of three experiments using Star-Oddi heart rate loggers.

Three experiments
The first experiment investigated the effectors of chronic social stress and elevated cortisol levels on critical thermal maximum (CT_max). The second looked at heart rate during a CT_max trial and aimed to provide more information on differential responses of dominant versus subordinate fish to acute warming. The third and final trial explored weather chronic social stress causes cardiac remodelling in subordinate trout.

In vivo heart rate measured during 5 days of social stress
Three groups (sham, dominant and subordinate) of juvenile rainbow trout weighing an average 143g were implanted with Star-Oddi's leadless DST micro-HRT (2.35% body mass). The loggers were programmed to record temperature, heart rate and electrocardiogram (ECG) traces during 5 days after a 20h recovery period.

Cortisol a major influencing factor
Results from the experiments showed that cortisol is a key driver to the lowered CT_max in rainbow trout experiencing chronic social stress. The authors propose additional research to identify the mechanisms linking changes in CT_max to circulating cortisol levels.

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