HPI reactivity does not reflect changes in personality among trout introduced to bold or shy social groups

Physiological stress responses often correlate with personalities (e.g. boldness). However, this relationship can become decoupled, although the mechanisms underlying changes in this relationship are poorly understood. Here we quantify (1) how an individual’s boldness (response to novel objects) in rainbow trout, Oncorhynchus mykiss, changes in response to interactions with a population of either bold or shy conspecifics and we (2) measured associated post-stress cortisol levels. Initially-bold trout became shyer regardless of group composition, whereas shy trout remained shy demonstrating that bold individuals are more plastic. Stress-induced plasma cortisol reflected the original personality of fish but not the personality induced by the treatment, irrespective of population personality. Change in boldness of bold trout may indicate preference towards initially subordinate behaviour when joining a new population. However, here we provide further evidence that behavioural and physiological parameters of coping styles may become uncoupled whereby behavioural changes are not correlated with stress responsiveness.