Environmental change alters personality in the rainbow trout, Oncorhynchus mykiss

Boldness is a personality trait that defines how individuals respond to risky situations and has clear fitness consequences. Since the adaptive value of boldness is context dependent, the benefit of a distinct personality is less clear when the environment is unpredictable. An ability to modulate behaviour can be beneficial, although as behavioural plasticity itself may be costly this depends on the levels of environmental stability. Both boldness and its plasticity are linked with physiological stress coping mechanisms, whereby animals with reduced glucocorticoid responses to stress are bolder and less flexible in behaviour. We investigated the behavioural changes made by bold and shy rainbow trout, and the magnitude of those changes, in response to predation risk and exposure to two environmental challenges. Behavioural and physiological responses under biotic (either no, predictable or unpredictable predation risk) and abiotic (temperature increase or hypoxia) factors were measured. Boldness was determined using a standard novel object paradigm. In general, after exposure to the treatments, fish exhibited less extreme bold or shy behaviour; the greatest change was observed in fish exposed to hypoxia, or those exposed to high risk particularly in shy fish held at a lower temperature. Higher risk also resulted in increased stress, suggesting that extreme bold or shy behaviour might have been maladaptive under a potential predator threat. These results represent novel evidence that boldness is flexible depending upon particular environmental challenges, with important implications for populations facing environmental extremes caused by anthropogenic activity and climate change.