Effect of noxious stimulation upon antipredator responses and dominance status in rainbow trout

A potentially painful experience may modify normal behavioural responses. To gauge the importance of pain relative to predation or social status, we presented competing stimuli, a predator cue or an unfamiliar social group, to two groups of noxiously treated rainbow trout, Oncorhynchus mykiss. In the predator cue experiment, fish were classified as bold or shy. Noxiously stimulated fish did not show antipredator responses, suggesting that pain is the imperative. In the social status experiment, noxiously stimulated fish held individually and undisturbed showed an increase in respiration rate and plasma cortisol. As a comparison, we used the dominant or subordinate fish in a group as the noxiously stimulated fish. After the noxious treatment, we returned this test fish to a familiar or unfamiliar social group. Neither dominants nor subordinates showed a negative change in physiology compared to their controls. However, in a familiar group the dominant was much less aggressive, suggesting a behavioural impairment in response to noxious stimulation. In an unfamiliar group, no reduction of aggression was seen, suggesting that maintaining dominance status took priority over showing signs of pain. These findings may reflect an ability to prioritize motivational drivers in fish, and as such provides evidence for central processing of pain rather than merely showing a nociceptive reflex.