Fear and avoidance responses in the domestic rainbow trout
Welfare is determined by the quality of an animal's subjective experiences; these experiences in fishes remain largely unknown. This thesis investigated avoidance behaviour and the nature of fear, a negative subjective experience, in a common species of farmed fish, the rainbow trout, 'Oncorhynchus mykiss.' By using a combination of cognitive, preference, avoidance and conditioning tests, the responses of fish to threatening stimuli were teased apart into those which appeared to be reflexive and those which suggested the presence of subjective experience. In the first study, trout learned that a light cue signaled an oncoming plunging dip net. Upon light cue, trout swam into a safe area to avoid the dip net (P < 0.0001). However, it was uncertain whether trout's avoidance behaviour was a reflexive startle response. Therefore the following study addressed the issue of conscious, purposeful behaviour. Using the conditioned suppression technique, trout learned the task of pressing a pendulum for food rewards, then in a separate instance learned to associate a neutral light cue with a plunging dip net. When the light cue was superimposed on a steady bout of pendulum-pressing, trout ceased operant activity completely (P < 0.05), but recovered activity upon termination of the light cue. It is suggested that this suppression of appetitively motivated behaviour was due to a negative motivational state. Then fish were trained to associate either a plunging dip net, an intense flashing light or no stimulus with different distinctly marked compartments. In the absence of the aversive treatments, fish were allowed to move freely amongst compartments. Trout generally chose to avoid the compartments associated with aversive treatments. In the final study, social stress of being confined with conspecifics of different body sizes was investigated. Although cortisol concentrations did not differ among fish exposed to different size treatment groups (P > 0.05), there was a large increase above baseline levels suggesting that encounters with unfamiliar fish were stressful, regardless of size. Results suggest that trout have at least a rudimentary level of conscious awareness allowing them to experience fear as a psychological phenomenon.