Avoidance learning in goldfish (Carassius auratus) and trout (Oncorhynchus mykiss) and implications for pain perception

This paper investigates spatially cued behavioural responses of two species of fish to an acute noxious stimulus and demonstrates response elasticity. Typical avoidance responses to a nociceptive stimulus were used to test (1) if fish learn to avoid spatial areas associated with a potentially noxious stimulus, (2) learning and memory improves with increased stimulus intensity and (3) a supposedly innate reflex behavioural responses change depending on the circumstances. Electric shocks with two different intensities, low (2.5 V and 3 V for trout and goldfish, respectively) and high (25 V and 30 V for trout and goldfish, respectively), were administered directly to the skin to stimulate cutaneous nociceptors. Goldfish (n = 8) demonstrated spatially cued shock avoidance and an increase in stimulus voltage, significantly improved shock-avoidance learning and memory. However, trout (n = 8) demonstrated shock-avoidance learning but no significant stimulus discrimination and little information retention. The presence of a conspecific significantly changed this behavioural response to a noxious stimulus. Trout were willing to remain in the vicinity of the conspecific while being subjected to low intensity shock stimuli previously shown to elicit avoidance. Goldfish tended to leave this area yet remain in the mid-tank area, adjacent to the stimulating zone, rather than at the end of the tank. These results suggest that shock avoidance in fish is not purely a reflex action. Fish were prepared to change the supposedly innate avoidance reaction according to a change in circumstances, an important concept in the ongoing debate on pain perception in fish.