From fright to anticipation: Reward conditioning versus habituation to a moving dip net in farmed Atlantic cod (Gadus morhua)

Using reward conditioning to change the cognitive interpretation of stressors has been suggested as a method to reduce stress in farmed fish. We studied the modification of behavioural and physiological responses of Atlantic cod to a stressor. Eight groups of cod were exposed to a moving dip net six times daily over 7 days. Half were rewarded with food 10 s after dip net exposure (trace reward conditioning), while the remaining groups were fed 30 min after dip net exposure (unrewarded). The swimming speed and oxygen consumption responses after dip net exposure in the unrewarded groups declined at a rate of 13% and 12% trial−1, respectively. The swimming speed response was lost completely, while 37% of the initial oxygen hyper consumption response remained after habituation. The rewarded groups learnt to associate dip net exposure with food delivery. When the dip net stopped moving, they approached it before gathering in the feeding area. Due to this anticipatory behaviour, swimming speed and oxygen consumption remained elevated after dip net exposure in the rewarded groups. Plasma cortisol and glucose levels remained low after dip net exposure on the last day in both groups. The experiment demonstrates that a stimulus that initially evokes a fright response can become a positive signal of a forthcoming reward.