Learned recognition of a novel odour by wild juvenile Atlantic salmon, Salmo salar, under fully natural conditions

The ability to learn context-specific information regarding local predation risk is of prime importance for prey individuals balancing the conflicting demands of predator avoidance with other fitness-enhancing activities. Many taxonomically diverse prey species must learn to recognize novel predators and make context-appropriate responses. Predator-recognition learning in freshwater prey fish is commonly facilitated through the association of a novel predator cue with a damage-released chemical alarm cue. While a multitude of laboratory studies have demonstrated the sophistication of chemically mediated predator-recognition learning, no field verifications of this mechanism have been conducted. We conducted an experiment to determine whether the single pairing of a novel odour (lemon essence) with a damage-released chemical alarm cue could function to facilitate learning in wild juvenile Atlantic salmon under fully natural conditions. Tagged juvenile salmon were initially exposed to either a paired stimulus of conspecific alarm cue and lemon odour or a control of stream water and lemon odour. As predicted, salmon exposed to the alarm cue during the conditioning trials showed a significant increase in antipredator response. When salmon were subsequently exposed to lemon odour alone, only those initially exposed to the alarm cue paired with the lemon odour showed a significant increase in alarm response. Those initially exposed to the stream water and lemon odour control did not show any change in behaviour (i.e. did not learn). As such, these data demonstrate that chemically mediated, acquired predator recognition can occur under fully natural conditions.