From fright to anticipation: using aversive light stimuli to investigate reward conditioning in large groups of Atlantic salmon (Salmo salar)

In this study, we demonstrate how an event that is initially frightening to Atlantic salmon is turned to a positive stimulus through habituation and associative learning. The study was carried out in four commercial sized tanks (5 m³) with near industry densities (>550 fish, 16 kg m⁻³), using a delay conditioning procedure with an aversive flashing light as the conditioned stimulus and food reward as the unconditioned stimulus. By using video image analysis of the distribution of the fish in the tanks, the changes in behaviour from trial to trial could be documented in great detail. The current study documents the change in behaviour across the individual conditioning trials, clearly showing the step-by-step nature of the transition. The salmon needed more than 26 trials to become fully habituated to the flashing light but showed clear anticipatory behaviour already after about 19 trials. This demonstrates that the learning process is a combination of habituation and associative learning.