Stress in the spotlight: impacts of stress on learning and memory in a cichlid fish.

Stress has been shown to both enhance and inhibit learning, which requires memory formation and memory retention. To date, the question of how stress influences learning and memory retention has been especially well studied in rodents and primates. Here, we expand our understanding of how stress influences memory formation and retention across vertebrates using the African cichlid fish, Neolamprologus pulcher. Fish were randomly assigned to either a control group or a stressed treatment group (with repeated chasing) and then trained to learn a foraging task, where they had to first learn to move a single disc off a food tray and then learned to lift a particular-coloured disc to receive the food reward. More unstressed (control) fish learned the final foraging task compared to the stressed fish although we did not detect a difference in the number of trials taken to reach the learning criteria or the number of mistakes between stressed and unstressed fish. Once the fish had learned the foraging task, we tested their memory for the task after 12, 24 and 48 days without reinforcement. We show that approximately 80% of the fish, regardless of treatment, remembered the task after 12 days, and 55% of the fish tested remembered even after 48 days. When we compared across all the memory trials, the stressed fish overall showed more memory compared to the control fish. Our results provide a memory decay curve and show that stress dampened learning while enhancing memory, thus expanding our understanding of fish cognition.