Marine biologists in Portugal observed the physiological and physical responses of European sea bass to predictable and unpredictable stressors. They first taught the fish to associate the sight of a black-and-yellow-striped card with a stressor, namely a mesh net like the ones used to catch fish in aquaculture operations. The fish in the predictable group always saw the card a minute before the net swept through the fish tank, and so it served as a signal of the impending stressor. The fish subjected to unpredictable stress had no such warning: the card was displayed randomly, before or after the net appeared.

During experimental trials, the researchers recorded changes in cortisol, a stress hormone, in response to predictable vs. unpredictable stressors. They also looked at the fish’s behavior, and measured activity in specific regions of their brains. The fish coped better with predictable stressors. In the unpredictable stressor group, they found stronger stress responses, including higher cortisol levels and more changes in activity in two areas of the fish’s brains that are most similar to the amygdala and hippocampus in human brains. Fish in the unpredictable stress group also made more attempts to escape compared to fish that experienced predictable stress or no stressful events.

[Reference: Cerqueira, M., Millot, S., Felix, A., Silva, T., Oliveira, C.C.V., Rey, S., MacKenzie, S., & Oliveira, R. F. (2020, in press). Cognitive appraisal in fish: Stressor predictability modulates the physiological and neurobehavioural stress response in seabass. Proceedings of Royal Society London B]