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Imagine you are quietly reading this story at home when—bang—you hear a loud crash nearby. You whirl your head toward the sound—a behavioral response—while your heart starts racing—a physiological response. These stress responses can mean the difference between life and death.
How these processes play out in the brains of humans and other mammals is well understood. But for fish, these pathways from the brain to behavior have largely been a mystery. Previous research shows that fish do, of course, react to external stressors. But new research reveals that fish’s brains regulate stress responses in a similar way to mammals’ brains.
Marco Cerqueira, a marine biologist at the University of Algarve in Portugal, investigated these stress responses by testing how European sea bass react to two types of stress: predictable and unpredictable. In training sessions, Cerqueira and his colleagues taught the fish to associate a visual cue, a black-and-yellow-striped card, with a stressor, 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, a signal of the impending stress. The fish subjected to unpredictable stress had no such warning: the card was displayed randomly before or after the net.
During experimental trials, the researchers recorded how the level of cortisol, a stress hormone, changed in response to the different types of stress. They also looked at the fish’s behavior, and measured activity in specific regions of their brains.
The key finding, says Carolyn Bauer, a biologist at Adelphi University in New York who was not involved with the research, is that fish are able to cope much better with predictable stressors.
The experiments showed that unpredictable stressors elicit stronger stress responses: fish spent more time stationary, but also made more attempts to escape compared to fish that experienced predictable stress or no stressful events. In the wild, periods of immobility would cost the fish foraging and traveling time.
Underlying these changes in behavior, the researchers measured higher cortisol levels and changes in activity in two areas of the fish’s brains most similar to the amygdala and hippocampus in our own brains. That these regions are responsible for processing stress responses suggests fish also possess the ability to evaluate potential stressors, says Cerqueira. This is why the response depends on a fish’s point of view and previous experiences, he adds.
The study raises major implications for animal welfare in commercial fish farming. “Maybe things need to be made really predictable for fish that we are raising for food or breeding,” says Bauer.