Most mammals, including humans, get stressed when something unpredictable happens. A lack of control or certainty sets off a cascade of reactions throughout our bodies known as the ‘stress response’. This can include not only changes in our behaviours but also fluctuations in our hormone levels and, in some cases, alterations even to the activity of our brains. Recent work by Marco Cerqueira and colleagues from universities in Portugal, France and the UK suggests that this sort of response to erratic environments is not limited to mammals – fish seem to experience it, too.

The team raised European sea bass in the lab to see whether the fish would change their behaviours depending on whether they could anticipate a stressful event – in this case, being briefly confined by a net to a small area of their tank. The researchers initially split the fish into groups. In the first group, they repeatedly showed the sea bass a yellow and black striped picture right before trapping them in the net. In the second group, the researchers showed the fish the same image, but at a random time either before or after the net started moving towards them. The idea was that fish from the former group should be able to anticipate the net's arrival while members of the latter group should not be able to tell when they would be trapped. Cerqueira and colleagues then set up cameras to watch how the fish responded to seeing the menacing yellow and black stripes.

The sea bass that could predict when they would be trapped seemed less stressed upon seeing the ominous striped picture, spending much less time either frozen in place or trying to escape. This idea was further supported when the researchers realized that these fish also spent much more time swimming around their tanks, ready to explore rather than cowering in fear. These differences in behaviour suggest that sea bass are less stressed when they are in predictable environments.

To confirm this, Cerqueira and colleagues looked to see whether the fish that anticipated the arrival of the nets had lower levels of a stress hormone, cortisol, in their blood. They collected blood samples from both groups and discovered that, as expected, there were lower levels of cortisol in the blood of sea bass that could foresee their incarceration.

As predictability seems to affect both the behaviours and hormone levels of sea bass, Cerqueira and colleagues had a hunch that stress could also affect their brains. The researchers looked specifically at a couple of different brain areas in the sea bass. One area they examined is just like the mammalian amygdala, sometimes referred to as the ‘fear centre’, which appears important for processing emotions and perceptions, in addition to detecting things in the environment that stand out from the surroundings. To determine the activity of this brain area, the team measured the expression of certain genes that turn on when the brain is active.

They discovered that sea bass that couldn't predict the appearance of the net showed more activity in this section of the brain. This suggests that this amygdala-like brain region functions the same way in fish as it does in mammals to perceive threats in the environment.

The fact that stressed sea bass show differences in their brain activity depending upon whether they can anticipate a stressor highlights that fish have feelings too. Like humans, predictability appears crucial for the fish stress response and the more predictable something is, the less stressed out the fish will be.