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Fish can be creatures of habit, too

October 24, 2011
Technical University of Denmark (DTU)
Behavioral experiments and brain research reveal surprising similarities between fish and humans. For example, some individuals are routine-bound creatures of habit, while others are better able to improvise, a new study shows.

Senior researcher at DTU Aqua, Erik Höglund, is one of Denmark’s leading experts on fish behaviour and fish neuroscience. He has researched differences in the personality of fish.
Credit: Photo by Martin Dam Kristensen


You need look no further than the lecture hall or your own circle of friends to see that some people prefer to stick to familiar routines, while others thrive on new challenges and change.

The same is true for rainbow trout according to tests in which fish have had to find food hidden in a maze, witnessed by whirring video cameras and timed by eager researchers with stopwatches. The results may prove to be of great significance for commercial fish-breeders and for companies using fish as guinea pigs in the pharmaceutical industry.

"The results of our study were unequivocal -- it seems that fish also demonstrate cognitive differences in how they react to stimuli," says senior researcher at DTU Aqua Erik Höglund, one of Denmark's leading experts on fish behaviour and fish neuroscience.

"Just like people, fish can more or less be divided into two personality types: proactive and reactive. Proactive fish are more aggressive and assertive, but also more fixed in their ways. Reactive fish are less aggressive and rather more timid, but at the same time are more open and flexible and seem to be able to adapt more easily, "says the senior researcher, who does research into both the behaviour of fish and how their brains work neurologically.

Better animal welfare

Researchers carried out a behavioural experiment using an aquarium with a maze where fish had to find food in different places while being timed. The experiment used rainbow trout as it is often bred on fish farms in Denmark. And the new knowledge about the different kinds of fish personalities may lead to both improved fish welfare and increased production in industry, believes the DTU Aqua researcher:

"If there are large individual differences in fish, then what is good for one individual is not necessarily good for all fish, and this can affect how you manage the conditions in, for example, ponds so that fish of one kind or the other thrive as optimally as possible," explains Erik Höglund.

Fish sort themselves out

Therefore work is currently being done at DTU Aqua in Hirtshals to develop methods to allow the fish to sort themselves out according to their personality types.

"We have done studies that show that fish with different response patterns react differently when the oxygen content in the water around them falls. We are also working on how we can get fish to "tell" what density of oxygen they prefer, and at the moment we are developing a system so the fish can demonstrate this," says the fish behaviour researcher.

Fish resemble humans

One of the things that makes it difficult to study exactly how fishes' brains work is that you cannot transfer what we know about the human brain directly. This is because everything to do with the brain both has different proportions and is positioned differently in fish's brains. For example, the ratio between the telecephalon (cerebrum) and the rest of the brain is six per cent in rainbow trout against 50 per cent in humans.

However, the latest research shows that fish brains are, from a purely physiological point of view, far more like the human brain than was previously thought, for example when it comes to reacting to danger.

"Our research has demonstrated that the neurotransmitter CRFR1, which plays an important role in people's reactions to stress and fear, is also found in fish. Inhibiting the transmitter in Crucian carp led to them reacting less to stress," the senior researcher explains.

To the age-old question about whether fish can feel pain, therefore, he answers with a cautious yes.

"It is becoming widely recognized that even fish have consciousness. And purely physiologically, they possess the same receptors as humans. But the question is still of course how they experience it," Erik Höglund explains.

Strong political interest

And it is because of this ethical aspect that Erik Höglund believes interest in fish behaviour and fish brains is rising so sharply -- even in the political arena.

"Animal welfare in general is a hot topic and, coupled with the growth in fish farming worldwide, fish such as for example the zebra-fish are starting to be used more and more as experimental animals for drug development. Therefore we need scientifically based facts that we can evaluate and base our decision-making on. But it is a new field of research and there are still only very few people who know anything at all about fish behaviour and how fish brains work," he says.

The fish behaviour researcher has therefore taken the initiative of bringing together the leading Nordic experts in fish behaviour and neuroscience in a cross-disciplinary network supported by the Nordic Council. The first seminar has just been held at DTU Aqua in Hirtshals and two more are planned.

How the scientists analyzed the fish's personalities

For the experiment conducted at DTU Aqua in Hirtshals, the research team used two strains of rainbow trout, one of which was bred to be proactive and one reactive. Each fish was placed individually in aquaria which contained an open area, a sheltered area and a T-shaped maze.

While the proactive and reactive fish were equally quick to learn where to find food in the initial stages, significant differences were revealed when Erik Höglund and his colleagues began moving food to other places in the aquarium:

While the reactive fish spent only 46 seconds on average to find and eat food when it was moved, it took the proactive fish an incredible 976 seconds.

In fact all the proactive fish swam right over the food without touching it in order to search in the place where they had got used to finding food in the first place, while all the reactive fish found the food at the new location before they examined the old ones. The reactive fish could also change their behaviour -- even when the researchers lowered a rubber barrier into the aquarium. They attempted to avoid it by remaining in the sheltered area, while the proactive fish swam off unconcerned in search of food.

Researchers interpret the results as showing that reactive fish are more flexible, while proactive fish are more likely to adopt and follow specific routines.

A video filmed by Maria de Lourdes Ruiz-Gomez is available at:

Story Source:

Materials provided by Technical University of Denmark (DTU). Original written by Line Reeh, DTU Aqua. Note: Content may be edited for style and length.

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Technical University of Denmark (DTU). "Fish can be creatures of habit, too." ScienceDaily. ScienceDaily, 24 October 2011. <>.
Technical University of Denmark (DTU). (2011, October 24). Fish can be creatures of habit, too. ScienceDaily. Retrieved March 29, 2017 from
Technical University of Denmark (DTU). "Fish can be creatures of habit, too." ScienceDaily. (accessed March 29, 2017).