The fish in the photograph is a few months old and is from Jörlandaľn on the Swedish west coast. Credit: Bart Adriaenssens

Personality is not just a feature unique to humans and pets. Scientists at the University of Gothenburg (Sweden) have revealed that also brown trout have individual characters and show different personalities.

Researcher Bart Adriaenssens from the Department of Zoology at the University of Gothenburg has for many years studied the behaviour of juvenile trout from watercourses on the west coast of Sweden.

"My results show that it are not just humans and other mammals that exhibit personality. Also brown trout differ among each other in their level of aggression and react differently to changes in their surroundings", says Bart Adriaenssens. "The release of a novel object in the aquarium causes very different reactions. Some individuals will immediately explore this object, whereas others will rather hide in a corner and try to avoid every contact."

"But it are not always the bold and aggressive fish who are most successful. When we marked trout individually and released them back in the wild, it were shy trout who grew most rapidly."

Which fish personality works best may also depend on the environment: if there is little protection available, as is the case, for example, in a tank at an aquaculture facility, large and bold fish are likely able to grab most of the food. But in the more complex environment of a stream in the wild, shy individuals can be more successful.

The question of why animals have personalities remains still to be answered.

"If a certain personality proves to work well, and individuals with that personality grow rapidly, survive in greater numbers and have more offspring, we would expect all individuals to behave according to that personality. This is not the case, however, and there is still a lot of work to be done in this area to explain why", says Bart Adriaenssens.

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