Personality in fish

Personality is not just a feature unique to mammals. 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 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.

Another study of fish showed that coral reef fish can undergo a personality change in warmer water, and suggests that climate change may make some species more aggressive .

Experiments with two species of young damselfish on Australia's Great Barrier Reef showed that some reef fish are either consistently timid, or consistently bold, and that these individual differences are even more marked as water temperatures rise.

The scientists used fish who were captured just as they were ending their larval stage in open water and had not yet settled onto the reef, and so were naive to social hustle and bustle of reef fish life. They then directly manipulated water temperatures in laboratory tanks at Lizard Island Research station.

Placed by themselves in tanks, the fish were free to explore or to take refuge in a short piece of plastic pipe. The scientists observed how far and how often the fish ventured from the pipe. In cooler water, individual fish differed greatly in their activity levels. They all became more active to varying degrees when the water was warmed.

A slight lift of just one or two degrees may have only a small effect on some fish but the behaviour of others can be transformed - leading them to become up to 30 times more active and aggressive.

"The idea that fish have personalities may seem surprising at first, but we now know that personality is common in animal populations, and that this phenomenon may have far-reaching implications for understanding how animals respond to ecological and environmental challenges." says Dr Peter Biro, of the UNSW School of Biological, Earth and Environmental Sciences, who led the study.