This finding was based on research by a team of biologists and mathematicians from Swansea University and the University of Essex. It involved 15 three-spined stickleback fish observed individually in a fish tank containing two, three or five plants in fixed positions.

Using high-resolution tracking data from video recording, the scientists made their observations on the following parameters: movement (time stationary, step length, turning angle and burst frequency) and behaviorial (distance travelled, space use, time in free water and time near objects).

The data revealed that each of all 15 fishes’ movements were individual and repeatable. So much so that the scientists were able to identify the individual fish based on its movement.

According to co-author Dr Ines Fürtbauer from Swansea University, “these micropersonalities in fish are like signatures—different and unique to an individual. We found the fish’s signatures were the same when we made simple changes to the fish tanks, such as adding additional plants.

She added that the signatures can change gradually over the animal's lifetime, or abruptly if it encounters something new or unexpected in its environment. 

Elaborating, she said that "tracking animals’ motion over longer periods and in the wild will give us this sort of insight and help us better understand not only personality but also how flexible an animal’s behaviour is."

According to the authors, more research with other species and contexts is needed to find out how general the phenomenon is, and whether the same patterns are seen with land animals or flying species.