For their experiment, researchers from the University of Exeter in Cornwall, United Kingdom, put a group of Trinidadian guppy (Poecilia reticulata) in different "stress contexts" and observed the behavior of individuals.
But individuals still retained their distinct personalities.
The addition of the fake predators affected the "average behaviour" of the guppies by making them more cautious overall.
Some scientists have previously argued that a fish's personality can been plotted on a basic spectrum measuring how risk-averse or risk-prone an individual is, but the latest research suggests a fish's disposition isn't so simply characterized.
Houslay and his colleagues induced mild stress by transferring a fish into an unfamiliar tank. But our research shows that the reality is much more complex. Introducing models of predators triggered a higher stress.
The researchers studied their coping strategies in situations created to trigger various levels of stress.
A mock-up image showing a Trinidadian guppy (the small fish), a blue acara cichlid and a model of a heron.
Scientists from Exeter University in south-west England studied how individual Trinidadian guppy fish behaved in various stressful situations and discovered wide differences in how they responded. "So, while the behaviour of all the guppies changed depending on the situation - for example, all becoming more cautious in more stressful situations - the relative differences between individuals remained intact".
Researchers hope further testing will help them understand how and why personality differences develop and are maintained as a species evolves.
The researchers have declared a new theory saying that the Trinidadian guppies appear to be individuals with complex personalities.
Next, the research will attempt to identify whether there are any genetic factors that underly these personality traits or to what extent the environment influences these traits.
"The goal is really gaining insight into evolutionary processes, how different behavioural strategies might persist as species evolve", said Professor Alastair Wilson, also from the CEC at the University of Exeter.
The bottom line is many animals people commonly see as mindless drones - seen one, seen them all - can be very different on an individual basis.