Express Your Personality or Go Along with the Group: What Determines the Behaviour of Shoaling Perch?

Behavioural syndromes, defined as correlated behaviours in different contexts, have been studied across species and taxa including humans as part of a personality concept. While most studies have focused on solitary individuals, less is known on how shoaling fish compromise between own personality and group behaviour. Risk-taking behaviour in 1-year-old perch (Perca fluviatilis) was observed to compare individual behaviour when in a group and when alone. An experimental design gave the fish the choice between foraging in an open area in the presence of a piscivore and hiding in the vegetation. We quantified the variation accountable by the effect of individuals being in a group, individuals alone and repeated measurements, using hierarchical mixed effects models. Within-group variances were low, but when individuals were later tested alone, individual differences explained most of the variation. Still, the individual best linear unbiased predictors (BLUPs) of time spent in the open area, extracted from the random effects of the mixed effects model, were positively correlated with the corresponding BLUPs when alone. The results indicate that individual behavioural traits are to some degree expressed also within groups. Most fish showed a shyer behaviour when alone, but bolder individuals changed less between treatments than did shyer ones, suggesting a more influential role of bold fish in the group.