Individual phenotypic variability in the behaviour of an aggregative riverine fish is structured along a reactive-proactive axis.

High phenotypic diversity should provide populations with resilience to environmental change by increasing their capacity to respond to changing conditions. The aim of this study was to identify whether there is consistency in individual behaviours on a reactive-proactive axis in European barbel Barbus barbus ("barbel"), a riverine and aggregatory fish that expresses individual differences in its behaviours in nature. This was tested using three sequential experiments in ex-situ conditions that required individuals to leave a shelter and then explore new habitats ('open-field test'), respond to social stimuli ('mirror-image stimulation test') and forage ('foraging behaviour test'; assessing exploratory traits). Each suite of experiments was replicated three times per individual (46 hours minimum time between replicates). There was high variability in behaviours both within and among individuals. The most repeatable behaviours were latency to exit the shelter, active time in the shelter, and the number of food items consumed. Principal component scores did, however, indicate a range of consistent behavioural phenotypes across the individuals, distributing them along a reactive-proactive axis in which most of individuals were more reactive phenotypes (shyer, less exploratory, less social). These results suggest that within controlled conditions, there is considerable phenotypic diversity among individuals in their behaviours, suggesting their populations will have some adaptive capacity to environmental change.