Is a cannibal different from its conspecifics? A behavioural, morphological, muscular and retinal structure study with pikeperch juveniles under farming conditions

Most studies carried out on personality recognized that personality is defined by behavioural traits consistent through time and/or contexts. In fish, most studies on personality were performed either on juveniles (aged between 6 months and 1 year) or adults, but very few focused on the early life stages. The main goal of this study is to characterize behavioural syndromes and to highlight the existence of a personality in young juvenile pikeperch, a species with a strong economic value. To study the consistency of behavioural responses of juvenile (50 and 64 days post-hatch) pikeperch Sander lucioperca (n = 41, total length = 5.8 ± 1.0 cm and mass = 1.6 ± 0.7 g), we performed three tests per fish in one day: exploration (cross-maze), dyadic and restraint test. In the cross-maze test, exploratory fish were more active and bolder. In the dyadic test, fish with the highest number of contacts, showed also more approaches, orientations and avoidance behaviours. In the restraint test, bolder fish were more active and tried to escape more often. Consequently, the investigation of the different behavioural responses of each fish highlighted behavioural syndromes in this species. Furthermore, for the first time, we showed, with a cross-context analysis, that young juvenile pikeperch, responded in the same way to exploration and dyadic test but their responses were opposite in the restraint test. Our results opened new opportunities for testing individual personality in very young fish that may help solving some aquaculture problems, such as intra-cohort cannibalism.