Social decision making is influenced by size of shoal but not personality or familiarity in Deccan Mahseer (Tor khudree)

Shoals formed by many piscine species are fission-fusion societies where decisions to leave or join a group can have consequences on the fitness of individuals. Some important factors that determine shoal choice are shoal size, familiarity and species composition. However, individuals and species often exhibit distinct shoaling preferences. Individual differences in shoaling preferences may also be related to personality traits such as boldness and sociability. In this study, we examined the link between shoaling decisions and personality traits in a hatchery reared population of an endangered megafish, the Deccan Mahseer ( Tor khudree ). We found that this fish exhibits a distinct preference for larger shoals at ratios of 1:2 or greater. However, they did not prefer to associate with an isolated familiar individual over unfamiliar ones or with a member of their own species over an invasive species. Moreover, shoaling preferences in individuals did not correlate with their boldness or sociability. These results suggest that hatchery reared mahseers which are reintroduced into natural habitats may shoal with invasive species, negatively affecting their viability. Modifying social behaviour of mahseers by amending rearing practices may be a useful strategy to improve outcomes of restocking interventions.