Social decision making is influenced by size of shoal but not boldness, sociability or familiarity in Deccan mahseer (Tor khudree)

ABSTRACT Many piscine species form fission-fusion societies where decisions to leave or join a shoal are influenced by factors such as shoal size, familiarity and species. Individuals exhibit distinct shoaling preferences based on behavioural traits such as boldness and sociability. In this study, we examined the link between shoaling decisions and behavioural traits in a hatchery reared population of an endangered megafish, the Deccan mahseer (Tor khudree). We found that this fish exhibits preference for larger shoals when presented with choices of 2 vs 4 and 2 vs 8 shoal sizes. However, no preference for isolated familiar or unfamiliar conspecifics over invasive heterospecifics was observed. Moreover, individual shoaling preferences did not correlate with their boldness or sociability. These results suggest that juvenile hatchery reared mahseers reintroduced into natural habitats may shoal with invasive species and modifying their social behaviour by amending rearing practices could improve outcomes of restocking interventions.