Group swimming significantly decreases time to passage success for a galaxiid species.

We conducted laboratory experiments to explore the potential benefits of group behaviour on passage performance for a small-bodied migratory species, inanga Galaxias maculatus. An artificial velocity barrier was created to compare the fish passage success of groups of fish to solitary individuals. Passage success was measured using several metrics, including binomial success models and time-to-event analysis. Active metabolic rates were also measured as indices of energetic expenditure. Our findings revealed that fish swimming in groups have faster entry and passage rates compared to solitary individuals, but there was no difference in the proportion of fish successfully passing. Successful fish in groups displayed lower metabolic rates, suggesting the potential benefit of reduced energy expenditure for groups of fish. While group swimming did not enhance overall passage success, it significantly reduced the time required for successful passage compared to solitary swimming individuals. These findings underscore the importance of designing fish passes that accommodate gregarious species, ensuring improved success for fish populations overall. By considering the advantages of group behaviour on passage performance, fish passage structures can be tailored to better facilitate the movement of diverse fish species through aquatic environments.