Sexual segregation in monomorphic minnows

Sexual segregation, where males and females use habitat in different ways, is widespread among animals including fish, and has important consequences for key aspects of population ecology including foraging success, predator avoidance and growth. However, currently, evidence for sexual segregation is based on observations of sexually dimorphic species or species with differing reproductive strategies. We used European minnows, Phoxinus phoxinus, to test the null hypothesis that sexual segregation does not occur in sexually monomorphic species. A large, seminatural stream channel equipped with passive integrated transponder (PIT) detectors monitored the activity of 70 fish for 98 days on ecologically relevant spatial and temporal scales. Sexual segregation was evident spatially (with males and females using different habitats within the stream), temporally (males switched patches more frequently than females at night, but not during the day) and socially (with males, but not females, demonstrating same-sex association preferences). Our results are the first to demonstrate sexual segregation in monomorphic species outside the breeding season. We discuss potential explanations for our observations and ways in which patterns of variation in activity, space use and social interactions have important implications for population dynamics. •We tested if sex affected freshwater fish behaviour outside the breeding season.•Sexes used different stream habitats, males switching more than females at night.•Males preferred the company of other males, but females did not have a preference.•These sex differences cannot be explained by mating behaviour.•Sexual segregation may be maintained by different motivation to interact socially.