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.