Sexual segregation in European cyprinids: consequence of response to predation risk influenced by sexual size dimorphism

Fish respond to predation threat by size/cohort-dependent presence in particular habitats and this may contribute to sexual segregation between habitats in species with sexual size dimorphism (SSD). The present study examines the validity of the "predation risk hypothesis" and importance of SSD on habitat (pelagic/inshore) segregation and dietary differences between sexes in three cyprinids with various magnitudes of SSD-roach (Rutilus rutilus), freshwater bream (Abramis brama) and bleak (Alburnus alburnus). Fish were sampled using gillnets over five consecutive years in the imov Reservoir. Habitat segregation with female overrepresentation in the pelagic habitat was found in the most sexually dimorphic species with 26% bigger females-the roach. When analysis of habitat segregation was size-controlled, this segregation ceased to confirm the importance of SSD. Freshwater bream sexes (2% SSD) differed in diet but did not differ in habitat occupation. Bleak sexes were not segregated. Larger roach individuals (predominantly females) are less threatened by gape-limited predators and consequently they can occupy the risky but optimal (for zooplankton acquisition) pelagic habitat. Our results demonstrate that habitat segregation is present in the species with the most pronounced SSD.