Patterns of Sexual Segregation in the Use of Trophic Resources in Breeding Imperial Cormorants

Many seabirds show sexual differences in different aspects of their trophic ecology. The Imperial Cormorant ( Leucocarbo atriceps ) presents sexual size dimorphism, with males being larger and heavier than females. They show sexual differences in feeding behaviour, but no studies have yet comprehensively quantified the sexual segregation in the use of food resources. Diet composition through the analysis of stomach content samples and isotopic niches of female and male individuals were assessed during the incubation and chick rearing stage of 2019 at Islas Blancas (44°46′S, 65°38′W) and Punta León (43°04′S, 64°29′W), Argentina. A total of 117 stomach content samples and 80 whole blood samples were analysed. The sexes differed significantly in their overall prey composition. Females consumed mostly benthic prey in all cases, while males consumed demersal, pelagic or benthic prey depending on the breeding stage and colony. Males presented a lower diet diversity and consumed larger prey items than females in all stages. The isotopic niche of females and males differed in position and/or amplitude, presenting variations in the axes, in agreement with the results based on conventional diet analysis. Females and males exhibited trophic resource partitioning, although patterns of segregation varied depending on the stages of the breeding cycle and the location of the colony. These patterns show the differential role of the sexes in marine food webs, and may expose females and males to different environmental and anthropogenic pressures.