Feeding ecology of Nannopterum brasilianum (Gmelin, 1789) and its trophic interactions with artisanal fisheries in the Laguna Estuarine System, Southern Brazil

Abstract Neotropic cormorants are abundant piscivorous birds in southern Brazil and present challenges for fisheries and aquaculture as potential food competitors. To evaluate this interaction, we address the species trophic overlap with artisanal fisheries in a productive estuary in southern Brazil (48°45’27’’S, 28°30’23’’W) with abundance data, pellet samples, taxonomic composition and fishery production during July 2021 until May 2023. Bird abundance showed a seasonal pattern with peaks during autumn-winter and a population decrease during spring-summer, primarily due to the migration of reproductive individuals and the establishment of a colony from March to September. Pellet analysis identified 31 prey types within 17 taxonomical families, most of which were fish from the Ariidae (35.9%), Engraulidae (15.6%), Scianidae (12.8%), Mugilidae (10.3%), and Gerreidae (10.1%) families. We characterized the cormorant population as opportunistic, as temporal variation and Amundsen’s analysis indicated the consumption of available and diverse resources. Through otolith-total length allometric regressions we estimated a mean fish biomass per pellet of 382.7g and prey size ranging from 26.6mm to 326.9mm. When compared to artisanal fisheries, cormorants consumed 12% the amount of fish exploited by artisanal fisheries during the sampling period, although the Pianka’s index of 0.78 indicated a high overlapping food resource. Cormorant predation on juvenile fishes likely plays an important ecological role at Laguna Estuarine System by regulating abundant prey species through density-dependent mechanisms. This study provides new and updated information on cormorant population, diet and interactions with artisanal fisheries of southern Brazil.