Pasture areas reduce the abundance and trophic niche width, but not the body condition of the Amazonian whale catfish

Land use changes modify aquatic ecosystems by changing their physical structure and biodiversity and generally reduce riparian vegetation, increase erosion of the margins, and negatively affect habitat-specialist species. Nonetheless, agricultural activities with different intensities affect those systems in the Neotropical region differently. For example, pastures drastically reduce the riparian vegetation alongside streams compared to oil palm monocultures. In turn, the riparian vegetation in oil palm monocultures supports the input of allochthonous materials into the stream, which serves as habitat or trophic resources to aquatic organisms. Herein, we test how different land uses (mature forest, pasture, and oil palm monoculture) in Eastern Amazon affect the abundance, diet, and condition factor of the whale catfish Helogenes marmoratus, an organism specialized in using floating or submerged debris as habitat. Pasture streams displayed a reduced abundance of H. marmoratus compared to those within forest and oil palm areas. In addition, the diet of the individuals captured in the pasture streams differed and included fewer items than the diet of the remaining areas. Nonetheless, these dietary differences were not related to any changes in the body condition of the individuals. Our results highlight that different types of land use have different effects on a habitat-specialist Neotropical fish and that pastures might have more marked effects by reducing their local abundance and trophic interactions with different prey.