The Small War: Trophic interactions between small-bodied non-native and native characoids in the marginal zones of Neotropical reservoirs.

Abstract

Small-bodied characoids represent a significant portion of Brazil’s freshwater fish fauna, yet they remain understudied. In reservoirs, these species are highly abundant, colonise the marginal zone and are important in the aquatic food webs. In the lower Paranapanema River, the neighbouring reservoirs Rosana and Taquaruçu differ in their environmental conditions, but both host native and non-native characoids. This study describes the diet, feeding ecology and interactions of between co-existing native and non-native small-bodied characoids in both reservoirs, as well as their trophic responses to seasonal climatic variations (dry vs. wet season). Samples were collected quarterly between September 2018 and September 2020, with trawls, sieves and cast nets collecting fish in marginal zones and aquatic macrophyte beds. The stomach contents of 416 individuals from seven native and non-native characoids were analysed, with their prey classified into several food categories and resources. In the Rosana Reservoir, both native and non-native characoids were strongly reliant on allochthonous resources and their diet composition showed similar shifts between the wet and dry season. Conversely, in the Taquaruçu Reservoir, the characoids were more reliant on autochthonous resources and showed weak dietary shifts between seasons. Niche breadth and trophic overlap indices indicated diet specialisation and segregation in the characoids of the Rosana Reservoir, while, in Taquaruçu, they overlapped significantly, especially in the dry season. These results reveal considerable differences in the trophic interactions between native and non-native characoids in two Neotropical reservoirs, suggesting high context dependency in the ecological implications of introducing non-native species of these small-bodied fishes.