Daily variation feeding of Astyanax lacustris (Lütken 1875) in a subtropical river

This study aimed to know the daily variation of Astyanax lacustris (Lütken 1875) feeding in a lotic environment. Fish were caught with a net for three days every three hours in the summer to capture ten individuals per hour in a stretch of the Ijuí River, Middle Uruguay River, Brazil. The captured specimens had their stomachs removed and the content analyzed and separated into seven food categories with the aid of stereomicroscope. For the analysis of food items were used the frequency of occurrence methods, volumetric method and applied the Alimentary Index (IAi). Two hundred stomachs were analyzed, of which 95% had food content. The most abundant items were algae and autochthonous insects. Astyanax lacustris feeds throughout the day especially early in the morning (9 hours) and reduces its feeding at night (24 hours and 3 hours). It was also verified variation of AI of different food items throughout the day. It is concluded that A. lacustris feeds throughout the day, especially in the daytime and that throughout the 24 hours it varies its diet due to photoperiod and food availability.

Seasonal and longitudinal variation in fish assemblage structure along an unregulated stretch of the Middle Uruguay River

Diversity patterns and their causes remain important questions, especially for ecosystems that preserve natural conditions. This is the case of fish diversity in large Neotropical rivers. In this context, we investigated fish diversity patterns along an extensive unregulated river section (ca. 450 km) in the Middle Uruguay River Basin. Sampling was conducted seasonally between May 2017 and March 2018, at six sites (patches) with contrasting environmental conditions. We collected 3,008 individuals belonging to 90 species. Nine were migratory, which summed relevant biomass in local assemblages (20 to 60%). We observed spatial variation in assemblage structure, but weak seasonal effects. Overall, biomass was similar among sites, but richness tended to increase downstream. Migratory fishes showed opposed trends, with higher richness and biomass upstream, particularly catfishes (Siluriformes). Ordination analyses separated sites in three groups based on variations in composition and abundance, and revealed associations between these groups and specific hydro-geomorphic conditions (i.e., flow, depth, channel width and the presence of riparian vegetation). Our study revealed that fish diversity distributes heterogeneously along the main channel, where hydro-geomorphic patches select for distinct assemblages along riverine gradients.(AU)

Os padrões de diversidade e suas causas continuam sendo questões importantes, especialmente em ecossistemas que preservam condições naturais. Este é o caso da diversidade de peixes em grandes rios neotropicais. Neste contexto, investigamos padrões de diversidade de peixes ao longo de um extenso gradiente fluvial (ca. 450 km) no médio rio Uruguai. As amostragens ocorreram sazonalmente entre maio/2017 e março/2018 em seis locais com condições ambientais distintas. Coletamos 3.008 indivíduos pertencentes a 90 espécies. Nove eram migradoras, as quais somaram relevante biomassa nas assembleias locais (20 a 60%). Houve variação espacial na estrutura das assembleias, mas fraco efeito sazonal. No geral, a biomassa foi semelhante entre os locais, mas a riqueza tendeu a aumentar em direção a jusante. Peixes migradores apresentaram padrão inverso, com maior riqueza e biomassa a montante, especialmente bagres (Siluriformes). Análises de ordenação separaram os locais em três grupos de acordo com a variação na composição e na abundância, e revelaram correlação entre esses grupos e condições hidro-geomorfológicas específicas (i.e., velocidade, profundidade, largura do canal e vegetação riparia). Nosso estudo revelou que a diversidade de peixes se distribui de maneira heterogênea ao longo do canal, onde características hidro-geomorfológicas selecionam assembleias distintas ao longo de gradientes fluviais.(AU)