The degradation of riparian vegetation in low-order streams can cause irreversible environmental damage, biodiversity loss and alter the structure of communities. In this context, the present study evaluated how the level of degradation of riparian vegetation influences the fish assemblage in three low-order streams in the Brazilian Amazon. In each stream, three different stretches were delimited, distributed in the environments of forest, edge and pasture, and the samplings were conducted in three hydrological regimes using dip nets. Influences of the stretches and streams on richness and abundance were tested with GLMM, on the composition with PERMANOVA and on indicator species with IndVal. Species richness was higher in forest environments and lower in pasture; while abundance was higher in pasture and lower in forest. Species composition differed between stretches, streams and in the interaction between stretches and streams, with a strong difference between stretches of forest and pasture environments. Six species dependent of the riparian vegetation were classified as indicators of forest environments; while a generalist species was indicator of pasture. These results reinforce the negative influence of riparian vegetation removal on biological communities and provide subsidies that can help in the conservation and management of these systems.

The trophic structure of fish assemblages associated with banks of aquatic macrophytes in abandoned meander lakes on the river Purus is investigated. Comparisons between lakes and between different periods of the hydrological cycle are undertaken. Fish samples were collected in six lakes during the rainy (February), ebb (May) and dry (September) season in 2012. Fish stomach contents were analyzed by frequency of occurrence and volume methods, combined in the Feeding Index (IAi). Fish species were included in nine trophic categories, based on IAi rates. The richness and abundance of fish species, by trophic category, was influenced by temporal variations, with no significant spatial variation. Some species changed their diets and were included in different trophic categories according to the period and lake under analysis. Specialist species have also been identified, but failed to change their diets. Differences may be due to the characteristics of macrophytes banks and to the flooding regime which change the availability of food resources.

The effect of the flood pulse directs productivity and interactions of biota in abandoned oxbow lakes, as these habitats provide refuge from predators and serve as nurseries and feeding grounds for many fish species. The present study was conducted in twelve oxbow lakes with varying degrees of connectivity, located in the floodplain of the Middle Purus River in Brazil. 8.647 individuals were collected among 157 species. Lakes with high hydrological connectivity were shown to have the largest richness of species (131 species), while lakes with medium connectivity had slightly smaller richness (123 species), and lakes with low connectivity had even less (113 species). The results show high diversity in lakes that are attributed different connectivity, thereby indicating the importance of studies focused on oxbow lakes that contribute with relevant information on the composition of fish species in the Middle Purus River.

Aims To evaluate the effects of replacing riparian vegetation with pasture in the diet of Apistogramma agassizii and Aequidens tetramerus. Methods The study was carried out in the municipality of Senador Guiomar, Acre. We sampled three streams divided into three stretches each (100 m), bordered by forest, pasture, and both forest and pasture (edge). Quarterly collections using fishnets were performed between January and October 2008. Results The species A. agassizii had a diet based on the consumption of adult insects in stretches of forest, different from the pasture environment, in which the consumption of insect larvae was higher. In the diet of A. tetramerus, the most important item was insect larvae in all environments, mainly in the pasture. We found low amplitude of trophic niche for A. agassizii in the forest environment, due to the consumption of adult insects, and for A. tetramerus in the pasture, due to the consumption of insect larvae. In the pasture areas, we found no difference between the items consumed by the species, which showed a high trophic niche overlap, influenced by the consumption of Diptera larvae. Conclusion Thus, we show that allochthonous items, such as adult insects, are food resources indicative of riparian vegetation conservation and are important for both species, mainly A. agassizii. However, this item has been replaced by autochthonous items, such as Diptera larvae, in the A. agassizii diet and become predominant in the A. tetramerus diet as the riparian vegetation deteriorates.

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