Dense macrophytes influence the horizontal distribution of fish in floodplain lakes

Fish species distribution is commonly influenced by aquatic macrophytes. Despite of the usage of these plants as habitats for refuge, feeding and reproduction by fish, too dense macrophyte stands make microhabitats unsuitable for certain fish species. Moreover, the distance from the open water within macrophyte stands may also affect fish species distribution because of increasingly harsh conditions. In order to test differences in species distribution of small-sized fish within macrophyte stands we sampled stands of Eichhornia spp presenting low and high levels of macrophyte density and at several distances from the open water (0, 3, 6, 9, 12 and 15 m). We measured depth, temperature, dissolved oxygen, conductivity and pH of the water column and attributes of the fish assemblages. We captured 1,167 individuals of fish belonging to 24 species. Oxygen was significantly higher in lower levels of macrophyte density and similar patterns were found for fish abundance and species richness. These results indicate that, in general, small sized-fish prefer less dense macrophyte stands. In addition, both depth and oxygen were significantly higher at the closest distance from open water, where the composition of fish species was also distinct from those found in other distances. In accordance with changes in species composition, different fish species showed divergent distribution along distances from the open water. Together these results demonstrate that oxygen content influences fish species composition, and indicate that fish species are able to use less suitable microhabitats most likely because of morphological, physiological and behavioral adaptations.