Fish constitute the most specious group among the Craniata, comprising about half of vertebrates known in the world. With regard to freshwater fishes, the greatest diversity is found in the Neotropical region, which includes, among other regions, the Amazon basin, which has a gigantic network of small streams, called regionally streams, responsible for much of this biodiversity. The waters of these streams are relatively acidic, poor in mineral salts and with low concentration of calcium and magnesium. These streams, especially of smaller order and on land and / or headwaters, usually have their courses covered by the canopy making their fauna dependent on the adjacent forest. The objective of this study was to know the structure of the fish assemblages in streams of the streams National Forest in two periods of the seasonal cycle, as well as to evaluate the effects of the El Nino climate phenomenon on the fisiography and ichthyofauna of these streams. For the first objective, collections were made in two distinct periods, the first in the dry season (from September to November 2015), and the second in the rainy season (March to May 2016). The second objective was to analyze samples of environmental and ichthyofauna variables from the drought of the year 2013 and from the recent drought surveys of 2015. The fish were collected with trawls and sieves in 22 1st and 3rd order
streams. During the dry season, 2,452 individuals distributed in 96 species were collected. In the
rainy season, 1,092 individuals were collected in 67 species. The 3,517 individuals are
distributed in 105 species that compose in 27 families and six orders. The greatest riches were in
the families Characidae, Cichlidae and Loricariidae. The results showed that there were no
differences regarding composition in the different periods sampled. As for wealth and
abundance, there were significant differences. The scientific knowledge generated through this
study may guide future management actions in the conservation unit. Actions that will subsidize
from the adaptation of the zoning of the unit for the preservation of rare or endemic species, to
the economic management of species for aquarium or observation dive.
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