Variação vertical e nictemeral do ictioplâncton na calha principal do Médio Rio Uruguai, Brasil

Abstract

The ichthyoplankton studies develops an extreme important role on rivers’ ecosystem, consisting an essential relation between species in organisms’ trophic web, and at its primary production in food chain. The various reproductive strategies of fish present an important study area, increasing many researchers curiosity during the last years in order to search and understand the association between full moon period, water temperature, strategies used for each species, spawning locations and horizontal, vertical and nictemeral distribution. Concerning to the research, it aims to evaluate the vertical and nictemeral variation of ichthyoplankton organisms’ density at Uruguay River channel, on the months of December 2016 to January 2017, on three sample stations located at Porto Lucena, Roque Gonzales and São Nicolau cities, all situated at Rio Grande do Sul state. To reach the objective of study, daytime and nocturnal collect were made, on surface and bottom of the river, materializing the collect of 24 samples on surface and 24 samples of the bottom for each research station, totalizing 288 samples during the two months of study. Along with the collect the following variables were measured: water speed, transparency, temperature, dissolved oxygen, electric conductivity, pH and river level. Samplings were accomplished with taper-cylindrical nets of 0.5 mm mesh. The collection instruments were fixed with 4% formalin. Eggs and larvae were triad on labs; afterwards the larvae were identified with the aid of specialized bibliographic references. It was utilized Analysis of Variance (ANOVA), Tukey’s post-hoc test and Pearson’s correlation to observed the organisms’ correlations with environmental variables. On the present study 2.856 larvae and 2.494 eggs were captured. Most of the captured larvae correspond to the Characiformes and Siluriformes orders. Among the most abundant species, the prominence is the Bryconamericus stramineus lambari and the Schizodon altoparanae vogue. During the study it was possible to conclude that exists a periodicity on the fishes’ eggs and larvae’s drift. These organisms are found mostly on surface during nocturnal period. The interaction of daytime with the water column position was significant in all larvae development stages, moreover, it was verified that the larvae prefers to stay on the bottom during daytime and migrate to the surface at night on the after-flexion stage. At this study, it was also registered the presence of migratory species of great size, such as: Megaleporinus obtusidens, Prochilodus lineatus, Rhaphiodon vulpinus, Pterodoras granulosus, Luciopimelodus pati, Pseudopimelodus mangurus, Sorubim lima and Pseudoplatystoma corruscans. It is conclude that ichthyoplankton organisms perform vertical migration, emerging mostly to the surface during night period. On the other hand, the research region is essential for many life cycles of various migratory large species, since many species were reported on this work.