Distribuição e abundância do icitioplâncton no Médio Rio Uruguai durante um ciclo anual no Município de Porto Lucena (Rs, Brasil)

Abstract

The ichthyoplankton provides us with important information to determine the period and spawning grounds of the fish, are fundamental for the knowledge of the taxonomy and ecology of the species, however these studies are scarce in the middle Uruguay river. In this sense, the objective of this work was to study the distribution and abundance of fish eggs and larvae during an annual cycle in Porto Lucena / RS. The collections were carried out monthly from October 2015 to September 2016 (an annual cycle), in the night period, from trawls using plankton net of a conical-cylindrical shape of 500 μm mesh. Four trawls were made with two nets, 15 minutes each, totaling eight samples per month. In parallel to the samplings, the following environmental variables were measured: water temperature, dissolved oxygen, pH, electrical conductivity, water transparency and river level. The collected samples were fixed in 4% formalin and taken to the laboratory, where the sorting (separation of eggs and larvae) was carried out, the captured larvae were identified with the aid of a stereoscopic microscope. To evaluate the temporal variation of the abundance of eggs, larvae, larval stages and the six most abundant taxa, one-way ANOVA was applied and, when these were significant, Tukey's a posteriori test was applied to to detect the differences. To verify the association of the environmental variables in the abundance of eggs, larvae and the main taxa, Pearson correlations were made. In order to reduce the dimensionality of the environmental variables a Principal Components Analysis (PCA) was applied. During the study period, 5.648 larvae and 1.813 eggs were captured, 95.6% of which belong to the order Siluriformes. By means of ANOVA, there were significant differences in the temporal distribution of eggs (p <0.05) and larvae (p <0.05), both of the highest densities were observed from October to December (p <0.05). The most abundant species were Pimelodus maculatus, Pimelodus albicans, Pseudoplatystoma corruscans, Parapimelodus valenciennis, Iheringichthys labrosus and Luciopimelodus pati. It is important to note the large number of larvae of migratory species captured, corresponding to 84.6% of the total larvae. It is concluded that in Porto Lucena there is a high reproduction rate, mainly of migratory fish, and their reproductive activity occurs mainly in spring, coinciding with the increase of photoperiod, temperature and water level of the river.