Distribuição espaço-temporal de ovos e larvas de peixes migradores em trecho lótico no médio Rio Uruguai, Brasil

Abstract

When planning the use and management of a river basin, it is fundamental to recognize the ecological processes and functional characteristics related to reproduction and consequently the restoration of fish stocks. The reproductive tactics of fish are diverse in nature and migratory fish movements are one of the most important among them. In the Neotropical region it is still uncertain how the reproductive movements happen. Considering this, the present study aims to obtain information about the reproductive pattern for the migratory species in the Uruguay river, in a lotic stretch. My specific goals are to evaluate: (i) the spatio-temporal variation in the abundance of eggs and larvae of fish between the trough environments of the main river and tributary mouth; (ii) the composition of ichthioplankton; and (iii) the relationships between environmental variables and abundance of eggs and larvae. Samplings were carried out during a reproductive period in a 120 km lotic stretch in the Middle Uruguay River between October / 2016 and January/2017. These samplings were conducted at dusk with plankton netting. A total of 11.519 eggs, 3.211 larvae were captured. Among the composition of the larval assembly, 14 migratory species were found. Eggs and larvae were recorded at all sampling stations, demonstrating that these migratory species are finding stimuli to spawn at different points in this ecosystem, a hypothesis that is reinforced by the fact that different stages of larvae can be found in the same sampling station. The presence of the initial forms of migrating fish in the main river channel and the tributary mouths show that these environments are relevant for reproduction. At the station at the mouth of the Piratinim river, a high density of larvae was found in the yolk larval stage, which, together with the high density of migratory eggs found at this site, confirms that this is an important river for fish reproduction, being a large tributary and less impacted than the others. The ichthyoplankton of Pimelodus maculatus and Salminus brasiliensis were mainly related to the stations located in the tributary river. On the other hand, Megaleporinus obtusidens, Schizodon altoparanae and Prochilodus lineatus ichthyoplankton had their greatest abundance in the main river. The temperature and the river width were the most important factors for the distribution of the larvae of the migratory species. Which seems to be related to the segregation of the larvae of the species that are mostly found in the main river and those that are mostly found in the tributaries, since the main river is characterized by having a greater width and temperature of the water than the three tributaries. In conclusion, in the studied section there is not only one reproduction site, nor is there a gradient of the stages of larval development along the river. What is verified is that all the river stretch is important for reproduction. I also verified that some migratory species select the tributaries to spawn and others the main river.