Distribuição espaço-temporal do ictioplâncton e mapeamento de áreas de desova na porção norte da região hidrográfica do Paraguai

Abstract

The reproductive dynamics of fish is responsible for the maintaining populations and can be influenced by environmental variables as well as by human activities adjacent to the riparian system. This study aimed to understand the reproductive dynamics of fish fauna in the Paraguay Hydrographic Region (Paraguay HR), analyzing the characteristics of spawning grounds, the effects of environmental variables and anthropogenic actions on the reproduction of the fish assemblage. Sampling were carried out over two reproductive periods (november to march 2017/2018 and from october to march 2018/2019), at 10 sites in the northern portion of the Paraguay-HR in the state of Mato Grosso (Brazil). Concomitantly with the collection of ichthyoplankton, abiotic variables such as water temperature (ºC), dissolved oxygen (mL-1), pH, electrical conductivity (µS cm-1), and transparency (cm) were measured. To analyze the use and cover in the areas adjacent to the sampling sites, Landsat images were used to obtain the land cover area adjacent to sampling sites. These data were categorized according to i) natural vegetation, ii) bodies of water, iii) agriculture, iv) pasture and/or v) planted forest. The reproductive migration to the headwaters in the Paraguay-HR was confirmed because the sampling site Formoso River presented one of the highest egg densities. In addition, the highest densities of eggs occurred in the months of november and january. There was positive effect of abiotic variables and land use/cover (natural vegetation, water bodies and agriculture) on egg production. Higher water temperature, pH and electrical conductivity promote better conditions for embryonic development. The flow, pasture and planted forest had negative effects on egg production, indicating that spawning is influenced by the surroundings of the sampling sites. Larvae belonging to 25 families and 55 taxa were captured, with migratory species of commercial interest Brycon hilarii, Hemisorubim platyrhynchos, Piaractus mesopotamicus, Prochilodus lineatus, Pseudoplatystoma spp., Salminus brasiliensis, Sorubim lima and Zungaro jahu. The places with the greatest interactions and connectivity were the Sepotuba, Paraguai, Jauru and Cabaçal rivers and, therefore, they should be considered critical areas for the conservation of longitudinal connectivity in the fluvial system, as migratory species have spawned upstream of the sampled sites. On the other hand, the use/cover by human activities causes indirect changes in water quality with the transport of sediments and nutrients to aquatic ecosystems, and riparian vegetation should be maintained as recommended in Brazilian legislation in order to mitigate the effects of adjacent land use. Thus, it is necessary to understand the effects of land use/cover on reproductive dynamics for sustainable fisheries management and water quality in Brazilian hydrographic basins, helping to mitigate the effects of human activities on riparian ecosystems. Furthermore, the reproductive migration routes along the dendritic network should be maintained for the free movement of fish and to seek alternative ways of producing electricity for civil society, preventing the blocking of migratory routes in the plateau rivers in the Paraguay-HR. In addition, the reproductive period of the ichthyofauna should be considered for measures to protect the spawning stock, subsidizing the guidelines for the closed season in the Paraguay River Basin according to resolutions issued annually by the State Fisheries Council of the State of Mato Grosso (CEPESCA).