Verde River basin: isotopic geochemistry and traceability of the ichthyofauna amidst anthropogenic intervention.

Abstract

<p>The Verde River basin covers several municipalities that belong to the Brazilian state of Mato Grosso do Sul. Its source begins at about 500 m of elevation, as does its main tributary, the São Domingos River. Dozens of smaller tributaries flow into the Verde River before it joins the Paraná River at an altitude of 260 m. Sedimentary rocks, such as sandstone, dominate the local geology; however, igneous rocks, such as basalt, are also found. The weather is governed by two seasons, dry (April-September) and wet (October-March).</p><p>The Verde River has undergone anthropogenic changes due to cattle farming, agriculture and, in the last decade, due to dams. Currently, there are three hydroelectric dams in the middle of the basin (Verde 4A, Verde4 and São Domingos) that compartmentalize the habitat and block the migratory paths of the local ichthyofauna.</p><p>This study analyzes the relationship between the local geochemistry, indicated by the isotopic ratio in water samples (N=52), and the isotopic ratio in fish otoliths (aragonite structure) in hundreds of specimens of different species. Isotopic analysis of otoliths is recognized in scientific literature as a tool that can provide information about the geographic range of fish in their environment.</p><p>The chemical analyses were performed using LA-ICP mass spectrometers (Neptune XT HR-Thermo) in the raster mode setting to track strontium levels from the core to the edge of each otolith. The <sup>87</sup>Sr/<sup>86</sup>Sr ratio in the water samples was always higher than 0.71; however, some sections of the basin have exceeded the isotopic ratio with values over 0.72, for example, the very first segment of the upper Verde River (0.7260), the upper and lower São Domingos River (0.7272), the Verde River after receiving water from São Domingos River, and most tributaries of the lower Verde River basin. Both the Paraná River and the middle Verde River, upstream of the São Domingos reservoir, had <sup>87</sup>Sr/<sup>86</sup>Sr ratios of below 0.72, the latter section being where we caught most of the fish.</p><p>The results for <sup>87</sup>Sr/<sup>86</sup>Sr obtained on the edge of the otolith showed consistent correspondence to the values in the water of the sites where the fish were captured. This indicates that the isotopic proportions of strontium obtained in otoliths all the way from the core to the edge must be related to the environments in which the animals grew up, which is the basis for tracing the individual spatial range. Supported by this correlation, we are now analyzing the potential, the previous (in the fish lifespan scale) and the current migratory and dispersal paths for the ichthyofauna. Considering all points, we expect that our results will contribute to a better understanding and management of the regional migratory ichthyofauna in the future.</p>