Mapeamento de isótopos de estrôncio (87Sr/86Sr) e sua aplicação no estudo da história de vida de peixes (Cichla temensis) em rios da Bacia Amazônica

Abstract

The 87Sr/86Sr isotope ratio has been used as a valuable tool to identify the origin and migration patterns of aquatic organisms, as well as supporting environmental and forensic research. In the Amazon basin, however, spatial modeling of this ratio is still incipient, limiting large-scale applications. To fill this gap, this study established a baseline of the 87Sr/86Sr isotopic signature in surface waters in the region, combining data from scientific publications and samples collected in the field. Using a geostatistical model based on spatial interpolation (IDW), it was possible to map the isotopic distribution in unsampled locations. The results revealed high heterogeneity in Amazonian waters, with lower values associated with Andean basins with more recent formations and higher values in areas of ancient crystalline shields, such as the Guiana and Brazilian Shields. The model's accuracy was confirmed by the strong correlation between predicted and observed values in fish otoliths (R² = 0.81), highlighting its potential for studies at various scales. Using this methodology, we investigated the movement of the tucunaré-açú (Cichla temensis), a key species for the local economy and ecology, in tributaries of the middle Rio Negro. Water samples and otoliths from 79 individuals in six sub-basins were analyzed during the low-water period. The analyses detected significant variations in the isotopic signatures between the tributaries: rivers on the right bank (Cuiuni and Caurés) showed lower ratios, while the Aracá River (left bank) showed high values, reflecting regional geological differences. The linear correlation between the isotopic ratios of water and otoliths (R² = 0.95) validated the method for tracking displacements. Most of the fish (64 individuals) remained in their sub-basins of origin, indicating resident behavior. However, transects in the otoliths revealed movements between sub-basins, such as between Aracá and Demeni, suggesting the existence of ecological corridors connecting distinct habitats. The results reinforce the importance of hydrological connectivity for maintaining genetic diversity and population resilience, especially in a scenario of increasing anthropogenic pressures. The study also points to the need to expand the collection of isotopic data and integrate new geospatial technologies to improve the accuracy of modeling. In addition, it highlights the importance of monitoring critical habitats and extending research to broader time scales, ensuring the effectiveness of management plans. By combining environmental modeling and isotopic analysis, this research not only advances understanding of the ecology of iconic species such as the tucunaré-açú, but also offers a methodological framework for the conservation and sustainable management of complex ecosystems such as those in the Amazon Basin.