Environmental effects on the gills and blood of Oreochromis niloticus exposed to rivers of Bahia, Brazil

• Assessment of xenobiotics on the gill tissue and hematological and biochemical parameters.
• Gill histopathologies are important biomarkers of aquatic pollution and sediment contamination.
• Stereological quantification has great potential in histopathological studies.
• Integrated analyzes are fundamental to the understanding of the factors acting on the environment.
• More knowledge of methodologies is needed to determine the complexity of environmental factors and employing biomarkers.

Through the integration of chemical, biochemical and morphological analyses, this study investigated the effects of multiple pollutants on environmental biomarkers, such as gill histopathological changes and hematological and biochemical parameters, in Oreochromis niloticus exposed to four sites in the Jacuipe and Subaé rivers over seven days. Sediment analyses identified Sapelba as the most contaminated site, followed by Oliveira de Campinhos, Santo Amaro and Jacuípe. Water analyses revealed aluminum, iron and manganese at all sites. Aluminum and other metal were also detected in the gills of fishes. Fish exposed to the Sapelba site exhibited significant necrosis formation, as well as higher hematological parameters and trend to increase of cortisol levels. However, filament epithelium proliferation was higher at the Oliveira de Campinhos and Santo Amaro sites, at which the lowest levels of the hematological variables were observed. Multivariate analysis grouped some gill histopathological changes together, such as epithelial detachment with edema and lamellar epithelial proliferation with the lamellar fusion of adjacent filaments, revealing relationships among them. Positive associations were identified between sediment contamination and necrosis and cortisol, while water contamination was related with filament epithelium proliferation, aneurism, lamellar fusion and several hematological parameters. Furthermore, relationships between blood parameters and gill histopathological changes demonstrated a joint physiological response that may have resulted from environmental variables such as dissolved oxygen. The results exhibited the direct influence of xenobiotics on these biomarkers but also highlighted the need to consider the complexity of environmental factors to optimize the adoption of these environmental predictive tools.