Mercury contamination in southern legal Amazonia: evaluating Prochilodus nigricans (Agassiz, 1829) as a bioindicator species and the risk of its consumption

Mercury (Hg), a highly toxic metal, continues to be widely used in artisanal and small-scale gold mining (ASGM), particularly in the Amazon region. Its improper use and disposal contribute to the contamination of aquatic ecosystems, impacting fish and the human populations that rely on them for food. This study evaluated Hg contamination in the Peixoto de Azevedo River Basin, located in the southern Brazilian Legal Amazon, by analyzing scale, skin, muscle, and liver tissues of Prochilodus nigricans , a key species in the local diet. Mercury was detected in all tissues and across all river stretches, with higher concentrations observed near mining sites, suggesting the association between ASGM and environmental contamination. The liver exhibited the highest Hg levels (1.581 mg·kg⁻¹), followed by scale (0.537 mg·kg⁻¹), skin (0.153 mg·kg⁻¹), and muscle (0.087 mg·kg⁻¹), indicating the potential for bioindication through less invasive sampling. Although muscle Hg concentrations were below the FAO legal threshold (0.5 mg·kg⁻¹), the estimated health risk (THQ > 1) suggests potential harm to riverine populations due to high fish consumption. These findings underscore the urgent need for continuous monitoring and effective mitigation strategies to protect both aquatic ecosystems and public health in ASGM-affected regions.