Water is essential for life and is the habitat of thousands of aquatic species. However, it faces contamination by heavy metals such as mercury, especially in the upper Nanay River, Loreto, Peru, where illegal mining has aggravated the problem. This study determined the mercury levels in the muscle tissue of five species of piscivorous and detritivorous fish during the periods of crescent and trough, and verified whether the levels are within the permitted parameters and suitable for human consumption. A direct mercury analyzer (DMA-80 evo) was used. The results showed that Cichla monoculus (2.43 ± 0.26 mg/kg) and Pseudoplatystoma punctifer (2.63 ± 1.06 mg/kg) presented the highest levels during the river flood, and also exceeded the limits recommended by FAO, the European Union and SANIPES. Semaprochilodus insignis showed the lowest levels in both periods. The CRmw, CDm and THQ values...