Nitrite toxicity in yellowtail tetra fish, Astyanax lacustris, at different temperature levels

Nitrite (NO₂⁻) is a toxic nitrogenous compound in aquaculture systems, particularly in recirculating aquaculture systems (RAS) operating at high densities, where its accumulation may cause severe physiological disorders in fish. This study evaluated the acute toxicity (LC₅₀) of nitrite and its sublethal effects on hematological parameters, oxygen consumption, and ammonia excretion in adults of Astyanax lacustris (yellowtail tetra fish), widely used as live bait in recreational fishing. Experiments were performed at three temperatures (20°C, 25°C, and 30°C). Nitrite toxicity increased significantly with both exposure time and temperature. After 96 h, LC₅₀ values decreased from 5.17 mg L⁻¹ at 20°C to 2.47 mg L⁻¹ at 30°C, indicating enhanced thermal sensitivity. Sublethal exposure promoted reductions in erythrocyte count and hemoglobin levels, consistent with hemolytic anemia, while plasma glucose levels increased, suggesting activation of stress responses. Metabolic effects included significant, temperature-dependent increases in oxygen consumption (up to 223.2 % at 30°C and 20 mg L⁻¹ nitrite) and ammonia excretion (up to 147.05 % at 25°C and 20 mg L⁻¹ nitrite), demonstrating a synergistic interaction between nitrite and elevated temperature. These results emphasize the importance of controlling both nitrite levels and water temperature in aquaculture systems, especially for species managed as live bait. Optimizing transport and holding conditions for A. lacustris requires monitoring of thermal regimes and nitrogenous waste accumulation, as their interaction intensifies physiological stress. Overall, this study provides practical guidelines to reduce mortality, improve animal welfare in the live bait trade, and support precise water quality management strategies in tropical aquaculture. • Nitrite toxicity to Astyanax lacustris increases with rising temperature. • Sublethal exposure alters hematology and metabolism in tropical fish. • Nitrite elevates oxygen demand and ammonia excretion under heat stress. • Combined nitrite and temperature stress impair gill ion regulation.