Effects on Channel Catfish of High Nitrite Concentrations That Occur in Fish Ponds

摘要

The concentration of nitrite lethal to one-half (LC50) of the channel catfish Ictalurus punctatus in a 96-hour, flow-through toxicity test was 13.7 mg/liter. A high percentage of methemoglobin caused the blood to be brown and was the most consistent effect of nitrite exposure. Chronic exposure to nitrite caused degeneration of erythrocytes and lactic acidosis. Histological lesions were not found in nitrite-exposed fish, but vacuolation of hepatocytes due to glycogen decreased in fish with methemoglobinemia. The percentage of methemoglobin decreased during chronic exposure, and this apparent acclimation may explain the ability of nitrite-exposed fish to grow at the same rate as controls during a 4-month experiment. Exposure to 5 mg/liter nitrite predisposed channel catfish to infection by Aeromonas hydrophila and Flexibacter columnaris, common bacterial pathogens of fish. Fish exposed to 4.5 mg/liter nitrite had lower antibody titers 14 days after vaccination than vaccinated controls.