Abstract

Nitrite (NO ₂ ^-) is toxic to fish. High levels of NO ₂ ^- may develop in natural aquatic environments or in aquaculture systems, a situation that is aggravated in tropical regions by high temperatures and low dissolved oxygen. The toxicity of NO ₂ ^- was evaluated in the Amazonian fish, Colossoma macropomum to define the environmental quality standards and management of culture system for this species. The acute toxicity of NO ₂ ^- was tested using a renewal bioassay system and a series of increasing NO ₂ ^- concentrations up to 0.4 mM NO ₂ ^- (5.6 mg l ^-1). Blood samples were taken after a 96-h exposure to NO ₂ ^- to determine the hematological parameters, methaemoglobin (MetHb) concentration and plasma nitrite bioaccumulation. The 96-h LC ₅₀ was calculated as 0.13±0.06 mM NO ₂ ^- (1.82±0.98 mg l ^-1), indicating high sensitivity of this species to nitrite. Nitrite bioaccumulation resulted in methaemoglobinemia (66-76%) and hemolytic anemia (depression in total hemoglobin (Hb) concentration, hematocrit (Ht) and red blood cells (RBC)), which compromise the transport of O ₂ through the blood from the gills to the tissues. The increase of mean corpuscular Hb concentration in fish exposed to 0.04 and 0.2 mM NO ₂ ^- suggested changes in the ionic transport across the erythrocyte membrane. The absence of ecomorphological responses such as lip formation and aquatic surface respiration (ASR) during NO ₂ ^- exposure indicates that the mechanism involved in nitrite-induced hypoxia differs from that induced by environmental hypoxia. These findings reveal that intensive aquaculture systems of C. macropomum should be managed so as to avoid even low levels of NO ₂ ^-, since the effects of nitrite may compromise the fish's performance in an environment in which high temperature and daily fluctuations in dissolved oxygen may aggravate nitrite intoxication.