Recovery of rainbow trout (Oncorhynchus mykiss) after subchronic nitrite exposure

The aim of the study was to assess the effect of elevated nitrite concentration on the haematological and biochemical profile of blood of rainbow trout and to evaluate the ability of fish to recover under varying conditions. Fish (n = 72) were exposed for 14 days to elevated nitrite concentration (1 mg·l-1). Thereafter the exposed fish were divided into four groups and monitored for 7 days in the following conditions: water containing 1 mg·l-1 NO2 (group E), water containing 1 mg·l-1 NO2- + 100 mg·l-1 Cl- (group R1), nitrite-free water with 100 mg·l-1 Cl- (group R2), or nitrite- and chloride-free water (group R3). Nitrite exposure was associated with a significant increase (P P < 0.01) in haematocrit and mean corpuscular volume was recorded during nitrite exposure of fish. These changes were eliminated by addition of chloride to the water. Other haematological and biochemical blood indices showed that all treatments resulted in moderate stress to fish with the least pronounced changes in group R1. Results of the present study confirmed that even low nitrite concentrations may cause physiological changes in rainbow trout and pointed to possible recovery treatments of fish impaired by nitrite exposure. One of the most important findings is that chloride added to the water leads to successful fish recovery even in the presence of nitrite