The ability of recovery in common carp after nitrite poisoning

The aim of the study was to assess the ability of recovery in common carp after nitrite poisoning and to distinguish the difference in nitrite poisoning of carp at two different chloride concentrations in water. Two groups of carp (group 1 and 2) were exposed to environmental nitrite concentration of 1.45 mmol/l NO2- at different chloride concentrations (group 1: 0.31 mmol/l Cl- and group 2: 3.73 mmol/l Cl-) for 48 hours. After that, they were transferred into nitrite-free water. In the group 1 mortality of 51% occurred during nitrite exposure and further 11% mortality was observed after 24 h in nitrite free water. No mortality occurred in group 2 and control. Nitrite and methaemoglobin concentrations (MetHb) markedly increased in group 1 (plasma: 10.5 ± 1.90 mmol/l NO2-, liver: 3.5 ± 1.15 mmol/kg NO2-, muscle: 1.5 ± 0.37 mmol/kg NO2-, and MetHb: 93 ± 6.1%) compared with control (plasma: 0.05 ± 0.04 mmol/l NO2-, liver: 0.02 ± 0.01 mmol/kg NO2-, muscle: 0.04 ± 0.01 mmol/kg NO2-, and MetHb: 3 ± 2.9%). After 24 h in nitrite-free water, the values mildly decreased but not significantly (plasma: 5.1 ± 1.49 mmol/l NO2-, liver: 1.8 ± 0.65 mmol/kg NO2-, muscle: 0.8 ± 0.23 mmol/kg NO2-, and MetHb: 84 ± 11.2%). After next 120 h the values decreased significantly and were compared with those found in the control group (traces of nitrite, MetHb: 3 ± 2.1%). Nitrite exposure caused also increase in plasma K+ (3.8 ± 0.29 mmol/l), ammonia (230 ± 92 µmol/l), urea (1.7 ± 0.28 mmol/l) and uric acid concentration (66 ± 54 µmol/l) in group 1. On the other hand, values of haematocrit, erythrocyte count and haemoglobin concentration were markedly lower than control values. Most changes were corrected by the end of the recovery period, only plasma potassium concentration dropped bellow the control values. Nearly no changes were found in the group 2 compared with the control during the whole experiment duration. This shows the positive effect of chlorides on the fish resistance against nitrites.