Consequences of High Levels of Ammonia Exposure on the Gills Epithelium and on the Haematological Characteristics of the Blood of the Siberian Sturgeon, Acipenser baerii

Ammonia toxicity in the Siberian sturgeon, Acipenser baerii, a fresh water fish, was studied under controlled conditions of pH, temperature and ammonia concentrations in water.The effects of ammonia during a 72-h exposure to lethal and sublethal doses were examined on the morphology of branchial epithelium. Some hypertrophies and necrosis were observed in proportion with the ammonia doses.Ammonia has no effect on haematological characteristics except for the [K+] plasmatic concentration and the erythrocyte volume for lethal levels of ammonia.Among the numerous hypotheses to explain ammonia toxicity on fish, some refer to oxygen transport and oxygen affinity of the blood. In fact, the partial pressure of oxygen in the blood (PaO2) decreases significantly for a lethal dose only. Moreover, the influence of ammonia on oxyphoric properties of red blood cells of Siberian sturgeon has been investigated in vivo (blood of fish exposed for 72 h to 4.5 mmol L−1 total ammonia, a lethal concentration) and in vitro (ammonia added to blood of control fish). Whatever the experimental conditions, ammonia did not modify the affinity of haemoglobin for oxygen illustrated by P50, Bohr effect, Hill number and percentage of methaemoglobin. Consequently, the toxicity of ammonia on Siberian sturgeon cannot be explained by an alteration of the gill morphology, the ionic exchanges or the oxygen-binding properties of the haemoglobin only.