Effect of acute hypoxia in trout (Oncorhynchus mykiss) on immature erythrocyte release and production of oxidative radicals

This paper evaluates the response of trout exposed to acute hypoxia between 8.7 mgO(2)/l (controls) and 1.4 mgO(2)/l. Temperature, pH, NH4, oxygen consumption rate and spleen-somatic index (SSI) were evaluated. Blood cell counts, oxidative radicals (ORs) production and immature erythrocytes were assessed to evaluate their relationship with the degree of hypoxia. Acute hypoxia induced a decrease in water pH (p < 0.05), oxygen consumption rate (60% less), SSI (55% less) and a significant increase of water ammonium concentration (p < 0.05). Haematocrit increased significantly starting at 4.8 mgO(2)/l, as well as an increase in the total number of leukocytes (up to 50% in some fish). Starting at 2 mgO(2)/l, swelling of erythrocytes was induced and a decrease in the potential production of ORs, showing a negative correlation to immature erythrocyte numbers. These results suggest that lower than normal oxygen concentrations may jeopardize ORs production and hence non-specific defence mechanisms in trout. High percentages of immature erythrocytes can be used as a simple and inexpensive indicator of both hypoxic events and impairment of ORs production in the absence of bleeding, haemolysis, anti-anaemia treatments and/or increased temperature.