The phagocytic activity of the blood leukocyte was determined according to the method of modified as follows: 0.2 mL of blood was dropped into centrifuge tubes,to which was added 0.1 mL of 5.0 × 106 S. aureus suspension. After incubation
at 27 °C for 30 min, the blood smears (five slides for each fish) were created and stained by Giemsa solution (Sigma). The phagocytic activity,defined as the phagocytic rate (PR) or phagocytic index (PI), was expressed as: PR(%) = [100 × (phagocytic leucocytes)(total
leucocytes)−1] or PI = [(phagocytic number of S. aureus)(phagocytic
leucocytes)−1].