2.3.4. Measurement of the immune response
After the last measurement (week 14) the fish from the three replications were pooled within each cross-type. Then 60 fish were taken from each cross-type and separately stocked in a 1.8m2 concrete tank for further study on immune response. A negative control group for immune response was established by taking 15 fish from each
cross and pooled in a 1.8 m2 concrete tank.
To study phagocytosis abilities of the peripheral blood leukocytes, the peripheral blood leukocytes (PBLs) were isolated from a pooled blood sample of three individuals/cross, counted by Haemacytometer and adjusted to a concentration of 2×106 cells/ml. Then each of the PBL samples was dropped on three cover slips and incubated for 2 h at 25 °C in a chamber. Then a yeast suspension (2×108 cells/ml) was added to each chamber. After 1 h of incubation at 25 °C, the cells were stained with Diff-Quick dye, and phagocytic activity (PA) and phagocytic index (PI) were calculated following the method of Puangkaew et al. (2004) with slight modification.