Based on central composite rotatable experimental design and response surface method, the interacting effects of temperature (20 °C-34 °C) and dietary protein level (25%-50%) on hematological parameters including red blood cell (RBC), white blood cell (WBC) and hemoglobin (Hb) of juvenile Oreochromis niloticus were studied under laboratory conditions. The experiment lasted for 7 weeks. After the feeding trial, fish were challenged with Streptococcus iniae and mortality was recorded for within 8 days. Results showed that the linear and quadratic effects of temperature on RBC, WBC and Hb were highly significant (P < 0.01). When the dietary protein level was 25%-50%, the RBC, WBC and Hb were increased firstly and then decreased, but the linear and quadratic effects of protein level were insignificant (P > 0.05). The interacting effects of temperature and protein level on RBC and Hb were significant (P < 0.05). The regression equations of RBC, WBC and Hb toward the two factors of interest were established, with the coefficients of determination being 0.870, 0.836 and 0.881, respectively (P < 0.01). These equations could be used for prediction in practice. After the challenge, the mortalities for the combinations of 22.1 °C/28.7% and 20.0 °C/37.5% were significantly higher than 27.0 °C/37.5% (P < 0.05). The optimal temperature/dietary protein level combination was obtained at 27.9 °C/38.1% at which the lowest mortality (13.76%) was attained. This value was close to the optimal temperature/dietary protein level combination (29.4 °C/41.9%) for the greatest levels of RBC (2.560 × 10(6) μL(-1)), WBC (270.648 × 10(3) μL(-1)) and Hb (92.851 g L(-1)). The results of this study indicated that preferred temperature/dietary protein level combination might strengthen the non-specific immunity and reduce susceptibility to S. iniae.
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