Rearing in intermediate salinity enhances immunity and disease-resistance of turbot (Scophthalmus maximus L.)

Studies were conducted to investigate the non-specific immune responses and disease-resistance of juvenile turbot Scophthalmus maximus, cultured at four different salinities (8, 20, 32 and 40). Three concentrations (3.75×10⁷, 3.75×10⁸ and 3.75×10⁹ CFU/ml) of Vibrio anguillarum suspension were employed at each salinity to determine the 4-day LD₅₀. The serum lysozyme activity, the alternative complement pathway activity (ACH50) and the phagocytosis percentage of head kidney in turbot were tested at 24, 48 and 72 h post-challenge of V. anguillarum (1.1×10⁸ CFU/ml, 0.1 ml), respectively, to evaluate the non-specific immune responses at the selected rearing salinities. Fish reared at salinity 20 had the lowest mortality, namely, the highest 4-day LD₅₀ value (8.88±0.17). Besides, the lysozyme activity, ACH50 and the phagocytosis of turbot were the highest at the salinity 20, but with the lowest at the salinity 40 treatment regardless of sampling time. In addition, the non-specific immune activities kept increasing within 72 h post-challenge of V. anguillarum, except that the lysozyme activity increased from 24 to 48 h, and then decreased from 48 to 72 h at 40 significantly. These results together indicate that rearing in intermediate salinity (20) was able to enhance the immunity and disease-resistance of turbot.