Effects of dietary arachidonic acid on growth performance, survival, immune response and tissue fatty acid composition of juvenile Japanese seabass, Lateolabrax japonicus

A 12-week feeding experiment was conducted to investigate the effects of dietary arachidonic acid (ARA) on growth, survival, immune response and tissue fatty acid composition of juvenile Japanese seabass (Lateolabrax japonicus) (mean initial weight 9.48 ± 0.09 g) in seawater floating net cages (1.5 × 1.5 × 2.0 m). An ARA-enriched oil was supplemented into the basal diet to formulate six isonitrogenous and isoenergetic practical diets containing 0.08% (the control group), 0.22%, 0.36%, 0.56%, 1.33% and 2.12% ARA of dry weight, respectively. All diets had the same total n−3 long chain polyunsaturated fatty acid (LC-PUFA) level and 22:6n−3/20:5n−3 ratio. Triplicate groups of 30 fish were fed to apparent satiation twice daily for 12 weeks. The water temperature ranged from 22.5 to 31.5 °C, the salinity from 28‰ to 33‰ and the dissolved oxygen content was approximately 6 mg l− 1 during the experimental period. The results showed that final weight (FW) and specific growth rate (SGR) significantly increased with increasing dietary ARA from 0.08% to 0.36% (P < 0.01) and thereafter declined. The feed efficiency ratio (FER) had the similar pattern with SGR, and fish fed the diet with 0.36% ARA showed significantly higher FER than the control (P < 0.05). No significant differences were found in survival rate among dietary treatments (P > 0.05). The hepatosomatic index (HSI) decreased significantly with the increase of dietary ARA, and HSI in fish fed the diets with more than 0.22% ARA content were significantly lower than the control group (P < 0.01). The serum lysozyme (LYZ), alternative complement pathway (ACP) and superoxide dismutase (SOD) activity were significantly enhanced by the supplementation of ARA, especially in moderate supplementation (0.36–0.56%). However, there were no differences in both respiratory burst activity of head kidney macrophage and serum catalase (CAT) activity among dietary treatments. The body composition analysis showed that whole-body protein first increased, then decreased with increasing dietary ARA, while whole-body lipid content followed the opposite pattern. The fatty acid composition of whole body and liver reflected closely those of the diets, while liver EPA levels were inversely related to dietary ARA. These results suggested that dietary ARA, especially moderate ARA level (0.22–0.56% d.w.), significantly enhanced growth and immune response and modified the chemical composition of whole body and liver of Japanese seabass juvenile.