Effects of dietary (n-3) and (n-6) polyunsaturated fatty acid ratio on the immune response of Atlantic salmon, Salmo salar L

To examine the influence of the dietary ratio of (n‐3) to (n‐6) polyunsaturated fatty acids (PUFAs) on the immune system of Atlantic salmon, Salmo salar L., two dietary trials were carried out in which parr were maintained on diets containing either fish oil [(n‐3)/(n‐6) PUFA = 5.2] or sunflower oil [(n‐3)/(n‐6) PUFA = 0.3] and assessed for differences in immunological parameters. There were no significant differences in blood cell counts, differential leucocyte counts or haematocrit values between dietary groups, and while no apparent differences were observed in the non‐specific immune parameters measured, there was a significantly higher number of B cells responding to Aeromonas salmonicida, in the kidney and spleen of vaccinated fish maintained on high (n‐3)/(n‐6) PUFAs diets. There was also a significant difference (P≤ 0.01) between the dietary groups in trial 1 and trial 2 when non‐vaccinated fish were challenged with Aeromonas salmonicida and Vibrio anguillarum, respectively, with the (n‐6) group succumbing to the bacterium before the (n‐3) group. The results suggest that Atlantic salmon fed diets with a low ratio of (n‐3)/(n‐6) PUFA may be less resistant to infection than those fed diets containing lipid with a high (n‐3)/(n‐6) PUFA ratio.