Vegetable lipid sources for gilthead seabream ( Sparus aurata): effects on fish health

Commercial feeds for gilthead seabream are highly energetic, containing fish oil as the main lipid source. The steady production and raising prices of fish oil encourage the inclusion of vegetable oils in fish feeds. Fish oil could be at least partially substituted by vegetable oils in diets for marine species, being this substitution resulted in good feed utilization and maintenance of fish health, since imbalances in dietary fatty acids may alter the immunological status and stress resistance in fish. In order to evaluate the effect of vegetable oils on gilthead seabream health, fish were fed different isonitrogenous and isocaloric diets for 101 days (Experiment I) and 204 days (Experiment II). In Experiment I, diets were formulated to contain 60% of the fish oil used in the control diet (FO) as soybean oil (Diet 60SO), rapeseed oil (60RO), linseed oil (60LO) or a blend of those oils (Mix). In Experiment II, the same diets plus two which contained 80% of the fish oil as soybean oil (80SO) and linseed oil (80LO), respectively, were assayed. At the end of both experiments, basal levels of different immunological parameters were determined, including both humoral immunity (alternative complement pathway activity and serum lysozyme activity) and cellular immunity (circulating neutrophil activity and phagocytic index of head kidney macrophages). In addition, response to a confinement stress was assayed in terms of variations in plasma cortisol. The effect of dietary vegetable oils on fatty acid composition of head kidney macrophages and circulating red blood cells (RBC) was also studied. No effects of dietary vegetable oils were found in fish fed the experimental diets for a medium period. Feeding dietary vegetable oils for a long period did not affect lysozyme or neutrophil activity. However, in Experiment II, the inclusion of soybean oil reduced both serum alternative complement pathway activity (from 249 IU/ml (FO2) down to 153.8 IU/ml (60SO2)) and head kidney phagocytic activity (from 25.75% (FO2) down to 14.58% (80SO2). Inclusion of rapeseed oil reduced phagocytic activity. Fish fed vegetable oil-containing diets showed different patterns of stress response, especially those fish fed the linseed oil diets that showed a significant increase in plasma cortisol level after stress. The fatty acid composition of head kidney macrophages reflected the fatty acids content of the respective diets, but a selective incorporation of essential fatty acids into these