Total substitution of fish oil by vegetable oils in gilthead sea bream (Sparus aurata) diets: effects on hepatic Mx expression and some immune parameters.

The use of vegetable oils in fish nutrition has been extensively studied; and recent work has focused attention on replacing fish oil with alternative fatty acid sources and their effect on the immune system. However, little is known about the effect of these oils on immune parameters such as the fish interferon system. In this study we evaluate the effect of two vegetable oils (linseed and soybean) on gilthead sea bream Mx expression and other innate immune parameters. Experimental diets were formulated where fish oil was totally replaced by vegetable oils or for a mixture of them (50% linseed and 50% soybean). Another diet prepared with pure fish oil was used as a control. Two experiments were carried out in order to evaluate growth, feed utilization, serum alternative complement pathway activity, serum lysozyme and phagocytic activity of head kidney leucocytes as well as Mx expression in the liver. In the first experiment fish were fed with experimental diets for 6 months and then, growth and feed utilization as well as immune parameters were analyzed. In the second experiment, fish from the previous feeding trial were injected with either a sub-lethal dose of Photobacterium damselae subsp. piscicida (94/99) or a synthetic dsRNA (Poly I:C) in order to stimulate an Mx response. The results show that total substitution of fish oil by vegetable oils decreased the growth of gilthead sea bream juveniles. Furthermore, both phagocytic activity and serum alternative complement pathway activity were significantly reduced by the inclusion of either vegetable oil individually in the sea bream diets, but the diet with mixed vegetable oils had no significant effect. There was no effect on serum lysozyme levels but the basal constitutive levels of Mx transcript expression in the liver were elevated in the fish fed the vegetable oil diets. The time-course of the Mx response to injection of Poly I:C was shorter in the fish fed the fish oil diet and the fish fed the diet based on a mixture of both vegetable oils showed a faster Mx response to bacterial injection. Following stimulation with Poly I:C or PDP the fish fed the vegetable oil based diets still maintained higher basal levels of hepatic Mx expression than the fish fed the fish oil diet which returned to undetectable levels.