Fish health and fillet quality following substitution of fishmeal (FM) and fish oil (FO) with vegetable meal (VM) and oil (VO) in the modern aquaculture.

Plant proteins in vegetable meals (VM) and oils are used in an increasing percentage in diets for carnivorous fish to replace fish meal (FM) and fish oil (FO) in order to reduce production costs and to face the limitation of marine resources and to be sustainable in respect to the protection of oceanic biodiversity. However, several studies report that fish respond to such replacements by reducing growth rate, feed conversion and fillet quality. It may be due to a deficiency or poor availability in VM ingredients of some essential amino acids, but also to the occurrence of anti-nutritionals and inflammatory compounds acting on the fish intestine. This can lead to economic losses for fish farmers, increased fish diseases occurrence, and negative environmental impact. This study focuses on European sea bass and Rainbow trout, as two among the most commercially important fish species in Italy. Aims of the study are to improve feed formulation and optimize feed additives such taurine, as well as butyrate, salt of a short chain fatty acid (SCFA), to protect fish intestine and evaluate the effects on fish fillet quality of the modern commercial diets where FM is partially substitute with VM. In that context, our experimental study investigated butyrate and taurine in the diet, as nutraceutics able to reduce or minimize the negative effects of VM inclusion in fish performances and health. After improvement of analytical methods for butyrate titration in fecal samples, the possibility that a butyric fermentation occurs in fish intestine has been experimentally evaluated. The protective effect of butyrate on fish intestine has been studied by following approaches including histology with light and electronic microscopy, then a molecular approach to study inflammatory cytokines, as well as the epigenetic action of this SCFA. In spite, dietary butyrate showed no significant differences in weight gain or SGR (specific growth rate) of sea bass, the protective effect on the intestine walls was highlighted, together with its epigenetic effects on hiperacetylation histone H4 at lysine 8, while no effects has been found on the histone H3 at Lys9. A butyrate effect on the liver, as an increased transcriptional expression of the genes Dicer1, ehmt2, and hdac11 and il-10 have been assessed, suggesting an anti-inflammatory as well as an antiviral role of butyrate in seabass receiving a 2% of Na-butyrate as diet supplement. The effect of a dietary supplementation of the sulphonic aminoacid taurine, almost absent in the VM, was studied on seabass growth performances and on its potential role as antioxidant. Seabass fed with 1.5% taurine addition showed an increase of the fish growth performance and a marked reduction of the ROS production under resting conditions that was enhanced following forced swimming performances. The synergistic effect of butyrate and taurine as feed additives was also explored in seabass feeding diet containing an elevate concentration of a soy protein. Different cytological protective mechanisms have been observed, indicating the existence of a synergistic action of the two nutraceutical compounds. The quality of fish fillet in trout fed with 6 different commercial feed where FM was differently substituted by VM, has been studied by assessing the fillet lipidograms. Trout was utilized in this study as is know for the capacity to elongate and desaturate the n-3 long chain fatty acid EPA in DHA. The EPA/DHA ratio in feed and in fillet were expected to be modified by the fish, with an increased restitution of DHA as consequence of a reduction in EPA. Actually, such a dynamic was observed in fish fed with only 4 of the challenged commercial feed and that was supposed to be due to an effect on the fatty acids metabolism, following an unbalancement of the essential aminoacids in the diets where higher substitution of FM with VM was operated. In conclusion, a number of the expected negative effects due the FM substitution with VM have been actually assessed. On the same time, the possibility to mitigate such negative performances with diet manipulation have been documented as well. Beside some practical results immediately applicable by the fish farming industry, indications for further researches are emerging from this study.