Growth Performance of Gilthead Sea Bream ( <i>Sparus aurata</i> ) Fed Low Fish Meal Diets With an Innovative Mixture of Low Trophic Ingredients

This study examined the growth performance, cellular signaling, and gut microbiome of gilthead sea bream (<i>Sparus aurata</i>) fed four diets using low-trophic-level ingredients for 65 days. Control (C) diet contained fish meal (FM) as the main protein source and fish oil (FO) as a lipid source. In the 0%FMFO diet all FM and FO present in the C diet was replaced with a combination of microalgae, insect meal (IM), and tunicate meal (TM). IM and TM diets were formulated to contain 20% of the novel protein ingredients, replacing 68.09% and 45.91% of FM in diet C, respectively. Compared to diet C, feed utilization and growth performance of <i>S</i>. <i>aurata</i> fed 0%FMFO diet was not different (<i>p</i> > 0.05) and TM diet had a significantly lower (<i>p</i> < 0.05) specific growth rate (SGR), higher (<i>p</i> < 0.05) feed conversion ratio (FCR) and decreased fillet lipid content (<i>p</i> < 0.05). In liver and foregut of fish fed TM diet the activity of lactate dehydrogenase (L-LDH) significant increased (<i>p</i> < 0.05), indicating higher glycolytic potential, whereas the decrease in Hsp70, Hsp90, AMPK, and p38 MAPK may indicate reduced stress response. Fish midgut microbiome included beneficial taxa for the host. The results suggested that the mixture of algae, insect, and TM could replace all FM and FO in gilthead sea bream diets without affecting the fish growth performance.