Fermented agar by-product and sunflower cake mixture as feedstuff for European seabass (Dicentrarchus labrax)

Industrial agar production from red seaweeds such as Gelidium sp. generates large quantities of byproducts (GBP), often discarded as waste. GBP and sunflower cake (SC) are under-valorized as feedstuff due to their high non-starch polysaccharide (NSP) content. Solid-state fermentation (SSF) may disrupt NSP and improve their nutritional value. This study evaluated the utilization of Aspergillus ibericus and Aspergillus niger for SSF of a GBP and SC mixture (1:1 ratio; Gmix). SSF with A. ibericus decreased neutral detergent fiber (NDF) content (P < 0.05) along with high beta-glucosidase activity (P < 0.05). SSF with A. niger increased crude protein, reduced NDF and acid detergent fiber (ADF) content, and produced higher xylanase, cellulase, and protease activities (P < 0.05). A 63-day growth trial with European sea bass (Dicentrarchus labrax) juveniles tested four isoproteic and isolipidic diets (44 % protein; 16 % lipids): a control diet without Gmix and three diets incorporating 10 % Gmix unfermented (diet Gmix), fermented by A. ibericus (diet Gmixibericus), or by A. niger (diet Gmix-niger). The Gmix and Gmix-ibericus diets promoted similar growth to the control, but the Gmix-ibericus diet supported higher feed efficiency, nitrogen, and energy utilization than the control diet (P < 0.05). The Gmix-niger diet negatively affected overall growth, feed intake, and utilization. Plasma metabolite levels, hepatic antioxidant enzyme activities, and lipid peroxidation were similar among diets. However, the Gmix-ibericus diet reduced total and oxidized glutathione levels (P < 0.05). Overall, G-mix or Gmix-ibericus diets did not compromise European seabass performance or oxidative status, with the Gmix-ibericus diet enhancing feed utilization efficiency by 25 %. Further studies are needed to understand the negative impact of the Gmix-niger diet on the European seabass.