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 by-products (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 β-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 Gmix-ibericus), 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.