Evaluation of fish meal and fish oil replacement by soybean protein and algal meal from Schizochytrium limacinum in diets for giant grouper Epinephelus lanceolatus

Sparing marine-origin protein and lipid was investigated in diets for giant grouper (Epinephelus lanceolatus), using various combinations of soybean meal (SBM), soy protein concentrate (SPC), and an algal meal derived from Schizochytrium limacinum (ALG) to replace fish meal (FM), squid meal (SQM) and fish oil (FO). Diets were formulated to contain 45% protein (dry weight) and 10% lipid. Three diets were derived from this formulation, using combinations of SBM, SPC, and ALG to replace 20% (FM80), 40% (FM60), or 80% (FM20) of the marine-origin protein sources; additionally, incorporation of the lipid-dense ALG resulted in complete FO replacement in the FM60 and FM20 diets. A feeding experiment was carried out in a seawater recirculating system with 12, 90-L tanks initially stocked with 6 fish each (individual weight = 45.9 ± 7.1 g) and maintained at 25 °C. Diets were assigned to tanks in triplicate, fish were fed to apparent satiation twice daily for 12 weeks, and growth rate, body composition, nutrient retention, survival, hepatosomatic index and intestinal integrity were subsequently evaluated. With the exception of retained lipid, the FM100, FM80, and FM60 diets all yielded equivalent (P > 0.05) growth and feed utilization. In comparison, weight gain, specific growth rate, feed intake, retained nitrogen and retained lipid were significantly reduced in fish fed the FM20 feed, whereas FCR was significantly elevated. Amino acid analysis of the diets indicated that methionine was limiting in the FM20 feed. Hepatosomatic index did not vary among treatments, but other aspects of body composition were affected. Carcass lipid and ash significantly decreased with marine ingredient inclusion; carcass protein also varied, though less appreciably and without a clear pattern. Fillet fatty acid profile reflected dietary composition. The FM100 diet yielded the highest fillet EPA content, whereas DHA content, DHA:EPA and n − 3:n − 6 ratios, and total long-chain polyunsaturated fatty acids increased with dietary inclusion of ALG. Histological analysis revealed progressive morphological changes to the distal intestine with higher levels of FM replacement, suggesting enteritis among fish fed the FM20 feed. It is concluded that a blend of SBM, SPC, and ALG can replace at least 40% of marine protein sources, and ALG can be used as the main lipid source in diets for E. lanceolatus without significantly affecting fish performance or condition. Statement of relevance The results of this work contribute to decrease the dependence on protein and lipids from reduction fisheries traditionally used in commercial feeds for carnivorous fish, potentially improving the sustainability of aquaculture production of marine fish.