Partial Replacement of Fish Meal with Tuna By-Product Meal, Black Soldier Fly Larvae Meal, Fermented Soybean Meal, and Their Two Combinations in Olive Flounder (Paralichthys olivaceus) Diets: Effects on Growth, Feed Utilization, Biochemical Compositi

Fish meal (FM) remains a primary protein source in aquafeeds; however, its high cost and limited availability necessitate the exploration of alternative protein sources. This study, thus, investigates the effects of replacing 10% FM protein, equivalent to 22.2% of FM, with tuna by-product meal (TBM), black soldier fly (Hermetia illucens) larvae meal (BSF), fermented soybean meal (FSB), and their two combinations in diets on the growth, feed utilization, and biochemical composition of olive flounder (P. olivaceus), as well as on economic efficiency. A total of 450 juveniles averaging 10.3 g were randomly distributed into 18 300 L flow-through tanks (25 juveniles per tank). In the control (Con) diet (45% FM-basal), 10% FM protein was replaced with TBM, BSF, FSB, or a combination of TBM and BSF or TBM and FSB at a 1:1 ratio, referred to as the TBM, BSF, FSB, CTB, and CTF diets, respectively. Each experimental diet was randomly allocated to three replicate groups of fish. The fish were hand-fed to apparent satiation twice daily for 12 weeks. The weight gain, specific growth rate, feed consumption, and feed efficiency of fish fed the TBM diet were comparable to those fed the Con diet, but significantly higher than those fed the BSF, FSB, CTB, and CTF diets. The protein efficiency ratio of fish fed the TBM diet was significantly higher than that of fish fed the FSB diet. The protein retention of fish fed the TBM diet was significantly higher than that of fish fed the FSB and CTF diets. The condition factor of fish fed the Con and TBM diets was significantly higher than that of fish fed all other diets. In contrast, dietary treatment had no significant influence on the viscerosomatic index, hepatosomatic index, blood chemistry, proximate composition, and amino acid profiles of the fish. The economic conversion ratio was the lowest in the TBM diet, while the Con and TBM diets led to a significantly higher economic profit index than all other diets. Among the five tested formulations, thus, TBM was the best-performing substitute for FM when 10% FM protein was replaced in a 45% FM-basal diet of olive flounder.