Use of mixtures of algal and vegetable oils as fish oil replacers in Asian seabass ( Lates calcarifer ) feeds and their effects on growth, digestive enzymes, immune biomarkers, fatty acid profiles, and expression of genes involved in fatty acid biosynthesis

An experiment was conducted to assess the potential use of algal oil (AO) and vegetable (soya and palm oil; 1:1) oil (VO) mixtures as replacers for fish oil (FO) in Asian seabass ( Lates calcarifer ) diets. Six iso-nitrogenous (45% crude protein) and iso-lipidic (10% crude lipid) diets were formulated incorporating FO, VOs, and AO at 100:0:0 (T1), 0:0:100 (T2), 50:25:25 (T3), 25:50:25 (T4), 0:50:50 (T5), and 0:75:25 (T6), respectively. Each diet was given to five tanks (100 L with 10 fish in each) of juvenile fish (3.89±0.23 g initial weight) fed to satiation for 8 weeks. At termination, five replicate groups of fish were used for survival rate (SR), final weight, weight gain, consumed feed, feed conversion ratio (FCR), and specific growth rate (SGR) determination. Proximate composition, nutrient utilization, digestive enzyme activities of the pyloric caeca and intestine, and immune parameters were assessed using fish from three replications. A pooled sample consisting of three replicates was used for the determination of fatty acid profile and lipid quality indices. The liver samples from four replicate groups of fish were pooled into two samples and analyzed for Δ6-desaturase and Elovl5 gene expressions. The SR, growth performance, and feed conversion ratio were not significantly different ( p >0.05) among treatments; however, feed consumption was significantly different ( p <0.05) among treatments. The highest feed consumption (36.45±2.01 g/fish), final weight (32.41±2.78 g fish −1 ), and weight gain (28.53±2.78 g fish −1 ) were observed in T4 feeding group. Moreover, substituting FO with AO and VOs significantly affected ( p <0.05) viscerosomatic index, intraperitoneal fat, carcass lipids, and lipid retention efficiency (LRE). The VSI (5.45±0.42) and IPF (1.53±0.03) were the lowest in the T4 feeding group ( p <0.05). The 100% AO feeding group (T2) showed significantly lower carcass lipid and LRE ( p <0.05). Specific lipase activity in pyloric caeca was significantly higher in T4 feeding group ( p <0.05). Docosahexaenoic acid (22:6n-3) and eicosapentaenoic acid (20:5n-3) contents of fish carcasses were higher in the 100% AO-fed group. There was a significant ( p <0.05) impact on the expression of Δ6-desaturase and Elovl5 genes in liver tissue. There were no significant differences ( p >0.05) in innate immune parameters among the treatments. The findings of this study indicate that 75% of FO can be replaced by a combination of 25% AO and 50% VOs in Asian seabass ( L. calcarifer ) diets without impairing growth performance, nutrient utilization, and health status. These results can be used to develop more sustainable feeds that are less reliant on marine resources.