Evaluation of marine algal ( Ulva spp./ Solieria spp.) extracts combined with organic acids in diets for olive flounder ( Paralichthys olivaceus )

The supplemental effect of marine algae ( Ulva spp. and Solieria spp.) extracts with/without organic acids was evaluated in olive flounder ( Paralichthys olivaceus ) diet. A diet containing 55% fish meal was considered as the control diet. Three other diets were prepared by supplementing 0.5% of algae extract from Ulva spp. mixed with a blend of organic acids and 0.3% or 0.5% algae extracts from Ulva spp. and Solieria spp. into the control diet (CON, AS5, AL3 and AL5). Total 640 fish (initial mean body weight: 26.5 ± 0.02 g) were randomly distributed into 16 tanks as 40 fish per tank. Quadruplicate groups of fish were hand‐fed one of the experimental diets twice a day for 12 weeks. Growth performance of fish was not significantly affected by experimental diets although feed utilization of fish fed AS5 diet was significantly higher than that of the CON group. The innate immune responses and activities of antioxidant enzymes were significantly increased by dietary supplementation of the additives. When fish were challenged with Edwardsiella tarda , survival was higher in fish fed the additive supplemented diets than the CON group. Catalase, lysozyme and superoxide dismutase activities were significantly higher in surviving fish fed the additive supplemented diets than the CON group after the challenge test. Goblet cell counts in fish intestine were significantly higher in AS5 group. Amylase and lipase activities and lipid digestibility were increased by the diets containing additives. The findings in this study indicate that algal extract and/or organic acid supplementation in diets increase innate immunity, disease resistance, intestinal morphology, digestive enzyme activity, diet digestibility and feed utilization of olive flounder.