Effects of dietary arachidonic acid supplementation in low fishmeal and fish oil-free diets on growth performance, inflammatory response, gut histology, and non-specific immunity in sub-adult rainbow trout, Oncorhynchus mykiss

Arachidonic acid (ARA) is becoming increasingly important due to significant reductions in fishmeal (FM) and fish oil (FO), the primary sources of ARA, in aquafeeds. ARA is an essential omega-6 fatty acid in some fish species and plays a wide range of physiological roles. However, studies reported to date have demonstrated contrasting findings concerning the effects of ARA in fish growth and health, regardless of the fish's trophic level. Additionally, limited research has addressed energy allocation during reproduction and growth in sub-adult rainbow trout as they approach reproductive maturity. Therefore, a study was conducted to evaluate the effects of different levels of ARA on growth performance, the fatty acid composition of whole-body and ovary, intestinal inflammatory gene expression, gut histology, antioxidant activity, and non-specific immunity in sub-adult rainbow trout (Oncorhynchus mykiss). Four isonitrogenous (51% crude protein), isolipidic (15% crude lipid), and isocaloric (23 MJ/kg) diets were formulated and supplemented with ARA-enriched oil (40%), resulting in analyzed ARA levels of 0.10 (ARA₀.₁₀), 3.95 (ARA₃.₉₅), 7.67 ARA(₇.₆₇), and 14.8% (ARA₁₄.₈) of total fatty acid, respectively. Rainbow trout (307 ± 3.64 g) were fed to apparent satiation three times a day for 12 weeks to assess growth performance, feed utilization, and intestinal inflammatory response. The fish were then exposed to acute stress to evaluate circulating cortisol, prostaglandin E₂, antioxidant activity, and non-specific immune response. The 12-week growth trial demonstrated that dietary ARA did not affect rainbow trout growth performance and feed utilization during the final grow-out. Whole-body fatty acid profiles reflected those of the diets except for eicosapentaenoic acid (EPA) content being significantly lower in the ARA₁₄.₈ group than in the ARA₀.₁₀ and ARA₃.₉₅ groups. EPA and docosahexaenoic acid (DHA) contents in the ovary were less affected by dietary ARA supplementation, likely reflecting the selective deposition of those fatty acids in embryonic development. The expression of tnf-α was significantly downregulated in ARA₇.₆₇ and ARA₁₄.₈ groups relative to other treatment groups, and only distal intestinal villus height was significantly increased in the ARA₃.₉₅ group, suggesting that dietary ARA supported intestinal health. Plasma superoxide dismutase, catalase, and lysozyme activity significantly increased in both pre- and post-stress groups due to incrementa