Administration of Red Macroalgae (Galaxaura oblongata) in the Diet of the Rainbow Trout (Oncorhynchus mykiss) Improved Immunity and Hepatic Gene Expression

This study is designed to evaluate the effects of dietary red macroalgae (Galaxaura oblongata) on growth performance, serum, and skin mucus immunological and antioxidant responses in rainbow trout (Oncorhynchus mykiss). For this, rainbow trout were fed diets containing different levels of G. oblongata (0 (ctrl), 0.5 (G1), and 1 (G2) %) for 8 weeks. Following the feeding trial, there were no significant differences in growth performance between the experimental treatments (p > 0.05). Total immunoglobulin (Ig) content and lysozyme (LYZ) activity in serum were increased in fish fed G. oblongata (p < 0.05), with the highest value at (0.5%). Regardless of the inclusion level, mucus total Ig levels were significantly increased in the G. oblongata groups (p < 0.05), and mucus LYZ activity was not changed (p > 0.05). All groups fed G. oblongata showed higher serum catalase (CAT), superoxide dismutase (SOD), and glutathione peroxidase (GPx) activities than the control group (p < 0.05). However, skin mucus SOD activity increased more in the group fed 1% of G. oblongata than the other groups (p < 0.05). Additionally, the skin mucus GPx activity showed higher values in the group fed 0.5 and 1% G. oblongata than in the control (p < 0.05). No significant differences were recognized between the experimental treatments in terms of CAT activity and malondialdehyde (MDA) concentration (p > 0.05). G. oblongata up-regulated gpx gene expression with the maximum value at the group fed 1% G. oblongata (p < 0.05). Additionally, interleukin 6 (il-6) and tumor necrosis factor-alpha (tnf-α) gene expressions were significantly up-regulated in fish fed 1% compared with the control and 0.5% groups. Based on the results, 0.5–1% G. oblongata can be used in the fish diet and enhance immunity without causing impairment in growth.