Condensed tannins enhanced antioxidant capacity and hypoxic stress survivability but not growth performance and fatty acid profile of juvenile Japanese seabass (Lateolabrax japonicus)

Condensed tannins (CT) have been shown a promising alternative to antibiotics for livestock, but little information is available about the effect of purified CT on the growth and health of fish. A 56-day feeding trial with 640 juvenile Japanese seabass (Lateolabrax japonicus) was conducted to assess the effects of CT on the growth performance, fatty acid composition, antioxidant capacity and hypoxic stress of L. japonicus. Four diets were formulated to contain 0 (CT0), 100 (CT100), 200 (CT200) and 400 (CT400) mg/kg CT isolated from grape seed, quadruplicate groups of 40 fish with initial body weight of 10.1 ± 0.11 g were fed to apparent satiation two times daily. Fish were dissected at the end of experiment to evaluate the somatic indexes. Blood samples were collected to analyze serum metabolites, antioxidant enzymes and non-specific immune indices. Muscle and hepatopancreas were sampled to determine fatty acid composition. At the end of the feeding trail, 15 fish from each tank were suffered with a 10-hr hypoxic stress. Blood and hepatopancreas of the suffered fish were collected to analyze antioxidant enzymes, and hepatic mRNA levels of nuclear factor erythroid 2-related factor 2 (Nrf2), heme oxygenase 1 (HO-1), catalase (CAT) and superoxide dismutase (SOD) were analyzed. Results indicated that growth performance, whole body compositions, serum biochemical parameters, and fatty acid profile in muscle and hepatopancreas of fish were not affected by dietary treatments. Intraperitoneal fat ratio of the fish was linearly decreased (P < 0.01) as the dietary CT increasing from 0 to 400 mg/kg. Serum total antioxidant capacity (TAOC) and CAT were linearly increased (P < 0.01), but tumor necrosis factor was linearly decreased (P < 0.05) and IL-6 was linearly and quadratically decreased (P < 0.001) with increasing CT. There was a linear increase (P < 0.05) in hepatic TAOC and anti-superoxide anion with increased dietary CT. Hepatic CAT was quadratically increased (P < 0.01), and GSH-Px and SOD were linearly and quadratically decreased (P < 0.05) with increasing CT. Cumulative mortality rate (CMR) after 10 h of hypoxic stress and serum MDA were linearly and quadratically decreased (P < 0.001), whereas serum CAT and GSH-Px were linearly increased (P < 0.01) and TAOC was linearly and quadratically increased (P < 0.05) with increasing dietary CT. Hepatic CAT and SOD after 10 h of hypoxic stress were linearly increased (P < 0.05) with CT increasing. The Nrf2 and HO-1 mRNA levels were up-regulated (P < 0.05) with increasing dietary CT, and CAT and SOD mRNA levels were up-regulated (P < 0.05) in CT400 than CT0-CT200. In conclusion, dietary CT up to 400 mg/kg can not only improve the antioxidant status of L. japonicus without affecting growth performance, but also reduce the CMR under hypoxic stress by activating Nrf2 signaling pathway.