Impact of Salinity Changes on the Antioxidation of Juvenile Yellowfin Tuna (Thunnus albacares)

To understand the impacts of salinity stress on the antioxidation of yellowfin tuna Thunnus albacares, 72 fishes (646.52 +/- 66.32 g) were randomly divided into two treatments (32 parts per thousand and 29 parts per thousand) and sampled at four time points (0 h, 12 h, 24 h, and 48 h). The salinity of the control group (32 parts per thousand) was based on natural filtered seawater and the salinity of the stress group (29 parts per thousand) was reduced by adding tap water with 24 h aeration to the natural filtered seawater. The superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), and malondialdehyde (MDA) from liver, gill, and muscle tissues were used as the antioxidant indexes in this study. The results showed that the changes of SOD and GSH-Px in the gills were first not significantly different from the control group (p > 0.05) and finally significantly higher than the control group (SOD: 50.57%, GSH-Px: 195.95%, p < 0.05). SOD activity in fish liver was not significantly changed from 0 h to 48 h (p > 0.05), and was not significantly different between the stress group and control group (p > 0.05). With the increase in stress time, GSH-Px and MDA activities in the liver of juvenile yellowfin tuna increased first (GSH-Px: 113.42%, MDA: 137.45%) and then reduced (GSH-Px: -62.37%, MDA: -16.90%) to levels similar to the control group. The SOD activity in the white and red muscle of juvenile yellowfin tuna first decreased (white muscle: -27.51%, red muscle: -15.52%) and then increased (white muscle: 7.30%, red muscle: 3.70%) to the level of the control group. The activities of GSH-Px and MDA in white and red muscle increased first (white muscle GSH-Px: 81.96%, red muscle GSH-Px: 233.08%, white muscle MDA: 26.89%, red muscle MDA: 64.68%) and then decreased (white muscle GSH-Px: -48.03%, red muscle GSH-Px: -28.94%, white muscle MDA: -15.93%, red muscle MDA: -28.67%) to the level observed in the control group. The results from the present study indicate that low salinity may lead to changes in the antioxidant function of yellowfin tuna juveniles. In contrast, yellowfin tuna juveniles have strong adaptability to the salinity of 29 parts per thousand. However, excessive stress may consume the body's reserves and reduce the body's resistance.