*2.1. Experimental Fish and Design*

The juvenile yellowfin tuna were provided by Lingshui Research Station of Tropical Fisheries Research and Development Center, South China Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences. The length and weight of the experimental fish were 28.97 ± 2.17 cm and 646.52 ± 66.32 g, respectively. A total of 72 fish were randomly distributed into six 5000 L fiberglass tanks with recirculating seawater systems for a 7-day acclimation. The fish were fed daily from 08:30 to 09:00. Fresh miscellaneous fish (4 cm × 2 cm pieces) were fed with 5–8% body weight daily by satiety. No feeding was conducted on the day before and during the experiment. During the experiment, all of the tanks were supplied with filtered seawater with a water exchange rate of 300% tank

volume per day. The ambient salinity of 32‰ was used as the control with three replicates, and 29‰ was used as the stress group with three replicates (Figure 1). The salinity of the stress group was gradually adjusted by adding tap water with 24 h aeration to the natural filtered seawater at 1‰ per 1 h. When the salinity of the stress group reached 29‰, the experiment began. The experiment lasted 48 h. The photoperiod was maintained at 14:10 h (light:dark). The salinity, water temperature, DO, and pH were monitored using an HQ40d portable multi-parameter (HQ40d18, Hach, Loveland, CO, USA), and the nitrite nitrogen and ammonia nitrogen were measured by a nitrite nitrogen testing kit (Zhecheng Biotechnology, Beijing, China). During the experimental period, the water temperature was maintained at 29.5 ± 0.5 ◦C, DO was >7.50 mg·L<sup>−</sup>1, pH was 7.93 ± 0.12, ammonia nitrogen was <0.1 mg·L<sup>−</sup>1, and nitrite nitrogen <0.05 mg·L<sup>−</sup>1.

**Figure 1.** Experimental design of the effect of salinity change on the antioxidant capacity of juvenile yellowfin tuna (*Thunnus albacares*).
