*3.2. Changes in Oxidative Stress Parameters in Organs of Juvenile Yellowfin Tuna under an Acute Temperature Rise*

The superoxide dismutase (SOD) in the gills of the HT group (Figure 5a) showed an increasing and gradually stable trend, and there was no significant difference at 6 h, 24 h, and 48 h. The activity of SOD in the gills in the HT group gills was higher than in the control group at all times (Figure 5b). There was no significant difference between the HT

group and the 28 ◦C at all times. The activity of SOD in the liver (Figure 5c) in the HT group first increased and then decreased with time, and the activity reached the highest value at 24 h, and there was no significant difference between 6 h and 24 h. At 6 h and 24 h, the liver SOD activity of the HT group and control group was significantly different (Figure 5d).

**Figure 5.** Changes in gill superoxide dismutase (SOD) (**a**), gill SOD value (**b**), liver SOD (**c**), liver SOD value (**d**), red muscle SOD (**e**), red muscle SOD value (**f**), white muscle SOD (**g**) and white muscle SOD value (**h**) in organs of young yellowfin tuna under acute high–temperature stress. The value is the gap of the experimental group minus the control group. Red means down and green means up. The significantly different between the control group and the HT group was represented by a -. Different and the same letters indicate a significant difference (*p* < 0.05) and insignificant difference (*p* > 0.05).

The activity of SOD in the red and white muscle of the HT group (Figure 5e,g) showed a decreasing and then increasing trend, with a significant difference between adjacent groups in the red muscle. The activity in the red muscle of the HT group was higher than the corresponding time concentration in the control group at 0 h, 6 h, and 48 h, and lower than the control group at 24 h (Figure 5f). There was no significant difference in the activity of SOD in red muscle between the HT group and control group at 0 h and 6 h, but there was a significant difference between 24 h and 48 h. The activity in the white muscle was no significant difference at 6 h and 24 h. The activity in the white muscle of the HT group was higher than the control group at 0 h and 24 h, and lower than the control group at 6 h and 48 h. There was no significant difference in SOD activity between the HT group and control group at the corresponding time at 0 h, but there had a significant difference at the other times.

After acute heating, the concentration of malondialdehyde in the gills of the HT group (Figure 6a) increased first and then decreased with time, and there was no significant difference between 6 h and 24 h. The concentration of malondialdehyde in the gills of the HT group was higher than that in the control group at 0 h, 6 h, and 24 h, and lower than 48 h (Figure 6b). There was no significant difference in the concentration of malondialdehyde in the gills of the HT group and control group all the time.

The MDA concentration of the liver in the HT group (Figure 6c) showed an upward trend with the prolongation of the temperature stress time. There was no significant difference between the concentrations at 0 h and 6 h, 24 h, and 48 h. The MDA concentration in the liver of the HT group was higher than that in the control group at 0 h, 24 h, and 48 h, and lower than that in the control group at 6 h (Figure 6d). At 0 h and 24 h, there was no significant difference in the concentration of malondialdehyde in the liver between the HT group and control group, but there was a significant difference at 24 h and 48 h. There was no significant difference in the concentration of MDA between 0 h and 6 h (Figure 6e) in the red muscle of the HT group, there was a significant difference between 24 h and 48 h. Concentrations in the HT group were higher than those in the control group at 0 h, 6 h, and 48 h, and lower than those in the control group at 24 h (Figure 6f). The MDA in the red muscle of the HT group was not significantly different from that in the control group at all times. The concentration of MDA in the white muscle of the HT group (Figure 6g) showed a trend of first decreasing and then increasing with the prolongation of stress time, and there was a significant difference between 0 h and 6 h, and 24 h and 48 h had no significant difference. At 6 h, 24 h, and 48 h, the concentration of MDA in the white muscle of the HT group was lower than that in the control group (Figure 6h). At 6 h, the concentration of malondialdehyde in the white muscle of the HT group had significant difference from that in the control group, but there had no significant difference at the other times.

### *3.3. Changes in Immune Indexes in the Liver of Juvenile Yellowfin Tuna under an Acute Temperature Rise*

The alkaline phosphatase activity in the liver in the HT group (Figure 7a) decreased at first and then increased with prolonged stress time. The concentration at 24 h was significantly lower than the other three time points, and there was no significant difference between 6 h and 48 h. After temperature stress, the hepatic alkaline phosphatase activity of yellowfin tuna in the HT group was higher than the corresponding concentration in the control group (Figure 7b) at 0 h and lower than the control group at the other points. At 0 h and 48 h, there was no significant difference in the concentration of hepatic alkaline phosphatase between the HT group and control group, but there was a significant difference at 6 h and 24 h.

**Figure 6.** Changes in gill malondialdehyde (MDA) (**a**), gill MDA value (**b**), liver MDA (**c**), liver MDA value (**d**), red muscle MDA (**e**), red muscle MDA value (**f**), white muscle MDA (**g**) and white muscle MDA value (**h**) in organs of young yellowfin tuna under acute high–temperature stress. The value is the gap of the experimental group minus the control group. Red means down and green means up. The significantly different between the control group and the HT group was represented by a -. Different and the same letters indicate a significant difference (*p* < 0.05) and insignificant difference (*p* > 0.05).

**Figure 7.** Changes in alkaline phosphatase (**a**) and alkaline phosphatase value (**b**) in the liver of young yellowfin tuna under acute high–temperature stress. The value is the gap of the experimental group minus the control group. Red means down and green means up. The significantly different between the control group and the HT group was represented by a -. Different and the same letters indicate a significant difference (*p* < 0.05) and insignificant difference (*p* > 0.05).

#### **4. Discussion**

*4.1. Changes in Serum Indexes of Juvenile Yellowfin Tuna under an Acute Temperature Rise* 4.1.1. Changes in Ion Concentration, Osmotic Pressure, Blood Glucose, Lactic Acid, and Cortisol in Serum of Juvenile Yellowfin Tuna under an Acute Temperature Rise

The change trend of osmotic pressure in the HT group was first increased and then decreased. This was because the fish maintained its own pressure by regulating its own osmotic pressure to regulate the concentration of ions after stress. The research showed that [35] the effect of environmental temperature stress on juvenile catfish plasma ion concentration and osmotic pressure concentration, and found that Na+, Cl<sup>−</sup> and osmotic pressure showed an upward trend, while K+ remained roughly unchanged. The results of Na+, Cl−, and osmotic stress were similar to those of this study, but K+ was different. It is found that the ionic permeability increases [36] when the temperature rises, so Na+, Cl<sup>−</sup>, and osmotic pressure show an upward trend. The change in K+ concentration may be due to the fact that after the increase in cell membrane permeability, K<sup>+</sup> needs to enter the cell to play a balancing role, leading to the decrease in the concentration. The Na<sup>+</sup> and Cl<sup>−</sup> concentration change at 48 h, which may be due to the balance of K+.

The research reported that blood glucose increased with the increase in temperature in carp and Senegalese sole (*Solea senegalensis* Kaup) under high temperatures [37–39]. The changing trend in this study is the same as that in previous studies, indicating that the metabolism is more vigorous and more glycogen is needed. In this study, the lactic acid content in the HT group is lower than the control group most of the time, and lactic acid in the HT group shows a downward trend. This indicates that to adapt to the environment under high-temperature stress, lactic acid was taken to the liver and decomposed into CO2 and H2O, reducing the lactic acid content. The research showed that the plasma cortisol concentration of the fish adapted to high temperature was about twice that of the control fish, which was consistent with the results of carp [40,41] and black snapper (*Acantopagrus schlegelii*) [42] adapting to the high temperature previously reported. The result of this study is that it decreases first and then increases, which is different from the results of other studies. This may be because the yellowfin tuna is a kind of temperate fish, which has a tolerance to temperature for a short period of time. The HT group rises at 48 h, which may be due to the stress response gradually enhanced with the extension of time.

4.1.2. Changes of Metabolic Indexes in Serum of Juvenile Yellowfin Tuna under an Acute Temperature Rise

The research showed that [43] the growth performance and metabolism of Roche Labeo (*Labeo rohita*) rohita under heat stress and found that the triglycerides and cholesterol concentrations in serum decreased gradually. The research showed [19] the physiological and biochemical reactions of fat short cap carp (*Piaractus brachypomus*) under temperature stress. The results showed that the contents of triglycerides and cholesterol in plasma decreased after heat shock. The results of this study showed that the triglyceride concentration of the control group and the experimental group decreased gradually after 6 h, which was consistent with the previous study. However, the concentration increased after 24 h, which may be due to the prolonged stress time, the body's adaptation to environmental changes, and the gradual recovery of triglyceride metabolism, leading to the increase in triglyceride concentration. Alkaline phosphatase (ALP) is an essential non-specific immune marker enzyme and metabolic regulator enzyme. This enzyme has detoxification, defense, and digestion functions and is also an essential metabolic regulating enzyme involved in phosphate group transport and metabolism and is a sign of fish health [44]. Environmental changes affect its content, reflecting the stress state of fish [45]. The research showed that [46] rainbow trout's physiological and biochemical reactions to heat stress. The results showed that the alkaline phosphatase decreased significantly after the temperature increased. This study showed that the serum alkaline phosphatase concentration in the HT group decreased first and then increased, similar to the previous study. This is because alkaline phosphatase promotes fat degradation and reduces concentration under high-temperature stress. Under long-term pressure, the body's triglyceride, cholesterol, and alkaline phosphatase concentrations increase. The results showed that acute heat stress could promote the lipid metabolism of fish in a short time.
