*Article* **Effects of Acute High-Temperature Stress on Physical Responses of Yellowfin Tuna (***Thunnus albacares***)**

**Hongyan Liu 1,2,3,†, Zhengyi Fu 2,3,4,†, Gang Yu 2,3, Zhenhua Ma 1,2,3,\* and Humin Zong 5,\***

<sup>1</sup> College of Fisheries and Life, Shanghai Ocean University, Shanghai 201306, China


**Abstract:** To understand the physiological reactions of juvenile yellowfin tuna (*Thunnus albacares*) under acute high-temperature stress, this study measured the changes in biochemical indexes of serum, liver, gill, and muscle of yellowfin tuna under acute high-temperature stress (HT, 34 ◦C) and a control group (28 ◦C) for 0 h and 6 h, 24 h and 48 h. The rising speed of water temperature in the HT group was 2 ◦C/h and the timing started when the temperature reached 34 ◦C. In the HT group, there was no significant difference between the four adjacent times in cortisol and lactic acid concentration. Serum triglyceride, cholesterol, and alkaline phosphatase concentration were significantly different from the four adjacent times. The superoxide dismutase (SOD) activity in the liver and gills increased at 6 h and 24 h, and the gills and liver had antioxidant reactions in a short time. The malondialdehyde (MDA) concentration in the gills changed significantly at 6 h, while that in the liver did not change significantly. The gills were more sensitive to temperature stress than the liver and muscle. Acute high-temperature stress affected yellowfin tuna's antioxidant enzymes and metabolic indexes, resulting negative trend in physiological indexes, indicating that yellowfin tuna juveniles are susceptible to elevated temperature.

**Keywords:** biochemical indexes; metabolism; serum ionic concentration; immune; oxidative stress parameters
