**1. Introduction**

Fish often live in various water environments. It is a significant physiological process for fish to adjust to environmental pressure. Temperature change is one of the most common environmental changes, which is considered the main abiotic factor affecting aquatic animals [1]. At present, there are many related studies on the impact of temperature on fish, including killifish (*Oryzias latipes*) [2], sardine (*Sardine pilchardus*) [3], grass carp (*Ctenophryngodon Idella*) [4], black head minnow (*Fathead minnow*) [5]. The change in water temperature can lead to changes in the immune system, metabolism, oxidative stress, and other physiological changes to adjust to the environment [6–9]. Temperature fluctuations cause physiological changes in fish that generally begin by causing stress and then metabolic rates change [1]. Metabolic changes cause the production of reactive oxygen species [10]. Excessive reactive oxygen species will damage DNA, protein, and lipids [11], resulting in oxidative damage. At the same time, temperature also affects fish feeding and digestive processes, thus affecting their metabolism [12]. Unsuitable water environment temperature also leads to slow growth and poor appetite of fish. Therefore, studying the impact of temperature on the physical responses in fish is crucial.

**Citation:** Liu, H.; Fu, Z.; Yu, G.; Ma, Z.; Zong, H. Effects of Acute High-Temperature Stress on Physical Responses of Yellowfin Tuna (*Thunnus albacares*). *J. Mar. Sci. Eng.* **2022**, *10*, 1857. https://doi.org/ 10.3390/jmse10121857

Academic Editor: Valerio Zupo

Received: 3 November 2022 Accepted: 24 November 2022 Published: 2 December 2022

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

The temperature change can cause stress, composed of components characterized by sympathetic nerve activation and the secretion of adrenaline and cortisol [13]. The second stage is the increase in plasma glucose and the disorder of osmotic pressure regulation [13]. Cortisol (COR) is a steroid hormone [14], which has many biological activities, including maintaining osmotic pressure, regulating blood glucose, and inhibiting immunity [15]. Fish can also adapt to temperature changes by changing the superoxide dismutase (SOD) activity and then influencing the content of malondialdehyde (MDA) [16]. SOD is an important antioxidant enzyme that catalyzes the disproportionation of free superoxide anion radicals to hydrogen peroxide, which is then converted into water and oxygen by catalase or glutathione peroxidase [17]. Elevated MDA levels are an indicator of lipid peroxidation, which results from oxidative stress damage caused by exposure of fish to environmental changes or pollutants [18]. SOD can alleviate the oxidative damage caused by MDA produced by lipid peroxidation. Heat stress can also lead to significant changes in the metabolism-related indicators of fish, such as triglyceride, cholesterol [19] and plasma components calcium and magnesium, alanine aminotransferase (ALT), and alkaline phosphatase (ALP) [20].

Tuna is a highly demanded marine fish. Its back muscle contains 26.2% crude protein and 0.2% fat. It has rich nutrition [21]. Additionally, it is one of the most important economic fish in the world [22,23]. Tuna products are exported to more than 60 countries worldwide, of which three major markets are Japan, the European Union, and the United States [24]. In the 50 years from 1950 to 2000, the total catch of commercial tuna stocks increased from 4000 tons to 3.9 million tons [25]. Yellowfin tuna is an important species caught by fisheries in the Pacific region [26] and the global average annual capture production has increased year by year since the 1960s, but with significant volatility after 2004 [27]. The reason was mainly due to the exhaustion of wild resources. Resource survey results show that since the 1970s, wild yellowfin tuna spawning has been in a long-term decline, and the fishing mortality of adults and juveniles has continued to increase [28]. Wild resources of yellowfin tuna in the Central and Western Pacific Oceans have been fully exploited [29], and resources are declining. Therefore, it is urgent to conduct research related to the artificial culture of yellowfin tuna.

Yellowfin tuna (*Thunnus albacares*) belongs to the mackerel family and the tuna genus [30]. It is a highly migratory fish species in the ocean. It can swim at high speed and in deep water. It can quickly dive to the cold-water area below the thermocline (20 ◦C isotherms) to feed, and the maximum depth exceeds 1000 m [31]. Tuna can automatically adjust the active water depth when encountering temperature changes in the sea [32], but in the cage or land-based culture, the active space is limited, and high temperatures cannot be avoided. The appropriate temperature for yellowfin tuna larval is 28.0 ± 1.0 ◦C [33], and after our observation, we found that the summer temperature of the land-based culture pond in the tropics can be as high as 34 ◦C. In fish farming, the aquaculture water temperature is easy to maintain at a high level in summer, and summer is the season of increased fish diseases [34]. Therefore, it is essential to explore the expression and change of fish's physiological and biochemical indicators, immune function, and oxidative stress parameters under acute high-temperature conditions and analyze fish's response to high temperature. In this experiment, the water temperature is raised to 34 ◦C. By measuring the relevant indicators of serum, gills, liver, and muscle of young yellowfin tuna at 0 h and 6 h, 24 h and 48 h after the change of environmental conditions, the effects of acute temperature rise on osmotic physiology and oxidative stress parameters of young yellowfin tuna are discussed, it provides a theoretical basis for in-depth study of the stress response of the organism caused by environmental changes, provides a reference for yellowfin tuna aquaculture.
