**1. Introduction**

*Thunnus albacares* belongs to Perciformes, Scomberia, Scombrida, commonly known as yellowfin tuna [1]. Yellowfin tuna is an oceanic migratory fish that lives in the vast upper middle waters of tropical, subtropical, and temperate oceans. As one of the high economic value tuna species, yellowfin tuna grows quickly among the tuna species with high flesh quality [2]. It is a preferred species for offshore aquaculture [1,3,4]. Yellowfin tuna has been cultured in Mexico, Panama, and Indonesia [5]. Mexico has established a large number of yellowfin tuna culture bases, but the limited supply of wild fry has seriously

**Citation:** Zhou, S.; Zhang, N.; Fu, Z.; Yu, G.; Ma, Z.; Zhao, L. Impact of Salinity Changes on the Antioxidation of Juvenile Yellowfin Tuna (*Thunnus albacares*). *J. Mar. Sci. Eng.* **2023**, *11*, 132. https://doi.org/ 10.3390/jmse11010132

Academic Editor: Dariusz Kucharczyk

Received: 22 November 2022 Revised: 22 December 2022 Accepted: 3 January 2023 Published: 6 January 2023

**Copyright:** © 2023 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/).

restricted the culture industry of this species [6]. The artificial cultivation of yellowfin tuna in China is still at the initial stage. The in-depth marine aquaculture technology and variety development innovation team from the Chinese Academy of Fishery Sciences has achieved indoor circulating water and offshore deep-water cage culture of yellowfin tuna in Lingshui Li Autonomous County, Hainan Province, and carried out a series of studies on aquaculture biology and disease prevention and control [7,8]. Up to now, open published biological data on yellowfin tuna farming are relatively scarce, which restricts the development of its artificial farming. Yellowfin tuna is a migratory fish in the ocean, and its migration route is closely related to salinity [1], and the physical responses of yellowfin tuna to ambient salinity changes are still unclear.

As an ecological factor, ambient salinity has a series of physiological effects on fish. As reported in previous studies, salinity is an important factor regulating fish growth, metabolism, and various physiological activities [9]. Ambient salinity can also directly affect the promotion of aquaculture [10]. An inappropriate salinity range will affect the physiology and biochemistry, immunity, growth and development, feeding, and reproduction of fish, and fish will show different adaptive states under various salinity conditions [11]. Recent studies have shown that salinity changes can cause physiological stress reactions in fish, accompanied by the production of excessive reactive oxygen free radicals, which can lead to oxidative stress reactions that damage the antioxidant defense system of fish [12,13]. Excessive oxygen free radicals produced by oxidative stress will attack biological membranes, proteins, and nucleic acids, causing oxidative damage such as cytoplasmic efflux, enzyme inactivation, and genetic replication errors [14]. Therefore, such processes will disrupt the normal physiological and behavioral activities of fish. Salinity fluctuations exist in coastal areas, with possible causes including heavy rainfall and river injection, which pose a threat to cage- and land-based culture that relies on naturally filtered seawater. The low salinity of 28.5 ‰ has been observed in the coast of Hainan, China [15], but the optimum growth salinity of yellowfin tuna is 31.2~33.3 ‰ [16]. Such temporary environmental fluctuations may pose unknown challenges to yellowfin tuna.

The antioxidant system in fish can resist oxidative damage [17]. Superoxide dismutase (SOD) is an essential antioxidant enzyme in the antioxidant system [18]. SOD is the primary substance for scavenging free radicals in organisms. It decomposes superoxide anion free radicals (O−2) through disproportionation. After scavenging O−2, SOD generates H2O2 [19]. Glutathione peroxidase (GSH-Px) is a critical non-enzymatic component [20]. As an antioxidant, GSH-Px can participate in the elimination of toxic peroxides [21]. Malondialdehyde (MDA) is a lipid peroxide, which can indirectly reflect the degree of lipid peroxidation in tissue cells [22]. In the present study, SOD, GSH-Px, and MDA were used as the antioxidant indicators to evaluate the impacts of salinity on the antioxidant system of juvenile yellowfin tuna. The purpose of this study is to understand the physiological response of yellowfin tuna to acute low salt in the process of salinity adaptation, in order to solve the change of seawater salinity reduction in a short time in subgrade culture or cage culture. The results from the present study will add biometric data to the culture of yellowfin tuna.
