Abstract:Low-temperature acclimation technology is necessary for transporting live fish because it induces a state of dormancy, reducing injuries and mortality during transportation. To explore low-temperature acclimation technology for Lateolabrax maculatus, this study investigated the effects of gradient cooling on the physiological and biochemical properties, muscle texture characteristics, and structural changes in the liver and gill tissues of L. maculatus. Gradient cooling allows for gradual adjustment of water temperature, preventing stress in fish caused by sudden temperature changes and minimizing damage while preserving live circulation. The fish were cooled at a rate of 1–2 ℃/h, and their behavioral characteristics at different temperature ranges were observed to determine their dormancy and critical temperatures. The fish were cooled using a gradient cooling method (when T > 20 ℃, the cooling rate was 3 ℃/h; for 10 ℃ < T < 20 ℃, the cooling rate was 2 ℃/h (for T < 10 ℃, the cooling rate was 1 ℃/h). Samples were taken after holding the fish at 24, 20, 16, 12, and 8 ℃ for two hours each. Serum biochemical, antioxidant, lipid metabolism, and cardiac enzyme indicators were measured, and muscle texture characteristics were assessed. Additionally, the microscopic structural changes in the liver and gill tissues of the fish were observed using an optical microscope. The respiratory rate of L. maculatus gradually decreased as the temperature dropped, with 16 ℃ being the dormancy temperature and 8 ℃ the critical temperature for L. maculatus. During the gradient cooling process (24–8 ℃), the malondialdehyde, lactate dehydrogenase, and creatine kinase contents significantly decreased at 16 ℃, whereas the catalase content significantly increased. The aspartate transaminase and alanine transaminase contents significantly increased at 8 ℃, and the triglyceride, total cholesterol, and high- and low-density lipoprotein cholesterol levels remained relatively stable during the later stages of cooling (16–8 ℃). After gradient cooling, the taste and quality of L. maculatus muscle improved; the degree of lipid peroxidation and cellular membrane damage was reduced, metabolic activity slowed down, and antioxidant capacity was enhanced at 16 ℃. After low temperature (8℃) stress, round vacuoles appeared in the cytoplasm of L. maculatus hepatocytes, which were irregularly arranged, and some nuclei were deviated or lysed. The gill filaments on the same side were neatly arranged, lengthened, and spaced, and a few epithelial cells of gill filaments were detached from the gill vesicles, and the number of chlorine-secreting cells was increased. Low-temperature stress causes changes in the structure of the liver and gill tissues of L. maculatus. This study serves as a reference for researching and applying gradient cooling technology in long-distance fish transportation and stress prevention.
