Investigation and research on the cultivation environment and health index of triploid Crassostrea gigas in the North Yellow Sea during summer

This study conducted three surveys in the floating raft cultivation area of triploid oysters in Wangjiadao Town (123.07ºE, 39.45ºN), Zhuanghe City, Liaoning Province, from July to August 2023, to elucidate the cultivation environment and health characteristics of the triploid Pacific oysters Crassostrea gigas in the North Yellow Sea during summer. The results revealed that the surface water temperature in the surveyed area ranged from 20.6 to 24.2 °C, posing high-temperature stress on C. gigas. The pH of surface seawater remained relatively stable, ranging from 7.70 to 7.83 across the three surveys. The average concentration of Chl.a in the water was 1.810 to 4.395 μg/L, which was lower than that reported in the same area for diploid oysters during the same period in 2021. The abundance levels of total bacteria in the water and in the gill tissues of triploid oysters ranged from 3.93×10⁷ to 2.89×10⁹ cells/L and 1.10×10⁷ to 1.64×10⁸ cells/L, respectively. The abundance levels of Vibrio species in the water and gill tissues of triploid oysters ranged from 8.40×10⁵ to 8.20×10⁶ cells/L and from 732.43 to 2.26×10⁴ cells/L, respectively, both of which were lower than those in the same period in 2021. During the three surveys, the glycogen content of the adductor muscle of triploid oysters increased gradually, with values of 6.41, 8.09, and 8.10 mg/g, respectively. The glucose content in the adductor muscle of triploid oysters was 4.72, 3.11, and 2.86 μmol/g, respectively, with no significant differences among samples (P > 0.05). The MDA content in the gills decreased significantly, with values of 17.40, 11.76, and 8.09 nmol/mg prot, respectively (P < 0.05). There was no sustained differential expression of cytokine mRNA in haemocytes. In summary, the water in the cultivation area of triploid oysters in the North Yellow Sea experienced high-temperature stress during the summer of 2023, with Chl.a concentration and Vibrio abundance in water and gill tissue lower than those in the same period in 2021. The glycogen storage level in the adductor muscle did not show a significant decrease, and there were no apparent abnormalities in the oxidative stress levels in the gill tissues and the expression of cytokines in haemocytes. These findings contribute to a comprehensive understanding of triploid oysters' health and environmental changes in summer cultivation, providing a theoretical basis for preventing and controlling large-scale mortality in summer and promoting the green and high-quality development of the shellfish industry.