Yellowtail kingfish Seriola aureovittata farmed in offshore net cage response to high density by regulating protein and glucose metabolism.

We evaluated the impact of stocking density on the growth performance and physiological parameters of 1-year-old yellowtail kingfish (Seriola aureovittata) farmed in offshore net cages. A 90-day experiment was conducted with high (HSD, 7.41 ind/m), medium (MSD, 5.93 ind/m), and low (LSD, 4.44 ind/m) stocking densities. Results showed that weight gain rate (WGR) and specific growth rate (SGR) were decreased with increase in density. Serum adrenaline, cortisol, and glucose levels and SOD and LZM activities were all increased with increase in density, and there were significant differences between HSD and MSD. HSD upregulated immune-related gene mRNA levels and activated the cytokine-cytokine receptor interaction pathway. These findings suggest that the fish in HSD suffered from crowding stress and exhibited immune stress, and the fish accelerated protein catabolism and phosphoenolpyruvate metabolism through significantly increasing AST, ALT, and PK activities to obtain more energy and cope with this stress. Meantime, the fish in HSD and MSD significantly downregulated gck expression and upregulated g6pc2 expression in glycolysis/gluconeogenesis to support the elevation of serum glucose levels. The fish in MSD experienced endoplasmic reticulum (ER) stress, which ultimately triggered the ER-associated degradation to restore the normal physiological function of ER. Considering the non-significant difference in AST, ALT, LZM, and PK activities between MSD and LSD, it is speculated that the stress level in MSD is lower than that in HSD. Interestingly, growth-related gene mRNA levels and K value in MSD were higher than those in HSD and LSD, indicating that the fish in MSD have greater growth potential. In this study, an MSD (5.93 ind/m) was appropriate for 1-year-old yellowtail kingfish farmed in offshore net cages based on welfare and growth performance. These findings provide a technical foundation for the stocking regulation and healthy culture of yellowtail kingfish farmed in net cages.