Effects of stocking density on growth, serum parameters, antioxidant status, liver and intestine histology and gene expression of largemouth bass ( Micropterus salmoides ) farmed in the in‐pond raceway system

In China, the in-pond raceway system (IPRS) recirculating culture is a new model of freshwater aquaculture. A 120-day feeding study was carried out to evaluate the influence of stocking density on growth, digestive enzymes activity, serum parameters, antioxidant status, liver and intestine histology and gene expression of HSP70 and Cu/Zn-SOD of largemouth bass farmed in IPRS. Largemouth bass, Micropterus salmoides (initial body weight, 4.50 ± 0.23 g), were randomly farmed in nine raceways stocked at three stocking densities of 0.2 (LSD), 0.4 (MSD) and 0.6 (HSD) kg/m3, and sampled at 30, 60, 90 and 120 days. The results showed that body weight, specific growth rate and weight gain of fish farmed at MSD group were higher than that farmed at LSD and HSD treatments on 30 days (p < 0.05), while fish farmed at MSD and HSD treatments had significantly lower growth than those farmed at LSD groups on 90 and 120 days (p < 0.05). There were no significant differences in serum cortisol, glucose and triglyceride levels among three groups on 30, 60 and 90 days (p > 0.05), whereas fish farmed at HSD group had significant higher values than that farmed at LSD group on 120 day (p < 0.05). There were no significant differences in serum lysozyme level on day 30, while it decreased with increasing stocking densities on after day 60. Fish reared at LSD group had significantly higher SOD activities than those farmed at MSD and HSD treatments, and had significantly higher CAT activities than those farmed at HSD group on 120 day (p < 0.05). MDA level increased with increasing stocking density on days 60, 90 and 120 (p < 0.05). Fish farmed at HSD group had significantly higher liver HSP70 mRNA levels on days 90 and 120 (p < 0.05). Cu/Zn-SOD mRNA levels showed a downward trend. No significant difference was observed on liver and intestine histology (p > 0.05). In conclusion, largemouth bass farmed at 0.4 kg/m3 had better growth and health status before days 30. After that, fish farmed at higher stocking densities in IPRS would inhibit growth and reduce immune function.