Evaluation of the effects of different stocking densities on growth and stress responses of juvenile hybrid grouper ♀ Epinephelus fuscoguttatus × ♂ Epinephelus lanceolatus in recirculating aquaculture systems

This study was aimed at evaluating the physiological and metabolic responses of juvenile hybrid grouper ♀ Epinephelus fuscoguttatus × ♂ Epinephelus lanceolatus to stocking density. Hybrid grouper juveniles (mean ± SE = 25.43 ± 2.36 g live mass) were stocked for 22 weeks in a recirculating aquaculture system (RAS) under four different densities: low stocking density (LD; 1.03 kg m⁻³), medium stocking density (MD; 2.06 kg m⁻³), high stocking density (HD; 3.09 kg m⁻³) and extra-high stocking density (EHD; 4.11 kg m⁻³). Biometric variables were recorded and plasma, liver, intestine and stomach samples were taken for biochemical analysis at the end of the experimental period. Final stocking densities were 6.27, 16.04, 23.77 and 28.32 kg m⁻³, respectively, with significant differences in growth performance. Our results showed that the best growth rates and feed utilisation occurred in the MD group. Higher plasma cortisol and glucose levels and lower triglyceride levels reflected the stress responses in the EHD group. Moreover, the activity of aspartate and alanine transaminases was elevated in the HD and EHD groups due to enhanced gluconeogenesis. The activity of the digestive enzyme pepsin significantly increased in the MD group. We found that 2.06–3.09 kg m⁻³ is the most suitable starting density for culturing juvenile hybrid grouper in recirculating aquaculture systems.