Growth optimisation and physiological performance of red-bellied pacu, Piaractus brachypomus (Cuvier, 1818) in...

Global freshwater scarcity, progressive salinisation, and increasing anthropogenic pressures on aquatic ecosystems pose major constraints to the expansion of freshwater aquaculture. Utilisation of marginal and low-salinity brackish water resources represents a sustainable alternative, provided that species-specific salinity tolerance is well understood. In this study, the salinity tolerance and long-term physiological adaptability of red-bellied pacu (Piaractus brachypomus) were evaluated. A 96-h minimum lethal salinity (MLS_96h) test showed no mortality at 10 ppt, while 100% mortality occurred at 15 ppt within 60 h and at 20 ppt within 12 h, identifying 15 ppt as the minimum lethal salinity. Based on these results, a 96-h acute toxicity bioassay was conducted across salinities of 11-15 ppt, including a control. Probit and logit analyses estimated the 96-h median lethal concentration (LC₅₀) at 12.54 ppt (95% confidence interval, 12.22-12.89 ppt). Subsequently, a 90-day growth and physiological performance trial was conducted using six treatments with triplicates: 0 (control), 2, 4, 6, 8, and 10 ppt. Growth performance and feed efficiency were significantly higher (p < 0.001) at 2 ppt, followed by the control, and declined progressively with increasing salinity. Survival remained 100% up to 8 ppt but decreased to 66.67% at 10 ppt. Haematological parameters (RBC, Hb, Hct) and serum protein levels peaked at 2 ppt, indicating effective osmoregulation, while higher salinities (> 6 ppt) induced physiological stress, immune activation, enhanced antioxidant enzyme activity, elevated LDH levels, and progressive gill tissue damage. Whole-body proximate composition and muscle whiteness index were also highest at 2 ppt (p < 0.001). Overall, P. brachypomus tolerates brackish water conditions; however, a salinity of 2 ppt is likely recommended as its optimal salinity, maintaining growth performance and mitigating physiological stress. A practical operational salinity range of 2-6 ppt may be recommended for commercial culture, demonstrating the species potential for sustainable aquaculture in low-salinity and marginal brackish water systems.