Interactive effects of stocking density and feeding rate in a biofloc system on growth, immune response, antioxidant status, and stress resilience of common carp (Cyprinus carpio)

Biofloc technology has emerged as a sustainable aquaculture strategy that improves water quality and enhances nutrient recycling, offering the possibility of reducing commercial feed inputs without compromising fish performance. Stocking density and feeding rate are two key factors shaping both microbial dynamics in biofloc systems and the physiological responses of cultured fish. This study evaluated the combined effects of these factors on Cyprinus carpio. Juvenile fish (21.55 ± 1.79 g) were reared for 60 days under five treatments: a clear-water control (10 kg m⁻³; fed 3 % of body weight per day) and four biofloc groups representing two stocking densities—low density (10 kg m⁻³) and high density (20 kg m⁻³)—each fed either 3 % or 2.1 % of body weight per day (LD-3 %, LD-2.1 %, HD-3 %, HD-2.1 %). The 2.1 % feeding rate represented a 30 % reduction in commercial feed input. Water quality remained within acceptable ranges across biofloc treatments. Fish fed 2.1 % of body weight per day exhibited improved feed conversion and protein efficiency ratios without reductions in growth or survival. The LD-2.1 % group showed the most favorable physiological responses, including higher digestive enzyme activities (protease, lipase), elevated glutathione peroxidase activity, enhanced immune status (lysozyme and alternative complement activity), and lower hepatic malondialdehyde concentrations. Prior to acute crowding stress, fish fed 2.1 % displayed lower baseline cortisol and glucose levels and recovered more rapidly after stress exposure. Overall, reducing the feeding rate by 30 % under biofloc conditions improved feed efficiency and physiological robustness, and this feeding reduction remained feasible even at higher stocking density.