Density-dependent effects on growth performance, feed utilisation, integrated health biomarkers, and economic efficiency of Amur carp, Cyprinus carpio haematopterus (Linnaeus, 1758) in inland cages for sustainable production

The present study evaluated the impact of different stocking densities (SD 10, SD 20, and SD 30 fish m −3 ) on the growth, survival, physiological responses, and economic returns in Amur carp ( Cyprinus carpio haematopterus ) reared in cages for 180 days. The juveniles of Amur carp (12.27 ± 0.31 cm; 35.6 ± 3.16 g) were stocked in galvanised iron cages in triplicate and fed with a floating pellet diet (28% CP, 4% CF) at a gradually reduced rate, from 5 to 3% of body weight per day during the culture period. Growth attributes, including final weight, weight gain, specific growth rate, and feed utilisation indices, declined significantly ( p < 0.05) with increasing stocking density. The survival rate was noticed to be lower at higher densities. Although biomass was higher at SD 30, physiological and biochemical analyses indicated crowding stress, with increased levels of serum glutamate oxaloacetate transaminase (SGOT), serum glutamate pyruvate transaminase (SGPT), glucose, cortisol, and antioxidant enzymes. Conversely, serum proteins, lipids, thyroid hormones, and immune markers were decreased with density. Poor digestive enzyme activities in terms of amylase and protease were noticed at higher densities, resulting in lower muscle protein content at SD 30. Haematological parameters like red blood cells (RBC), haemoglobin, haematocrit, and platelets increased with density, whilst white blood cells (WBC) remained unchanged. Water quality parameters remained within optimal ranges with minimal variation between inside and outside the cages. The calculated benefit–cost ratio was highest at SD 10 (1.38). Correlation analysis and Integrated Biomarker Response (IBR) indices confirmed density-induced stress, identifying 10 fish/m 3 as the optimal stocking density for Amur carp in tropical Indian reservoir cage culture. These findings support evidence-based management strategies that enhance fish welfare and farmer profitability whilst minimising ecological impacts, thereby promoting responsible aquaculture and advancing sustainable food production in line with global Sustainable Development Goals (SDGs).