Integrated optimization of common carp (Cyprinus carpio) biofloc culture using ANP–PROMETHEE II: effects of carbon source, C/N ratio, and stocking density

Abstract

This study investigated the individual and interactive effects of carbon source, carbon/nitrogen (C/N) ratio, and stocking density on the production performance, physiological status, tissue integrity, and economic efficiency of carp (Cyprinus carpio) reared in a biofloc-based system. The experiment included eight different groups in a 2 × 2 × 2 factorial configuration consisting of combinations of two carbon sources (starch and molasses), two C/N ratios (10 and 20), and two stocking densities (8 and 16 kg/m³). Water quality, growth and feed conversion performance, somatic indices, hematological and biochemical indicators, antioxidant enzyme activities, carcass proximate composition, intestinal histomorphometry, liver histopathology, and unit partial variable cost were evaluated. To comprehensively evaluate eight culture scenarios, the weights of 48 criteria were determined using the Analytic Network Process, and alternatives were ranked using the PROMETHEE II method. While basic water quality conditions were generally maintained among the groups, significant differences emerged in terms of growth, stress, antioxidant status, histological structure, and cost. The C/N 10 with molasses and 16 kg/m³ combination showed the highest growth performance and the lowest unit partial variable cost. In contrast, C/N 20 with starch and 16 kg/m³ combination showed the most unfavorable physiological profile, with low growth, poor feed conversion ratio, high cortisol, glucose, and MDA levels, and severe liver necrosis and vacuolization. While the interactions between stocking density and C/N ratio were particularly decisive in intestinal histomorphometry, mucosal fold development and mucosal structure were found to be more favorable in some combinations using molasses. In the ANP–PROMETHEE II analysis, the combination of molasses, C/N 20, and 8 kg/m³ ranked first with a net outranking flow of 0.1764, exhibiting both the highest positive and lowest negative positive and negative outranking flows. The results demonstrate that biofloc operating conditions should be selected not solely on the basis of growth or cost, but through a holistic evaluation of environmental, physiological, histological, nutritional, and economic indicators, and that the molasses, C/N 20, and 8 kg/m³ combination constitutes the most balanced operating protocol for common carp farming in biofloc technology.