Effects of C/N Ratio for Bi‐Culture of Litopenaeus vannamei (Boone, 1931) and Macrobrachium rosenbergii (de Man, 1879) in a Molasses Based Biofloc System

Abstract

This study evaluated the effects of different C/N ratios (C10–N, C15–N, C20–N and C25–N, compared to C0–N as control) on growth, water quality, proximate composition, bacterial loads and total haemocyte counts (THCs) of shrimp ( Litopenaeus vannamei ) and prawn ( Macrobrachium rosenbergii ) post larvae in a biofloc‐based nursery bi‐culture system, aiming to optimize C/N ratios for sustainability and production efficiency over 6 weeks. Molasses was used as a carbon source, and water salinity was maintained at 15 ppt. In total, 15 cylindrical plastic tanks (125 L water‐holding capacity, 59 cm × 56 cm) were used and filled up each 100 L water for rearing 1500 PL of the two species (750 L . vannamei and 750  M . rosenbergii ). At the end of the experiment, significantly higher ( p  < 0.05) specific growth rate (SGR, % per day) (9.77) and survivability (95.33%) for L. vannamei were recorded in C10–N compared to the other treatments. For M. rosenbergii , the significantly higher ( p  < 0.05) SGR (% per day) (5.77) was recorded in C25–N, whereas the higher survival rate was found at C10–N (90.67%). The best feed conversion ratio was obtained at C10–N for L. vannamei (1.58), whereas for M. rosenbergii , it was obtained at C25–N (2.43). Significantly the higher ( p  < 0.05) THCs ( L. vannamei  = 40.63 × 10 5 and M. rosenbergii  = 31.25 × 10 5 ) of both species were obtained at C10–N. The experimental C–N ratios led to a significant increase ( p  < 0.05) in total microbial loads (116.6–355 × 10 9  CFU mL −1 ) compared to the control (18.40 × 10 6  CFU mL −1 ). Moreover, all tested water quality parameters remained within the optimal range across the treatments. The study concluded that a C:N ratio of 10:1 provides the highest economic return, making it the optimal ratio for biofloc‐based nursery bi‐culture of L. vannamei and M. rosenbergii .