A 16-week indoor culture trial was conducted to evaluate the effect of varying C:N ratio on growth performance, physico-chemical parameters, microbial dynamics, feed utilization, and immunological parameters. The experiment comprised of five biofloc treatment groups (with varying C:N ratio 5:1, 10:1, 15:1, 20:1) and a control with three replicates each, having 100 nos/m 3 as stocking density in 500 L tanks with constant aeration. The C:N ratios of the treatments were manipulated using molasses as an organic carbon source whereas there was no carbon source added in control. The water quality parameters monitored throughout the experiment were found to be within permissible limits in shrimp culture. At the end of the experiment, it was observed that there were significant differences between the treatment groups and the control regarding absolute growth, SGR, FCR, PER, and FER.Furthermore, a considerable difference in immunological parameters, namely, THC, phagocytosis, and PO activity (17.5 × 10 6 cells per ml, 43.5%, 0.112 Units min −1 mg min −1 ), was recorded among the treatments compared to that of the control groups (6.2 × 10 6 cells per ml, 31.5%, 0.051 Units min −1 mg min −1 ) respectively. Enhanced growth and survival with substantial disease resistance were recorded in C15 treatment. The results indicate that the CN15 ratio coupled with minimal water exchange is optimal for improved survival, growth, and immune activity. K E Y W O R D S C:N ratio, molasses, water quality, zero-water exchange 1 | INTRODUCTION Availability of specific pathogen-free shrimp has resulted in increased shrimp farming in terms of culture as well as production. The choice of Litopenaeus vannamei over Penaeus monodon is primarily due to enhanced production, SPF availability, higher yield after processing, and higher market demand. Of late, biofloc technology (BFT) has gained momentum and positive response in shrimp and tilapia farming. In India, farmed shrimp production increased from <1 lakh tonnes in 2009 to 3.5 lakh tonnes in 2014. In 2016-2017, the production was over 5 lakh tonnes, accounting for 38% in quantity and 64.5% in value (Rs. 24,426 crores) of the total Indian seafood export worth 5.78 billion dollars (Rs. 37,870 crores) (MPEDA, 2017). The expanding culture system influences water quality and environmental factors. Due to intensification with higher stocking densities, there isHighlights Growth, physico-chemical, and microbiological parameters were substantially higher in carbon and nitrogen (CN) ratio treatments compared to control. Optimization of C:N ratio in L. vannamei culture revealed an optimum ratio of 15 to be ideal for a biofloc-based system. Challenge study revealed higher mortality in control compared to CN-treated groups when challenged with the pathogen Vibrio parahaemolyticus (MTCC 451). Carbon supplementation appears to influence heterotrophic bacteria and provides immunity and protective response under BFT-based rearing. Immune responses like THC, phagocytic activity, and proPhenoloxidase activity we...
