Effects of two commercial feeds for semi-intensive and hyper-intensive culture and four C/N ratios on water quality and performance of Litopenaeus vannamei juveniles at high density in biofloc-based, zero-exchange outdoor tanks

A two-factor experiment was conducted to evaluate the effects of two commercial shrimp feeds and four different C/N ratios on biofloc development, water quality, growth performance, feed utilization and input cost in an outdoor tank system stocked with Litopenaeus vannamei juveniles under high-density and zero-exchange conditions. The two feeds used in the study were: a less expensive feed (US$0.99 kg−1) formulated for semi-intensive production systems and a more expensive one (US$1.75 kg−1) designed for hyper-intensive systems. Organic carbon (molasses) was added daily to provide calculated carbon to nitrogen (C/N) ratios of 12:1, 15:1, and 18:1 based on the carbon-nitrogen content of the feed and the carbon content of the molasses throughout the experiment. No molasses was added in the control group which had a C/N ratio of 9:1. Each treatment had four randomly assigned replicate tanks, and each tank filled with 500 L of biofloc-rich water. Juvenile shrimp (2.21 ± 0.11 g) were stocked into tanks at 150 shrimp tank−1 (300 juveniles m−3) and cultured for six weeks. The results showed that both feed and C/N ratio had significant effects on biofloc development, water quality, shrimp performance, feed utilization and input cost. Initiating the culture with biofloc-rich water and then adding a small amount of organic carbon regularly could effectively sustain the continued development of mixotrophic bioflocs in zero-exchange high-density shrimp culture systems. The best water quality, shrimp performance, and FCR results were achieved with the C/N ratio of 12:1 for both feeds. Analysis of feed and variable costs suggest improved economic benefits when using the less expensive feed of SI-35 under the conditions of this study. Furthermore, input costs could be decreased using a C/N ratio of 12:1 due mainly to reduced molasses and NaHCO3 usage.