Optimal Dietary Protein/Energy Ratio and Phosphorus Level on Water Quality and Output for a Hybrid Grouper (Epinephelus lanceolatus ♂ × Epinephelus fuscoguttatus ♀) Recirculating Aquaculture System

Abstract

In a recirculating aquaculture system (RAS), feed is critical to the growth of fish and is the main source of nutrient pollutants in aquaculture water. An eight-week feeding trial was conducted to investigate the role of feed on the growth efficiency of hybrid grouper (Epinephelus lanceolatus ♂ × Epinephelus fuscoguttatus ♀) and water quality in a RAS. Five commercial feeds with different respective dietary protein/energy (P/E) ratios and available phosphorus levels were selected (LNLP, 31.97 g/MJ, 0.96%; LNMP, 32.11 g/MJ, 1.54%; MNLP, 36.26 g/MJ, 0.98%; MNMP, 36.53 g/MJ, 1.58%; and HNP, 41.54 g/MJ, 1.97%). The results showed that HNP had the highest growth efficiency and MNLP provided the best economic benefit. The trend in water quality within 6 h after feeding was similar among the five groups. The relative concentrations of ammonia nitrogen, total nitrogen, active phosphate, and total phosphorus reached a maximum 2 h after feeding, and the relative concentration of nitrite reached a maximum 1 h after feeding. The high P/E ratio feed increased the concentrations of total ammonia nitrogen and nitrite nitrogen. The total ammonia nitrogen concentration in HNP was much higher than those in the other treatments. The dietary P/E ratio had no significant effect on total nitrogen concentration. High dietary phosphorus levels increased the total phosphorus concentration in the water, but no significant effect on the active phosphate concentration was observed. Considering the growth efficiency, economic benefit, and water quality, it can be concluded that MNLP is the most suitable feed for RAS breeding hybrid grouper. The results of this study supplement the gap on the effects of feed on RAS water quality and provide data support for the sustainable development of RAS industry.