Assessment of economic and ecological benefits for mesh enclosure isolated white shrimp-common mullet polyculture

In order to explore a new ecological culture mode for shrimps, we developed a white shrimp (Litopenaeus vannamei)-common mullet (Mugil cephalus) polyculture system in shrimp ponds. The stocking density of common mullets for each treatment was 0 (M0), 250 (M250), 500 (M500) and 800 (M800), respectively, in which M0 was the control without mesh enclosure inside the pond. The results show that the feed conversion ratios (FCR) of treatments with common mullets were all lower than the FCR of the control, but no significant difference was found among treatments (P>0.05). Treatment M250 had the highest shrimp production value (￥30.30×10⁴ yuan·hm^-2) and gross production value (￥31.02×10⁴ yuan·hm^-2), while treatment M800 had the lowest shrimp production value (￥20.96×10⁴ yuan·hm^-2) and gross production value (￥23.30×10⁴ yuan·hm^-2), though there were no significant differences among treatments (P>0.05). TOC value in treatments with more common mullets (M500 and M800) decreased by about 20 mg·L^-1 at final culture stage but changed very little in treatments M0 and M250, and no significant difference was found among treatments (P>0.05). Treatments with common mullets had higher concentration of NO₂-N than the control at final culture stage, and M250 had the highest NO₂-N and NH₄-N concentrations among all treatments. The results indicate that the polyculture system with fewer common mullets (M250) may obtain better shrimp production, while it is more feed-saving and has better water quality control with more fish stocking (M500 and M800). The economic and ecological benefits might be improved by stocking more fish at early culture stage and decreasing the fish stocking density at later culture stage with this mesh enclosure isolated white shrimp-common mullet co-culture system.