Effects of stocking density of gray mullet Mugil cephalus on water quality, growth performance, nutrient conversion rate, and microbial community structure in the white shrimp Litopenaeus vannamei integrated system

An experiment (75 days) was conducted in twelve fiberglass tanks (5 m(3)) to study the effects of stocking density of gray mullet (Mugil cephalus) on water quality, growth performance, nutrient conversion rate, and microbial community structure in the white shrimp (Litopenaeus vannamei) integrated system. Gray mullet (body weight of 1.50 g) were added into nine shrimp tanks with three different densities, namely at 10, 20, and 30% of the initial shrimp biomass as poly-culture treatments. White shrimp (size of 0.50 g) were stocked at a density of 300 shrimp.tank(-1). Three tanks stocked only with shrimp served as monoculture treatment (control treatment). Shrimp were fed four times a day using 38%, 35% protein (feed #1, #2, respectively) commercial pellet (Grobest Co. Ltd. Vietnam) with feeding rate was 3 to 10% of shrimp biomass. No external feed was given to the fish during the experiment. The results showed that the mean values of water quality parameters were not significantly different (p > 0.05) between treatments, except TSS, TAN, NO2-N concentration. The best shrimp growth performance, total biomass, and FCR (0.14 g.day(-), 2928.68 g.tank(-), 1.06 respectively) were found in the low gray mullet density treatment (10% of the initial shrimp biomass), which were significantly better than those in control and other poly-culture treatments. The nutrient conversion rate in poly-culture, which retained 30.41-40.32% of total nitrogen feed input and 25.26-33.54% of total phosphorous feed input, was significantly different between treatments. DGGE bacterial profile from the cultured water illustrated a dynamic microbial community structure in the water column of the experimental tanks, and a considerable difference in community structure between treatments. The phylogeny of the selected band sequences obtained from the DGGE gel was affiliated within the (gamma)- Proteobacteria, alpha- Proteobacteria, Bacterioidetes, Planctomycetes, and Terrabacteria group. The results indicate that gray mullet can be co-cultured with white shrimp at the fish stocking density of 10% shrimp biomass to improve water quality, overall productivity and nutrient utilization efficiency