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This study examined the effect of stocking density on growth and survival of tilapia cultured in biofloc technology system. Three different stocking densities cultured in biofloc technology were 6 fish/m3, 8 fish/m3 and 10 fish/m3 for 86 days in triplicate for each treatment. The stocking density of the control lot was 3 fish/m3 cultured without biofloc technology. Initial stocking weight ranged from 2–3 g/fish. The water quality parameters were monitored and regulated in the suitable ranges for biofloc technology and for the growth and development of tilapia. The results showed that specific growth rate of fish cultured at a density of 6 fish/m3 was higher than that in the treatments of 8 fish/m3 and 10 fish/m3 with the average values of 5.72%; 5.62% and 5.43%, respectively, and the specific growth rate of fish in the control treatment was 5.71%. Daily growth rate of fish cultured at a density of 6 fish/m3 was higher than that cultured at densities of 8 fish/m3 and 10 fish/m3 with average values of 3.19 g/day, 2.98 g/day, and 2.55 g/day, respectively; and the daily growth rate of the control treatment was 3.27 g/day. Survival rate of tilapia cultured at densities of 6 fish/m3 and 8 fish/m3 was 100%, whereas survival rate of tilapia cultured at a density of 10 fish/m3 was 95.75%, and it was 88.9% for the control lot. The research results provide a scientific basis to propose tilapia culture technique in biofloc technology in brackish water, with the density of 6–8 fish/m3.

Building on a set of criteria used as a scientific basis for conservation zoning of natural marine resources is essential. In this study, we propose the set of criteria to select the suitable natural clam beds of Lutraria rhynchaena for protection priority from 16 sites in Cat Ba - Ha Long bay. Research results have built a set of 13 different criteria, including area, density, benthic substrate, food organisms, turbidity, water flow, water depth, predators, salinity, benthic organisms, pollution, area location and zoning conflicts. The criteria are divided into 5 groups, in which the area and density have the highest coefficent of 5, followed by the zoning conflict with the coefficient of 4, the pollution with the coefficient of 3, the efficient of benthic substrate and  water flow is 2, the remaining criteria have a same coefficient of 1. The highest points are evaluated at Tung Sau bed (station 12) with a score of 254 points, followed by Van Boi beach (station 7) with a score of 235 points, the bed is in Trinh Nu Cave (Station 13) with a score of 229 points, which are proposed to be prioritized sites for conservation.

Density and nitrification potential of indigenous microorganism in mangroves (Tien Yen - Quang Ninh and Bang La - Hai Phong) and seagrass (Ha Coi, Dam Ha - Quang Ninh and Tam Giang - Thua Thien Hue) in the north of Vietnam were evaluated through 4 sampling times in the dry and rainy seasons in the years of 2017-2019. The analytical results showed that the average density of nitrifying bacteria was 4.6 ± 1.8 × 102 MPN/ml, in which the density in mangroves tended to be higher than that in the seagrass beds (P < 0.05) in both the rainy and dry seasons. The average nitrifying rate was 2.7 ± 0.6 µgN/g wet soil/hour, in which the rate in mangroves tended to be higher than that in seagrass beds in the rainy season (P < 0.05). Nitrifying density and rate are not only correlated with substrate concentration but also with other environmental factors such as P-PO4, BOD5, total phosphate in sediment and total bacterial density.

The regulation effect of substrate concentration and sediment features on nitrification and nitrate reduction potentials was assessed based on study results at 6 sites with different sediment characteristics and aquaculturing options in two coastal culturing zones in Hai Phong city, in April and August 2013. Nitrification and nitrate reduction rates were measured in slurries of field samples and enrichment experiments using the acetylene inhibition techniques. Nitrification and nitrate reduction rates ranged from 1.5 - 8.6mgN/g wet weight-1h-1 và 11.0 - 54.0mgN/g wet weight-1h-1, respectively. Both of these processes depended on substrate concentrations and sampling sites. Substrate availability stimulated inorganic nitrogen transformation activity but only within retricted range, further increase in substrate availability inhibited the activities. The thresholds of substrate concentrations for nitrification and nitrate reduction were in ranges of 0.55 - 1.00 mgN/l and 1.53 - 2.82 mgN/l, respectively.