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.

This paper presents some main characteristics of the distributions of temperature, salinity, chlorophyll-a, and sound speed, which were measured in the Da Nang and Quy Nhon waters in July, 2010. The CTD was deployed for measuring profiles of temperature and salinity at stations. The data showed that in the Da Nang water, concentration of chlorophyll-a ranged from 0.07 mg/l to 1.52 mg/l and decreased toward the offshore region. In the Quy Nhon water, concentration of chlorophyll-a varied from 0.05 mg/l to 1.43 mg/l. This concentration was quite small and homogenous from the surface to 28 m depth. The speed of sound in seawater was maximum with value of 1,543.8 m/s in the Da Nang transect while the water layer with the sudden change of sound speed was from 5 m to 10 m deep in nearshore region (from stations D1 to D4), and from 10 to 15 m deep in offshore region (in D5 and D6 stations). Its minimum value in the Da Nang transect is just 1,014 m/s in the bottom. In the Quy Nhon transect, the maximum value of sound speed in seawater was 1,545.2 m/s and the minimum value was 1,515.2 m/s. The nearshore sound velocity reaches its maximum in the surface layer down to 30 m deep and the offshore one is similar down to about 40 m deep, under which, the sound velocity declines steadily.

Delft3D model employed to simulate the distribution and transport of suspended sediment and black carbon in Ha Long bay shows outcomes meeting with results from previous experiment studies. In the rainy season, suspended matter in surface layer is mainly in waters of western and southwestern Cat Ba island regions, and from Cua Luc toward the south nearshore areas with concentration of 50–130 g/m3. The concentration of suspended setdiment in the waters from Cua Luc to the north nearshore area is from 20 g/m3 to 50 g/m3 and that of offshore areas is 2–20 g/m3. In the dry season, the average concentrations of suspended matter are lower, approximately 110–150 g/m3 compared to the rainy season. In the rainy season, the total particulate carbon in surface layer is 0.0016–0.0028 kg/m3 and in the dry season, it ranges from 0.0001–0.005 kg/m3.

The Con Co Island plays an important role in the economic and security development strategy of Vietnam. The need for petroleum consume is more increasing in the world leading to the increase of petroleum transportation. Located near the petroleum transport line from Middle East to North East Asia, Con Co Island will be affected if oil spills happen. By using Delft3D model and nesting technique, some scenarios of oil spill (100 tons) in different wind directions in Con Co area are simulated. Calculating results show that, with the NorthEast wind case, the oil slick spreads in 9-14 days; in the case of southern wind, oil slick spreads in 6-9 days and the case of South Wets, oil slick spread in 12-14 days. The results are reference for marine management agencies and related sectors.

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