In recent years, the movement of raising tilapia for exporting has developed significantly, especially in freshwater fish farming areas in the northern provinces such as Bac Giang, Bac Ninh, Hai Duong, Quang Ninh, Thai Binh and Thanh Hoa, etc with the aim to diversify exports to international markets.
In order to develop raw material areas for export processing and open new directions for aquaculture in the North as well as develop sustainable tilapia farming, the Ministry of Agriculture and Rural Development has issued Decision No. 1639/QĐ-BNN-TCTS dated May 6, 2016 on approving the planning on development of tilapia farming until 2020. Besides, with orientation to 2030 with the goal of developing tilapia farming into a large, efficient and sustainable production industry to meet the need for both the domestic and international market. In Vietnam, people often raise this kind of fish in fresh and brackish ponds. Nile tilapia (Oreochromis niloticus), pink tilapia (Oreochromis sp) is mainly cultured in the northern and the southern provinces.
In recent years, aquaculture development in the northern provinces has contributed to improve people’s lives. However, it might cause some negative impacts on the environment that deteriorate the quality of water. That led to serious epidemics recently. The main reason behind is the degraded quality of the farm water, which reduces the self-cleaning ability of the pond. Moreover, the improper use of antibiotics could reduce species’ disease resistance. Thus, they are easily vulnerable with the environment changes.
Currently, the climate change situation such as El Nino and La Nina is more complicated so erratic weather conditions can have a big impact on aquatic species. Besides, pollution from sewage, industrial zones’ wastes, etc is dangerous for aquaculture and human health. As a result, it is urgent for authority to assess water environment quality in aquaculture areas and the effects of the aquaculture process on the environment.
Environmental monitoring plays an important role for the environmental management system: the monitoring process results in the data and environmental information that will be checked and evaluated by environmental managers. After that, it could give a rise of management measures, environmental management plans such as preventing and controlling environmental pollution and degradation issues
Aquaculture development should be associated with environmental monitoring to help farmers plan and take initiative in managing environmental factors and preventing diseases effectively as well. From 2015-2018, the General Headquarter of fisheries approved the plan of implementing environmental monitoring tasks. Through that, they assign the Research Institutes of Aquaculture I, II and III and the center for aquaculture testing, conservation and verification (3K) to coordinate with some localities the aim to implement the plan in aquaculture areas focusing on key farming objects (clam, brackish shrimp, lobster, catfish, tilapia) at North, Central and South. The monitoring results report is informed and advised promptly to local authorities and farming establishments (for monitored farms) via email, directly calling. As a result, people can limit the environment changes that affect the development of the cultured species (201 issued news reports for brackish shrimp, 70 news reports about catfish, 26 news about clam, 100 news about shrimp, 31 news about fish cages, 22 news about tilapia and giant freshwater shrimp).
The Directorate of Fisheries actively cooperates with Institutes I, II and III and related units to organize unexpected missions to farming areas where mass death phenomenon occurs to grasp the situation and organize sampling to identify causes, guide and support localities to restore production.
Environmental changes in farming areas
According to the environmental monitoring results in some northern areas for tilapia, caged fish showed that temperature and pH of cage fish culture area ranged from 24 - 31.5oC and 7.4 - 8.3, respectively. In particular, the temperature in the spring is the lowest (24oC) and the highest temperature in summer range from 27 - 31.5oC and the temperature in the autumn ranged from 25.5 - 27.7oC. Comparing to the allowable limits in QCVN 02-22: 2015/BTNMT, the temperature and pH recorded at the monitoring points are suitable for fish development.
COD content ranges from 5.6 to 39.6 mg/l, of which 82.86% of the samples exceed the permissible limit. However, according to ANZECC (2000), the COD content in freshwater aquaculture is below 40 mg/l, the COD content in the cage culture area does not affect the quality of fish culture environment.
For indicators of NH4-N and PO4-P content in fish cage culture areas in the river ranged from 0.00 - 0.10mg/l and 0.00 - 0.30mg/l, recorded at 28.57 % of NH4-N samples and 14.29% of PO4-Pv samples exceeded the permitted limit under QCVN02-22-MT: 2015/BTNMT.
The target value of NO2-N ranges from 0.01 to 0.87 mg/l, recording 80% of samples with NO2-N exceeding the permissible limit according to QCVN08-MT: 2015/BTNMT). However, according to Boy (1998), water quality for aquaculture with NO2-N content <0.5 mg/l is considered good and from 0.5 to 2 mg/l is considered average, then all samples cage fish farming area, tilapia are good and medium quality.
- NH3 and H2S content during the observation period ranged from 0.00 - 0.13mg/l and 0.00 - 0.06 mg/l, all of which have average values suitable to aquaculture environment (<0, mg/l).
The density of phytoplankton in tilapia ponds is high, ranging from 2.65x107 - 4.4x107 tb/l, in which density of phytoplankton belonging to the algae phylum dominates from 1.4x107 to 3.3x107 tb/l. Populations of algae thrive will cause night and early morning hypoxia when algae turn to respiration. This has warned farms to manage appropriate feed to avoid excess food because they are nutrition for the development of phytoplankton.
For the toxic algae species in the monitoring tilapia ponds, two species of Microcystis aeruginosa and Microcystis botry were recorded. Microcystis aeruginosa recorded over the limit of 1.5x106 tb/l in the collection periods of April, June and August. Especially, the Microcystis aeruginosa density in August exceeded 17 times.
The density of phytoplankton in the river is low, ranging from 3,4x102 - 2,5x104 tb/l, in which density of phytoplankton of Green algae is dominant from 2,3x102 - 1,4x104 tb/l and Khue algae is 1.4x102 - 1.1x104 tb/l. Thus, the phytoplankton population fails to affect the aquaculture water quality.
For toxic algae species in cage culture ponds, one species of Microcystis aeruginosa was recorded in the collected sample in August with a density of 2.2x103tb/l, lower than the limit of 1.5x106 tb/l so the impact is little.
Water samples for fish cage culture and tilapia have the level of density Streptococcus sp. lower than 103CFU/ml. Thus, Streptococcussp. fails to affect cultured fish.
Lê Mai