VietNamNet Bridge - The authorities of Dong Nai province have announced that the low dissolved oxygen content in the water of Dong Nai River is the cause of the death of more than 200 tons of fish at fish farms in Tan Mai and Buu Hoa communes in Bien Hoa City, Dong Nai Province.

The Centre for Monitoring and Environmental Engineering of the Department of Natural Resources and Environment of Dong Nai said that a sudden change in the tidal regime limited the diffusion of oxygen in the water, leading to the decline of dissolved oxygen (DO) content.

The analysis shows that the concentration of dissolved oxygen in the water in early January ranged from about 1,51mg/l to 1.92 mg/l while the needed ratio is 4 mg/l. At the end of 2015, the DO content declined to about 2 mg/l. The average rate in December 2014 was 2,6mg/l and it reduced to 2,3mg/l in December 2015.

In the perspective of the Department of Natural Resources and Environment of Dong Nai, the density of fish cages is too high, affecting the living environment of fish. Also, fish breeders apply breeding techniques based on their experience and not in a certain mode.

According to the Department of Agriculture and Rural Development of Dong Nai, the density of fish cages in the area is very high, exceeding the planned density. There are a total of 272 fish cages owned by 215 households.

Breeders still use homemade feed processed from trash fish, snails, mussels, animal feed by-products... The spilled feed might decompose, polluting the water.

However, many fish breeders did not agree with the conclusion, saying that the main reason leading to the massive deaths of fish must be water pollution but the conclusion did not mention this.

They said there are dozens of pipes discharging waste water from factories to the river.

The Department of Agriculture and Rural Development has asked the Department of Natural Resources and Environment to strengthen monitoring to have measures to manage water quality in area.

The department also recommended the relevant units to strengthen inspection and strictly punish acts of environmental pollution, and inspect, supervise, and advise farmers to properly perform farming techniques to reduce the occurrence of similar phenomena.

As reported previously, on January 3-5, fish in many fish farms unexpectedly died. The total amount was up to more than 200 tons, causing heavy losses to farmers. By January 8 fish stopped dying.

Linh Nhat