More than a hundred tons of fish bred by Quang Ninh farmers have died over the past ten days by heat and the Streptoccus sp. bacterium.
Most of the dead fish in Yen Duc, Hoang Que and Kim Son Communes of Dong Trieu District, where more than 140 hectares of fish farms have been affected as of Tuesday, were of the tilapia species.
Initial investigations showed that the fish were killed by Streptococcus sp. following a prolonged hot spell, local officials said.
According to a 2006 research by experts from the Department of Oceanography, Pukyong National University, Korea, "streptococcus sp. is gram-positive coccus that causes streptococcal infections in fish due to intensification of aquaculture and caused significant economic losses in fish farm industry."
The level of toxic gas nitrogen dioxide (NO2) in local ponds has increased beyond the permitted level and that of oxygen has dropped, the officials said.
Many farmers have lost their entire stock, they added.
Nguyen Trong Thuong, vice chairman of Yen Duc Commune which suffered the heaviest loss in the district, said 40.6 of 140.5 hectares of fish farms in the commune have been affected, and more than 113 tons of fish have dead.
Thuong said the bacterium is spreading.
Agriculture officials have instructed farmers to bury the dead fish and clean their ponds. They are also working to find farmers more money to start breeding fish again.