Low dissolved oxygen threatens bangus in 2 P’sinan towns

BOLINAO, Pangasinan, Philippines – The chief of the Bureau of Fisheries and Aquatic Resources (BFAR) in Dagupan City has advised fishpond owners in the coastal towns of Anda and Bolinao to harvest their bangus (milkfish) early to prevent losses due to low dissolved oxygen based on the agency’s monitoring.

Dr. Westly Rosario told The STAR the other day that bangus growers must also watch out for the neap tide on May 24-26, saying this poses a serious threat to their produce because almost all fish kills in Pangasinan happen during this occurrence.

Bolinao and Anda towns produce big quantities of bangus, mostly sold in Metro Manila and other areas.

“So it’s better if they harvest early,” Rosario said.

During neap tide, there is no mechanical production of oxygen because there is no water flow and the current is calm, according to Rosario.

Neap tide occurs when the difference between high and low tide is least. It comes twice a month, in the first and third quarters of the moon.

Rosario said their monitoring last April 25 showed a problem on low dissolved oxygen at the bottom of the coastal waters in Anda and Bolinao with the bottom level being critical. The measurement invariably ranges from the lowest at 2.51 parts per million (ppm) and the highest at 3.88 ppm.

“It’s very consistent that there is low dissolved oxygen at the bottom. There is a possibility of fish kill in these areas,” he said.

The normal dissolved oxygen level for bangus survival is 5 ppm, and if it is below 3 ppm, this is stressful to the bangus, he said.

The BFAR was to conduct another monitoring yesterday.

Rosario said the low dissolved oxygen could be attributed to the organic waste deposited in these areas that is about 20 centimeters thick. It is deeper in some areas, reaching about a meter, as the waste has not been washed out, he added.

When the temperature is hot, there is fast decomposition of organic waste, leading to faster consumption of oxygen and competing with the oxygen consumption of bangus, he added.

Rosario advised bangus producers to lessen their feeds when there is low dissolved oxygen, and also reduce the density of their bangus stocks. He said artificial aeration could help.