The Bureau of Fisheries and Aquatic Resources (BFAR) said the recent fish deaths in Manila Bay may possibly be caused by an "oceanographic phenomenon".

"It is highly possible that an oceanographic phenomenon called upwelling or overturn caused the depletion of dissolved oxygen causing fish mortality of estimated 10 kilograms of fish on September 15, 2020 in Manila Bay, particularly in Baseco area," the BFAR said.

"Upwelling or overturn happens when deep, cold seawater rises or 'wells off' from below and replaces lighter and warmer water on the surface after a rainfall event," it added.

BFAR explained that "the upwelling water activity draws up sediments from the sea bottom bringing up organic load which competes with fishes in consuming available dissolved oxygen."

"When this happens, dissolved oxygen level which is important for fish to survive becomes depleted," it stated.

It can be recalled that the result of the water quality testing conducted by the BFAR in Baseco Area shows very low level of dissolved oxygen (DO) at 0.11 mg/L. It is lower than the acceptable level for marine waters which is 5 mg/L.

Among the fish that were found floating at the bay includes biya (goby), kanduli ( Manila sea catfish), asohus ( silver banded whiting), and tilapia.

BFAR said the "fish species affected were either bottom and midwater dwellers which easily succumbed to choking as a result of dissolved oxygen depletion." Robina Asido/DMS