THE fish mortality of an estimated 10 kilos of fish in Baseco Area, Manila Bay last Tuesday might have been caused by oceanographic phenomenon called upwelling or overturn that resulted to the depletion of dissolved oxygen.
This was diclosed Saturday by the Department of Agriculture-Bureau of Fisheries and Aquatic Resources after it conducted a water quality testing in the Baseco area.
Based on the test, the water quality indicated indicated very low level of dissolved oxygen (DO) at 0.11 mg/L. The acceptable level for marine waters is 5 mg/L.
It can be recalled that based on the report, residents found biya, kanduli, asohos, and tilapia gasping for air following a heavy rainfall. Fish species affected were either bottom and midwater dwellers which easily succumbed to choking as a result of dissolved oxygen depletion.
The DA-BFAR explained that 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.
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.