'Upwelling' due to rain likely cause of Manila Bay fish kill — BFAR
MANILA, Philippines — Deep seawater rising to the surface, a phenomenon called upwelling, very likely caused the fish kill in the Baseco area of Manila Bay, the Bureau of Fisheries and Aquatic Resources said Saturday.
BFAR said this as its water quality testing found a low level of dissolved oxygen in samples collected in the area.
"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," BFAR said in a statement to reporters.
BFAR said that upwelling happens when deep, cold water wells up and replaces warmer water on the surface after rainfall.
"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 also said.
A 2009 article in Fish Physiology note that while habitats with low dissolved oxygen occur naturally, anthropogenic (or caused by human activity) activities "related primarily to organic and nutrient enrichment have led to increases in hypoxia and anoxia in both freshwater and marine systems."
READ: Manila asks BFAR: Probe bay fish kill
BFAR said that its testing found that dissolved oxygen in the samples was at 0.11 mg/L, much lower than the acceptable level of 5 mg/L for marine waters.
"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," it said.
A similar incident in October 2019 was also attributed to low levels of dissolved oxygen and "higher levels of ammonia and phosphates than the standard level." Massive fish kill in Taal Lake, where 605 metric tons of tilapia died, in June 2019 was also attributed to low levels of dissolved oxygen.
The BFAR statement on Saturday did not mention ammonia and phosphates levels.
The Department of the Environment and Natural Resources this week rejected assertions that its dumping of crushed dolomite along a portion of the Manila Baywalk for "beach nourishment" contributed to the fish kill, saying Baseco is far from where the "white sand" project is.
The department as well as the Palace said that the fish kill may have been caused by sabotage. Presidential spokesperson Harry Roque incorrectly said that tilapia being among the fish found dead was suspicious because it is a freshwater fish.
READ: Yes, Harry Roque, 'tilapia' can be found in Manila Bay
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