Davao Gulf has been found to be an important spawning and nursery ground for tuna, according to a study commissioned by the Philippine Fishery Resources Management Project of the Bureau of Fisheries and Aquatic Resources (BFAR).
The report said the gulf is rich in nutrients and its waters' physico-chemical properties are close to optimal levels. It said nutrients pour into the gulf from the Davao, Hijo, Kingking, Tagum, Madaum and Digos Rivers. The gulf scored high in nitrites, phosphates, and chlorophyll, which use sunlight to sustain plant life. Samples of its zooplankton (minute sea animals) biomass, taken during the southeast and northwest monsoons, showed values "higher than those found in highly productive inland areas like the Visayan Sea." The physico-chemical properties of its waters were also found at near-optimal levels.
Dissolved oxygen was normal although at times the water became oversaturated due to wave action; water temperature was optimally warm at an average of slightly over 23 degrees centigrade, conducive to primary production and high metabolic animal activity; and salinity, pH which measures acidity and alkalinity, and density of sea water were all within normal range and devoid of abnormal conditions.
Even the bad fishing days, pegged at 60 days a year, were found good for the gulf and the fish - although not too good for the gulf's 18,393 municipal and 866 commercial fishermen operating all sorts of hooks, nets and traps to catch some 22,000 metric tons of fish a year.
Monsoon winds blowing six to eight months annually result in prolonged bad sea conditions and, in extreme conditions, can whip up waves of as high as three meters.
But the bad weather acts as a natural stopgap to fish exploitation, the report said.
The dynamic interaction of the winds, plus the high rainfall that brings in nutrients and the currents and eddies that mix up the surface waters- â€these create the conditions that make Davao Gulf waters high productive,†it said.
And there's more good news. Because the gulf is very wide, the final report estimates that it covers 6,550 square kilometers or 655,000 hectares and very deep with depths exceeding one kilometer, thousands of fish larvae caught by the study showed that they belong to 74 families and 14 orders, mostly with commercial importance.
Plankton catch were also found to possess oceanic and deep water forms not normally present in coastal shallow waters. Larvae of deep-sea fishes harvested from the gulf showed 11 families. The challenge is how to keep it that way.
The final report was submitted by the University of the Philippines to the Visayas Foundation, Inc. after a 15-month Resource and Social Assessment of Davao Gulf.