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Thermal manipulation induces milkfish spawning during off-season in the Philippines

September 9, 2021  By Ruby Gonzalez

Monitoring water temperature in a heated tank.

Heated tanks hold the potential to ease the Philippines’ tight supply of locally-produced milkfish fry.

The Southeast Asian Fisheries Development Center Aquaculture Department (SEAFDEC/AQD) said milkfish broodstock normally spawn only from March to October when the weather and consequently, the water is warmer. Thermal manipulation, they said, induces broodstock to spawn during November to February, when the cool weather would normally deter these from being productive.

“Water temperature manipulation in tanks of milkfish breeders allows the fish to spawn during the cold and off-season. Water temperature control using the installed water heaters gave way to the increase in the water temperature making the breeders spawn,” Dr. Roger Edgar Mamauag, scientist and head of Technology Verification and Extension Division at SEAFDEC/AQD, told Hatchery International.

“Along with the expansion of hatcheries and improvement of milkfish breeders, this technology will further increase the production of milkfish in the Philippines and reduce the dependence of milkfish fry imports.”

The national milkfish industry requires at least 2.5 billion fry annually, according to the Philippine Milkfish Industry Group. Per DA-BFAR, only 1.1 billion is locally produced. Indonesia and Taiwan are major sources of imports.

The experiment used two 500-ton broodstock tanks. The treatment tank, (an average temperature of 29.9 C), had 76 broodstock and control, 119. From November 2019 to February 2020, approximately 23.6 million of good eggs were produced and 12.5 million hatched into larvae. Control had nil production.

Excess feeds and wastes were siphoned off and water was changed 20 per cent daily to maintain good water quality. The tanks were provided with a flow-through water system maintaining an eight-feet water depth. 

“The thermostat of water heaters installed at the water source were adjusted to the highest temperature setting at 35 C at the water source. Since the hatchery practices a flow-through system, water temperature subsides as it passes from the source to the broodstock tank through the canal-in,” he explained.

Mechanisms to prevent over-heating were in place. Temperature from the water source, canal-in and broodstock tank were monitored every two hours daily for calibration. Water heaters have a built-in thermostat to maintain the water temperature. When the temperature rises to its upper limit, the water heaters will automatically turn off. In contrast, when the temperature approaches its lower limit, the water heaters will turn on. 

The “promising innovation” was brought to the SEAFDEC/ AQD demonstration activity from November 2020 to January 2021. A 500-ton heated tank with 100 broodstock. Out of the 2.9 million good eggs produced almost 1.7 million larvae were hatched. 

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