OpinionsEnvironment ConnectionFish mortality in Taal Lake

Fish mortality in Taal Lake

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Television reports yesterday (January 30, 2019) showed dead fish from Taal Lake. The announcer explained that the Bureau of Fisheries and Aquatic Resources (BFAR) stated that the death of fishes was caused by the “upwelling of sulphur” in the lake. But this explanation is definitely incomplete because it did not include the physical cause of this “upwelling.”

I would like to provide the basic scientific reason for the mortality of fishes that were killed by the sulphur compounds coming from the deep portions of Taal Lake.

In tropical regions, the water in lakes and oceans during most of the year is stratified into a shallow warm layer and a deeper cool layer, and there is a lack of circulation between the two layers. When temperatures of the upper layer become cooler (e.g. 9-10 degrees C.) during the northern winter months of December, January and February, the upper layer becomes more dense and heavier, and as a result it sinks, replacing the deeper layer that holds the toxic sulphur compounds. The deeper layer then rises to the surface and kills the fish. This phenomenon is termed “lake over-turn.”

Water has a peculiar characteristic. It is densest and heaviest at about 4 degrees C. and lighter at 0 degree and at other higher temperatures. That is why it sinks at 4 degrees C. and it floats as ice at 0 degree and at, say, 20-30 degrees C.

Fish kills in lakes such as Sampaloc Lake in Laguna occurred in the late 1980s, resulting in mass mortality of tilapia. Lake over-turn can be expected when temperatures become lower at certain months.

What BFAR should do is to make use of temperature forecasts to warn fishers to harvest their fish during periods of low temperatures, such as 9-10 degrees C., during the winter months.

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Author’s email: [email protected]

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