Massive Fishkill in Laguna Lake

(Photo from abs-cbnNEWS.com

After a long time, it was only last July 19,2010 right after typhoon Basyang did massive fishkill happened in Laguna Lake. Some 5,000 hectares of fishpen and fishcages were reportedly affected by this phenomenon.

Laguna Lake Development Authority (LLDA) chairman Ed Manda issued a statement saying, “fishkill happens every rainy season when the lake's oxygen level goes down due to pollution. The pollution is caused by wastewater and garbage thrown into the creeks and rivers that flow into Laguna Lake”.

My question is, if the lake is polluted why do low oxygen levels in the lake effecting fishkills happen only during the rainy season? Further, the strong wind brought about by typhoon Basyang should have brought turbulence to the water thus increasing the oxygen level in the lake. Obviously, this did not happen because LLDA said that low oxygen level caused the massive fishkill.

The phenomenon called upwelling where the ambient temperature suddenly drops down due to sudden rain causing the warm bottom water to surface could have been the culprit. Prior to the typhoon, the temperature has been very warm. Furthermore, the turbulence caused by the strong wind could have stirred the bottom water in Laguna Lake causing anoxia to the fish.

The oxygen level in the water diminishes with water depth. Below three meters depth, the oxygen level is zero. So, even without pollution, fishkill could happen due to this occurrence.

Of course, the situation has been aggravated by the pollution in the lake caused by the indiscriminate dumping of wastes by industries and people living along the shoreline of Laguna Lake and its tributaries.