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Study seeks to curb Taal fish kill losses

Thursday, April 23, 2015 02:05 AM    Views : 2246by:The Philippine Star
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Milkfish (bangus) pens are fixtures around Taal Lake in Talisay, Batangas.  A team of researchers from academe has determined an early warning system to minimize fish kill losses by predicting lake overturn. STAR/Val Rodriguez

MANILA, Philippines - Fish kill continues to be a bane to Taal Lake and the recurring phenomenon has far reaching impact on the area's ecosystems, according to a team of researchers.

"Development of an early warning system for fresh water fish kill in Taal Lake" is a Department of Science and Technology-funded research on the only caldera lake in the world, done by scientists and researchers from University of the Philippines Los Baņos, UP Visayas and Central Luzon State University.

"Some of the reported causes of fish kills include lake overturn, sulfur upwelling, sudden strong rain, shifting monsoon winds, parasitism by an isopod, volcanism, etc.," wrote Macrina Tamayo-Zafaralla, one of the authors of the research and professor emeritus of UP Los Baņos, in an article sent to The STAR entitled "Taal Lake fish kill and its ecological ramifications."

The research came up with scientific data and observations that shed light on ecological relationships of the phenomenon in the northern lake basin during the cool months December to January, Zafaralla of the UPLB Institute of Biological Sciences said.

She pointed out Taal Lake waters' yearlong cycle of stratification and destratification gives background on the mechanics of a fish kill. "Taal Lake is stable when the lake water forms horizontal layers, meaning it is stratified. The occurrence of layers prevents mixing from top to bottom, making the lake stable," she wrote.

According to Zafaralla, three layers of epilimnion, metalimnion and hypolimnion comprise a stable lake from top to bottom, each with its own temperature and chemical makeup.

Topmost is the epilimnion, which is warmest due to exposure to sunshine and also most aerated nearest the surface, with oxygen continuously replenished through algae photosynthesis. "A strong nutrient absorption by dense populations of phytoplankton make the epilimnion poor in nutrient content," the study said.

Mid-layer is the metalimnion or thermocline, narrower and where air cools abruptly and thus, denser and heavier. "Lighter materials like bits of debris float in it; heavier materials sink down through the sediments," Zafaralla in her article said.

The hypolimnion is at bottom, where sediments are deposited. This layer is coolest and most nutrient rich but oxygen poor. The layer is further broken down into the anoxic (oxygen content zero) and the hypoxic (oxygen less than 2mg/L).

"Anoxic conditions result when oxygen-requiring microorganisms of decay exhaust oxygen supply. Exposure to anoxic waters kill fish and other organisms that need oxygen to survive," the study said.

Decay in the sediments reduces oxygen to zero, releasing various substances some of which are toxic to fish, such as hydrogen sulfide gas and ammonia. In Taal Lake, which is about 198 meters deep, the researchers found that only a lake overturn or tremendous mixing "can bring toxicants, plant nutrients and water lacking in oxygen to the surface of the lake."

The study explained that the process of loss of stratification is called destratification, an unstable lake prone to mixing through blowing wind. This usually occurs during the cool months of December to February. The research was conducted from Dec. 19, 2013 to Feb. 28, 2014, or a period of around 70 days.

Thus when the amihan blows, fish pen owners should be on the lookout for a lake overturn, and decide whether to harvest early or move their fish pens to a less vulnerable location.

In a lake overturn, dissolved carbon dioxide erupts from deep lake water, choking wildlife, livestock and even humans.

"Fish cage operators save their fish from adversity through an early harvest or by towing away their cages to safer places about a couple or more kilometers away and out of the path of wind," Zafaralla in her article said.

The first sign of impending lake overturn is a slight drop in water temperature, Zafaralla said, in this case within a period of three weeks in the northern basin in Barangay Sampaloc. Further, there forms a zone of blue green water caused by sulfur rising to surface, or sulfur upwelling.

And while fish in cages and in the open lake die when blue green waters with poisonous sulfide traverse the area, dark green water is just as deadly because it is loaded with toxicants causing the death of fish en masse.

The dark green or blue green duration varies anywhere from an hour to three days, according to the study, adding that a gradual return to normal conditions occur with the re-stratification of the lake water at peak of summer.

Aside from sulfur compounds naturally present in a volcano lake, Taal Lake also has to cope with human activities such as aquaculture as well as wastewater effluents from the livestock industry.

The team of scientists has advised local government units of Talisay on some of the indicators to watch out for to mitigate effects of a lake overturn to give fish cage operators enough lead time to decide whether to harvest or tow. However, more research is needed to fine-tune the prediction system.

The research concluded that provisions of the Taal Volcano Protected Landscape Management Plan should come up with scientifically sound solutions to diminish sulfur upwelling, mitigate economic losses, initiate lake restoration measures, and preserve lake ecology based on sustainable levels.

Source: http://www.philstar.com/science-and-environment/2015/04/23/1446744/study-seeks-curb-taal-fish-kill-losses

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