OUR country is endowed with many inland waters in the form of rivers, swamps and lakes. These ecosystems provide economic benefits to us being a source of our food, our water for domestic consumption and irrigation and our use of these waterways for navigation. An example is the role of the Seven Lakes in San Pablo city in supporting the everyday lives of the lakeshore community.
Although not appreciated by the majority, inland waters perform other important ecological services such as flood control, bio-geological chemical cycling, groundwater recharge and waste treatment that support the everyday lives of the surrounding community. On the other hand, inland waters also become sinks of solid waste and effluents from domestic and industrial sources.
There are 422 principal river basins in the country with a drainage area that ranges from 41 to 25,649 square kilometers. The country is endowed with approximately 70 lakes covering an aggregate area of 200,000 hectares. According to the 2006 Integrated River Basin Management and Development Master Plan (IRBMDMP), twenty priority river basins have been identified for research and conservation. In 2010, Walpole pointed out that our inland waters, which includes peatlands and swamps such as the Agusan, Candaba, Liguasan, Sab-A and Buguey, are home to an assemblage of flora and fauna.
The classification of the rivers and lakes in the country has been anchored on the Philippine Clean Water Act (1994) and DENR DAO (Department Administrative Order) 34 using parameters such as dissolved oxygen (DO), pH, biological oxygen demand (BOD) and total coliform. DO is the amount of oxygen in the water to support life while BOD is a measure of the amount of oxygen used by microorganisms to decompose organic waste. pH is the basicity or acidity of the water vital for the survival of organisms.
The Laguna Lake is the biggest lake in the country whose outlet is the Pasig river which drains into the Manila Bay. This lake has several inlet rivers (which carry pollutants) that contribute to the poor water quality of the lake. Lake Taal, our third largest lake, is an ecotourism spot with a volcano island and small lake within it. Its outlet is Pansipit river which drains into Balayan Bay. These lakes are now primarily used for aquaculture.
These lakes used to be a common resource for all. Open fishing was the practice fifty years ago for both lakes where the fisherfolk in Laguna Lake had an annual fish catch of 80,000-82,000 tons in 1961-1964 and about 240,000 tons of harvest of shrimps and molluscs in the 1968 survey (Lacanilao, 2011).
With the projected growth of the Philippine population, aquaculture has been one of the innovations to feed the growing population. Tilapia was introduced into the country and reared in fishcages and pens in the lakes together with bangus and other fish. Investors found aquaculture to be a lucrative business. There was also an observed decline in the productivity and some of the fisherfolk became workers in the aquaculture industry.
Fish cages/pens should ideally occupy only 10 percent of the total area of the lake but the prescribed stocking density in cages or pens has been largely ignored. Overfeeding of caged tilapia and bangus became a practice on the perception that overstocked fish will grow faster and bigger. Unutilized feeds settled at the bottom of the lake decompose, using up the dissolved oxygen in the water. Solid wastes from inlet rivers also decompose which aggravates the problem of low dissolved oxygen.
Sediments taken from the West Bay part of Laguna Lake 15 years ago started to have the smell of a rotten egg or “burak” just like those found in dirty canals in Metro Manila. We have similar observations of sediments taken from the area near the fish cages in Taal lake and in Sampaloc lake in San Pablo City.
The phenomenon of overturn takes place when there is uniform distribution of temperature throughout the lake column that leads to the complete mixing of lake water which happens during the cold season. Overturn can bring to the surface water the unutilized nutrients that will enhance the growth of phytoplankton. This will also cause a total mixing of the lake water that can bring to the surface the bottom water with very low dissolved oxygen ( 0- 2 mg/L). This phenomenon can cause massive fish kills.
The seasonal overturn in deep lakes is normal and its impacts can be minimized with better regulation and solid waste management. Food security for an archipelagic country like us is vital, more so with the projected impacts of climate change. Aquaculture practice should be assessed including solid waste management of industry and community in the lakeshore area. There should be harmonization of policies of local government units and national agencies on the sustainability of aquaculture areas. These polices should regularly reviewed based on prevailing conditions and solid evidence.
Dr. Perez is a professor at the Ateneo de Manila Environmental Science Department. She is a long-time member of AGHAM-Advocates of Science and Technology for the People.
