Friday, September 07, 2007

Counteracting [phosphorus-induced] phytoplankton blooms with filter-feeding fish

Yang, S.L., 1982. Fish culture and reservoir management in the Republic of Singapore. In: Soewardi, K.Mahar, C. (eds.), Proceedings of the Seminar on Production and Exploitation of Open waters (with special emphasis on reservoirs), Bogor, 15-17 June, 1982. SEAMEO-BIOTROP, Bogor (Indonesia), 1987. Biotrop Special Publication (Indonesia) (ISSN 0125-975X), No. 27. Pp. 89-101.

Abstract - The primary function of impounding reservoirs in the Republic of Singapore is the collection and storage of rain water for the nation's water supply. The quality of reservoir water is a prime concern and the Public Utilities Board of the Republic has a responsibility to ensure that any activity in these reservoirs does not lead to deterioration of water supply. The objective of fish culture and reservoir management, apart from protection, is therefore to improve the quality of the raw water in these reservoirs.

A Biology Unit was established toward the end of 1970 to undertake these biological activities. The project on fish culture in reservoirs was initiated in 1972 at Seletar Reservoir. The feasibility of rearing the bighead carp (Aristichthys nobilis) in floating net-cages without supplemental feed was studied and the growth rates of the carp at different stocking rates were compared.

The stocking rate of 300 carp per net-cage of 6 m (length) x 4 m (width) x 5 m (depth) was found to have an optimum yield of about 680 kilograms at Seletar Reservoir. At this stocking rate, the carp attained an average weight of 2.0-3.0 kilograms during a grouping period of 12 months. A pilot project of 48 net-cages introduced at Seletar Reservoir in 1973 resulted in a harvest of 20.000 kilograms of carp in 1974. The project was then expanded at Seletar Reservoir and extended to other reservoirs. By the end of 1981, a total of 342 net-cages were operated in 4 reservoirs with a total harvest of 88.000 kilograms of carp.

The bighead carp which feeds solely on planktonic organisms in the reservoirs acts as a natural biological filter and thus helps to remove the excessive nutrients in the reservoirs. The carps, after growing big, are sold. Proceeds from the sales help to defray the cost of operating the Biology Unit.


I thought this was a pretty neat exercise. We used to gaze out to cages in Kranji Reservoir and hear about it and later tell students, so its nice to see this abstract; the NUS Science Library is digging this up and there might be a couple more papers that I hope to get. Now that I'm using it in class, I'd like to cite a paper, since its possible, rather then my memory!

It was not the only measure exercised to reduce eutrophication in our reservoirs, of course. Besides relocation of farms near rivers, the government even discouraged phosphorus-rich residential soap water runoff into drains as that eventually led to the catchment water. So I remember them removing backyard tap heads in the 1970's! The backyards of residential area were where pre-washing machine clothes were laundered, and the spent soap water was poured into drains. By removing the tap heads in backyards, families redesigned their backyard areas. From then on, waste soap water was directed into the sewage system instead.

Nowadays you can find biodegradable, phosphorus-free soap powder for sale.


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