Water Quality and Planktological Approach to Monitor Eutrophication by Cage-Culture of Red Tilapia (Oreochromis sp.) at the Sermo Reservoir, Yogyakarta, Indonesia

摘要

Water quality and plankton monitoring were conducted at the Sermo reservoir in Yogyakarta, Indonesia to characterize water quality, mostly on the relationships among phosphorus and nitrogen nutrients and plankton species, as a proposal to monitor theeutrophication of the reservoirs used for fish cage-culture. Monthly water samples were taken from six stations (September 1996 to February 1997). During monitoring, chemical and planktological analyses were applied and fish culture of red tilapia (Oreochromis sp.) was carried out for three months. Two units of floating net cages, 6 x 6 m~2 and 3 x 3 m~2 respectively, with a water depth of 2 m, were stocked with 40 to 50 gram size fish at the rate of 200 and 100 kg each. Commercial feed was given dailyat the rate of 3% of the total fish weight. Phosphate in water was 0.033 to 1.050 mg 1 that means within the categories of eutrophic, it has a positive relationship with the plankton density. Chlorophyta, Cyanophyta and Chrysophyta were identified as dominant species. Fish culture produced 1,082 and 486 kg on the average in each cage, a survival rate of 93 to 97% and feed conversion of 1,41 to 1 46.