MODELING AQUACULTURE CARRYING CAPACITY IN THE CIRATA RESERVOIR, JAVA, INDONESIA  

The Ciratum Reservoir in central West Java, Indonesia is a man-made reservoir constructed in 1988 for the use of hydropower.  It has a surface area of 6,200 ha in a watershed that includes two other man-made reservoirs: the upstream Saguling (surface area=5,607), and the downstream Jatiluhur (surface area=8,300 ha). To supplement farmers in the inundated areas, aquaculture was introduced. Of these three water bodies, the Cirata Reservoir is the most heavily polluted from an abundance of fish aquaculture cages, local agriculture and pollution from the heavily populated city of Bandung. Aquaculture has grown exponentially in the Cirata Reservoir, reaching nearly 56,000 cages, which has caused severely deteriorated water quality and periodic fish kills.  A mass balance approach is useful to determine the aquaculture carrying capacity (ACC) based on the amount of phosphorus [P] released to the environment through uneaten feed, and fecal loss based on estimates from feed quality and quantity, feed conversion ratios (FCR), and digestibility of the feed and fecal composition. The Beveridge model uses a modification of the Dillon & Rigler and Vollenweider models to determine phosphorus loading into a water body at environmental capacity.

Three Indonesian lakes and reservoirs were investigated as part of a larger project on modeling ACC. This section of the project investigated [P] levels in the Cirata Reservoir by modeling variations in FCRs, P content of feed, the use of floating feeds, the cultivation of water spinach and biomanipulation. With acceptable [P] modeled at 250 mg/m³, total permissible production for the Cirata Reservoir was estimated to be 400,000 metric tons per year. Data collected found FCRs to range from 1.3 to 4.7. If the FCRs are lowered to 1.2:1, the total permissible production (TPP) reaches nearly 500,000 metric tons per year. The average [P] content of the feed was estimated to be 17 kg/metric ton. The TPP at 10 kg/metric ton yields 600,000 metric tons. The use of floating feeds will allow farmers to better assess satiation, thus lowering the FCR and improving the TPP. Based on literature, the cultivation of water spinach has the ability to lower total phosphorus levels by up to 44% and can then be harvested for human consumption. Biomanipulation, or the deliberate addition or removal of a species in an ecosystem, has the ability to improve water quality in temperate lakes with the introduction of silver carp (Hypophthalmichthys molitrix), but should be preceded with caution in tropical lakes such as the Cirata Reservoir due to longer growing seasons and higher fish energy expenditures.