48 DECEMBER 2016 • WORLD AQUACULTURE • WWW.WAS.ORG data; rather, it relies on physical data, knowledge of actual and specification of desired P levels, and information about the performance of fish being cultured (FCR) and the feed composition. Beveridge (1996) suggested levels of P that would be appropriate for various uses of a water body: 250 µg/l would be the maximum for tilapia culture; 50 µg/L the maximum for normal fisheries; and 10 µg/l the maximum for human activities such as tourism. We regarded these levels, then, as roughly corresponding to PCC, ECC, and SCC. Indonesian lakes and reservoirs have not been as well studied as have many in more developed countries of the Northern Hemisphere. In the absence of sufficient data to parameterize complex ecosystem models such as Ecopath, we chose to use the P mass balance approach as the most applicable means of calculating ACC for selected Indonesian lakes and reservoirs. These included: 1) Lake Toba in North Sumatra, the world’s largest volcanic crater lake, which has long been valued for tourism, but has seen increased aquaculture production in recent years, uniquely amongst our sample targeted at export as well as domestic markets; 2) Lake Cirata in West Java was constructed in the 1980s, primarily for hydropower, and has experienced increasing aquaculture since then; 3) Lake Jatigede, also in West Java, is under construction primarily for hydropower and flood control, but has not yet been filled, providing an opportunity to forecast ACC for the future reservoir; and, 4) Lake Jatiluhur in West Java, the largest man-made reservoir in the country and the first of which is subject to removal of fish cages by government decree due to eutrophication caused by cage aquaculture. Mass-balance models of P for ACC requires information on lake area, volume, turnover rate, feed conversion ratio (FCR) by fish, P content of fish and feed, and some estimate of the proportion of P lost to the sediment. One identifies the initial (background) level of P prior to aquaculture and the desired final level of P with aquaculture, then calculates the amount of P entering the water per kilogram of fish produced by aquaculture of a given species, and finally calculates the number of tons of fish that can be produced in that water body given its physical characteristics, especially depth and water turnover rate. Fish farmers cannot change the physical characteristics of the water body in any substantive way, but they can try, in cooperation with input suppliers, to optimize FCR and feed P content to minimize P losses to the water column. An additional strategy to limit P in the water might be to raise hydroponic vegetables to take up nutrients. In the case of Indonesia, if regulators demand the reduction of fish production by removal of fish cages, installation of hydroponic vegetable production might mitigate the loss of income to fish farmers and the eutrophication problems resulting from overproduction. We therefore sought to model ACC under varying levels of FCR and feed P content and also to investigate whether hydroponic vegetable production might serve as a mitigation strategy for Lake Cirata. Those efforts required an understanding of current practices and the willingness of stakeholders to change practices in the future. Participants from the four universities in the project met at IPB in June 2014 to organize our work and begin the project. Three teams were formed, one for each lake (Toba, Cirata, Jatigede). Teams consisted of members from participating universities so that students could learn to work with international partners and faculty, who were distributed among the teams as much as possible. Work proceeded in June and July of 2014; US and UK students then returned to their home institutions while Indonesian students developed theses based on biological data gathered from the lakes. All met again in June 2015 at IPB to review project status and to gather more information, including new information about Lake Jatiluhur. Participants finally met again at the Asia-Pacific Aquaculture 2016 conference in Surabaya, Indonesia to present results. Lake Toba Dr. Francis Murray (UoS) led the effort on Lake Toba with a team consisting of Dr. Agus Samosir (IPB) and graduate students Dimitar Taskov (UoS), Irina Timonina (UoS), Joshua Oakley (URI) and Arif Rahman (IPB). Dimitar and Irina studied the aquaculture industry, markets and economics of the fish culture industry in Toba. They found large farms owned by international interests that export tilapia as a third-party certified product to international markets, along with many small family-owned farms that sell non-certified product, primarily in North Sumatra. The extreme depth of this natural water body (up to 505 m) limited unregulated encroachment of small-scale farms to shallower and less exposed embayments. This was in sharp contrast to the far more extensive spread of such farms in the shallower man-made reservoirs of West Java that were also part of the study. Total aquaculture production in Lake Toba was estimated to be about 79,000 t, with about 47,000 t from small farms (about 27,000 t from Haranggaol Bay alone) and about 32,000 t from large farms. FCR ranged from about 1.4 to 1.6 for large farms and about 1.7 for small farms; however, FCR at small farms ranges from about 1.1 to about 3.3, so there is substantial variation, probably due to feeding differences associated with tilapia-carp polyculture stocking practices and cage configurations. Farmers in one of the most intensively farmed embayments adjacent to Haranggaol village remarked that water quality seems to have deteriorated over the last ten years and it now takes 6-7 months to grow fish to market size compared to 3-4 months ten years ago. Previous efforts have been made by three agencies of the national and provincial governments to estimate ACC in Lake Toba but the estimates varied substantially. One problem with Lake Toba is that the lake is very large and, because aquaculture production is concentrated in a few specific areas, an ACC calculated for the whole lake may not be appropriate or useful. Josh Oakley analyzed the input data for the previous models to develop new estimates of ACC Densely crowded fish cages in Jatiluhur Reservoir.
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