AQUACULTURE CARRYING CAPACITY OF LAKE TOBA, INDONESIA

Lake Toba in northern Sumatra is the world's largest volcanic crater lake.  It has long had oligotrophic water and a history of tourism.  In recent years, aquaculture in the lake has expanded, leading to questions about the impact of aquaculture on water quality and therefore to an assessment of Aquaculture Carrying Capacity (ACC).   Aquaculture in the lake consists of a limited number of large farms whose operations have been certified for international markets plus an increasing number of small family farms producing fish for domestic consumption without certification.  In addition, land use around the lake is changing.  Water quality, at least in some parts of the lake, has changed from oligotrophic to mesotrophic.  Because of the large volume of the lake and the limited inflow and outflow, hydrologic residence time (HRT) of the lake is approximately 80 years; thus, any declines in water quality could take a long time to rectify.  Indonesian authorities arrived at widely varying estimates of ACC in 2014.  As part of a larger project on ACC in Indonesian lakes and reservoirs, we calculated ACC in Lake Toba using mass-balance modeling of phosphorus (P) based on the Dillon-Rigler model and a variety of potential scenarios.  A key element of this study was to calculate ACC for both the whole lake and for the aquaculture-intensive Haranggaol Bay. Because P inputs to the lake can come from land and aquaculture, we assessed watershed inputs via a land cover classification using satellite imagery and standard estimates of P loading for different land uses.  We then calculated ACC based on aquaculture only versus aquaculture plus watershed inputs.  

Of the several important variables in the mass-balance model, we chose to examine the effects of variations in three on ACC:  water depth, feed conversion ratio (FCR), and P content of feed.  Some variables like lake volume and HRT cannot be changed by farmers or regulators.  However, certain aquaculture practices, like siting aquaculture at certain water depths, improving feeding practices to optimize FCR, and optimizing P content of the feed can all be mandated by regulators and carried out by farmers.  With acceptable [P] set at 10 mg/m³, increasing lake depth from 75 to 225 m approximately tripled the ACC.  Reducing FCR from 2.0 to 1.2 approximately doubled the amount of aquaculture that could be conducted in the lake (current average FCR for all farms is about 1.7).  Reducing the P content of the feed from 2.6% to 1.1% similarly allowed a near tripling of the ACC.  In general, including watershed inputs reduced the ACC by about 40% compared to aquaculture alone with no watershed inputs.  Overall, to maintain oligotrophic water quality, ACC for all of Lake Toba should be about 35,000 tons of production per year, about half the current estimated production of 76,000 tons.  Haranggaol Bay should have about 52 tons of production, not the estimated 27,000 tons it is currently estimated to have.