56 SEPTEMBER 2017 • WORLD AQUACULTURE • WWW.WAS.ORG quality and quantity in the estuary was affected by rains, floods and dry season low flow. Water quality in ponds varied by season and was unpredictable. Algal blooms were sporadic and shrimp diseases common. During 1996-1997, the farm was seriously affected by White Spot Syndrome Virus with resultant mass mortalities. To minimize such effects, water exchange was reduced and water was contained, increasing water stability and improving water quality control. From 2010, the farm gradually converted to zero water exchange, using BFT principles and installing vertical substrates in the ponds. Aeration was increased and the pond dikes lined with HDPE sheets to minimize erosion. Suspended biofloc was practiced in a few ponds that were completely lined. Pond Management with Vertical Substrates Vertical substrates were made of black shade netting commonly used in horticulture and typified by 50 percent light transfer. Sheets were 50 cm wide and 20 m long (Fig. 2). Buoyancy and vertical positioning of each sheet were maintained by connecting empty plastic soft drink bottles to the upper edge of the shade cloth as floats and sand-filled bottles tied to the bottom edge as anchors. With this setup, nets were suspended vertically 10 cm below the water surface and about 50 cm above the pond bottom. Sheets of netting were placed in parallel lines, 30 cm apart, to form a battery (Fig. 3). A battery contained about 30 rows of nets and was about 10 m wide. A typical pond of 6,600 m2 had four such batteries adding a surface area of about 3,000 m2, accounting for both sides of the netting. A paddlewheel aerator was placed in front of each battery (Fig. 3) to ensure a significant flow of pond water along the lines of substrate, provide ample oxygen to attached organisms, and maximize the potential for particle interception by substrates. Similar to operation of BFT systems, addition of carbohydrate (molasses) was used to control ammonia uptake by heterotrophic bacteria on vertical substrates. Dilute molasses was introduced through a hose from a molasses tank to a point in front of the aerator (Fig. 3) to ensure proper mixing and dilution of the carbohydrate supply. The daily ration of molasses addition was controlled in accordance with results of monitoring pond ammonia concentration. In addition to the basic management procedures of zero water exchange and installing vertical substrate, other steps were taken. Efforts were made to improve the sludge drainage system, full pond lining was gradually installed, PL and feed quality were continuously improved, and biosecurity and Best Management Practices (BMPs) were followed. As a result, water quality was better and more stable, carrying capacity and production rates increased and infectious disease outbreaks were not common. FIGURE 2. Shade netting as vertical substrate, 50 cm x 20 m sheets. Note bottles used as floats in the background. FIGURE 3. A 30 m x 20 m battery of vertical substrates in front of a paddlewheel aerator. Note molasses tanks and hoses with outlets between paddlewheel and substrate battery.
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