Building Freshwater Prawn Ponds
Pond construction planning must take into account engineering requirements, the biological needs of the species that is being cultured, and the cost of construction. When exceptions are made in these areas, catastrophic results can occur. Some examples of pond failures include: dam failure due to improper compaction or soil type, rapid sedimentation and levee erosion when levee slope is to steep, and bankruptcy when pond construction costs are excessive for the potential return from aquaculture. Experts must be consulted when constructing a pond in order to minimize the risk associated with pond construction errors. The Soil Conservation Service, now the National Resource Conservation Service, has been the best source of this information in the past. Now, fewer NRCS engineers are available for assistance, but this agency is still a very good source of assistance. The Cooperative Extension Service can provide information about pond construction requirements for aquaculture. Many county extension agents have a great deal of knowledge of pond construction. The ultimate responsibility for pond construction decisions falls on the potential pond owner. In large pond projects, hiring a licensed engineer may be the best decision.
We are fortunate, in Georgia, to have several competent and ethical contractors who are currently building ponds for aquaculture. The ponds that have been constructed are built in an efficient manner and exceed the minimum engineering requirements without excessive cost. Soils on each site are considered and worked so that seepage is minimized. These contractors consider the requests of the land owner, counseling when the owner makes a poor construction decision.
Levee height and top width are variables that can improve the quality of the pond or add to the cost of pond construction. Minimum engineering requirements are recommended as follows: Top width at least 8 ft for levees less than 10 ft tall, freeboard of at least 1 ft on levee ponds (not watershed filled ponds) that are less than 656 ft long, levee slope of 3:1 (unstable soils require 4:1 to prevent excessive erosion) (Wheaton 1985). Levee-type ponds require 1,100 to 1,200 yards of dirt per acre when built on level land (Wellborn 1988a). Pond levee top width is recommended as 16 ft to allow heavy vehicle passage (Wellborn 1988b). A 3 ft increase in levee height must be added at the bottom of the levee, so that a change from 6 ft to 10 ft levee height adds at least 1,798 cubic yards of dirt per acre to the soil needs for building a pond. When amortized over a 10 year period, the added cost of this additional pond depth would amount to an additional $300 to $450 per acre per year in cost of operation. Similar relationships can be shown for increases in levee width or levee slope.
A source of electrical power is needed at each pond that will provide economical power to aerators used in prawn culture. Larger ponds may require a source of three-phase power in order to power larger aerator motors. It is common to provide one to four horsepower of aeration per acre of water.
Items Unique to Prawn Ponds
Prawn ponds are generally narrow and usually about 50 ft in width. They are also relatively small, a 0.5 to 1.0 acre size is common. The narrow width is justified to allow more efficient feed distribution to all areas of the pond. Boats must be used to distribute feed evenly over ponds that are wider.
Prawn ponds are also equipped with catch basins. The rule of thumb specifies a catch basin of 0.1 of the area of the entire pond. Side slopes of the catch basin follow the same rules as for the pond itself. Erosion and sedimentation will eventually fill catch basins, proper construction slows this process. The pond drain should be placed in the bottom of the basin. A four inch drain may be adequate for 0.25 acre ponds, but eight or 10 inch drains should be used for 0.5 and 1.0 acre ponds (Tidwell et al. 2002). Remember to allow for the depth of the basin and slope of the drain when laying out the pond elevations. Most catch basins will be constructed 12 to 18 inches below the elevation of the pond bottom. Since the water quality in the catch basin will be poor, a source of freshwater near the basin will help during harvest. Flushing the catch basin dilutes the nutrients and suspended material so that water quality can improve. External catch basins can be used if drains are large and can be flushed with fresh water to move the prawns though the drain.
The average depth of prawn ponds should be no less than 4 feet. Prawns do not like light. Shallow areas expose the prawns to predators. Light penetration into shallow water encourages aquatic weed growth that will make harvest difficult and also will reduce pond productivity.
The addition of structure to prawn ponds will increase prawn yield. Usually 50% of the surface area of the pond bottom is covered by structure. The plastic construction barricade material serves the purpose of structure by being durable and having wide openings to allow feed, shrimp, and water to pass through. Structure should be horizontally suspended so that it is about 12 inches off the pond bottom at the center of the pond. The ends of the structure should be anchored to the pond sides just below the water line. The center of the structure should be weighted to prevent floating. Structure that is exposed above the water line will encourage filamentous algae growth.
Proper pond construction methods must include a good site, adequate clay to reduce seepage, proper soil compaction, and economic design. Small narrow ponds with catch basins are appropriate for prawn production. Structure in the pond placed horizontally over at least 50% of the pond bottom will improve prawn yield.
Tidwell, J.H., S. Coyle, R.M. Durborow, S. Dasgupta, W.A. Wurts, F. Wynne, L.A. Bright, and A. van Aarnum. 2002. Prawn Manual. Kentucky State University, Frankfort, KY.
Wellborn, T. L. 1988a. Site selection of levee-type fish production ponds. SRAC Publication No. 100. Southern Regional Aquaculture Center, USDA. 2 pp.
Wellborn, T. L. 1988b. Construction of levee-type ponds for fish production. SRAC Publication No. 101. Southern Regional Aquaculture Center, USDA. 4 pp.
Wheaton, F. W. 1985. Aquaculture Engineering. Robert E. Krieger Publishing, Malabar, FL. pp. 414-462.