To be profitable, an aquaculture pond must be sited properly and designed for efficiency. An inaccessible location, leaks in the pond, poor seining conditions, or lack of good quality water will doom an aquaculture enterprise to failure.
Ponds do not have to be built on land as flat as the delta regions of western Mississippi, southeastern Arkansas or northeastern Louisiana. There are more than 20,000 acres of catfish production in slightly hilly areas of Mississippi, Tennessee, Alabama, Georgia and Illinois.
On rolling terrain the annual rainfall may be enough to completely fill and periodically recharge production ponds. Such watershed systems are not so dependent on groundwater.
They also act as flood control reservoirs and can greatly reduce erosion on land previously scarred with unsightly gullies.
Watershed ponds are constructed by building dams across valleys to form reservoirs that store rainwater (Fig. 1). Some aquaculture facilities have large reservoirs that, in turn, fill smaller ponds with captured runoff.
Watershed ponds can be cheaper to build than levee ponds on flat land and they don’t require expensive wells.
A disadvantage of deep watershed ponds is that they are prone to stratify. However, most aquaculture production ponds are routinely aerated and adequate aeration will break up daily stratification.
Water supplies:
Watershed ponds are usually filled by surface runoff from an area above the dam. This area, the watershed, can be estimated by drawing a line on a topographic map that follows the ridge lines forming the perimeter of the watershed (Fig. 2).
The watershed area and pond acreage can be roughly estimated using a planimeter. Field engineers with the Natural Resources Conservation Service (NRCS) should be able to help landowners with this calculation.
The entire watershed area of a proposed pond must be investigated to determine whether runoff might be polluted. Large chicken and hog farms, extensive areas of row crops, grazing livestock, industrial sites and other water quality hazards in the watershed could preclude the operation of a watershed pond.
Good watersheds contain well established, undisturbed vegetative cover such as timber or grass. A buffer zone of grass or sod should surround the pond, especially if there are croplands, concentrated feed lots, or large denuded areas in the watershed.
Springs or streams can be used as a water source. Large streams flowing through watersheds may require some kind of diversion device. Streams can be contaminated with wild fish or man made pollutants, so it’s a good idea to get the water tested before construction begins.
Also, large inflows of soft water in acid soils may hinder any long-term remedial effects of liming.
Proper site selection:
When choosing a pond site, consider the soil type, topography, characteristics of the watershed and, of course, safety. A power supply for aeration equipment must also be available.
Soil type:
The suitability of a pond site is mostly dependent on whether the soil will hold water when compacted by heavy machinery during the construction process. Soil composition ranges from pure sand to heavy clay. Soil type can vary drastically within a single site.
The pond area should contain a relatively impervious layer of clay or silty clay soils. Coarse soils containing large amounts of sand and/or gravel are unsuitable. If the soil can be formed into a tight ball that maintains its shape or is moldable, it is suitable for pond construction. A rule of thumb is that soil must contain at least 20 percent clay.
If there is poor soil over a portion of the pond, large amounts of clay may have to be trucked into make it impervious, and that could make construction costs too high. Also beware of limestone areas. They may have sinkholes or caverns just beneath the surface.
Ponds built in limestone areas have been known to drain completely overnight! The type of soil in the dam area is especially critical for safety reasons. Dam seepage can lead to dangerous dam failures.
The NRCS and some private agricultural and civil engineers have hydraulic probes that can sample soils several feet down. A backhoe swipe also will help in analyzing soil. Cutoff trenches and dam cores must be located where soil has a high clay content so they will remain structurally sound during the life of the pond.
Topography:
The objective is to locate a pond where the largest storage volume is obtained with the least amount of earth fill needed for the dam. Locating a dam between two gently sloped ridges in front of a broad section of valley is ideal.
The less pond excavation needed, the more feasible the site and the lower the construction costs.
Safety:
Do not locate large dams where their failure could cause loss of life, injury to people or livestock, damage to buildings, or the interruption of the use of railways, highways or public utilities. Aquaculture ponds are not as deep as public reservoirs and dam failures are rare. However, if the site involves some degree of risk to the public, a professional engineer with experience in dam construction should be consulted.
Almost every state has a dam safety law. Contact an Extension fisheries specialist or aquaculturist for more information. The location of buried pipelines, electrical lines or telephone wires should be researched before any construction begins. Breaking a fiber optic communication cable will put most pond construction budgets into the red!
Determining an adequate watershed area:
The amount and quality of water entering the pond from the surrounding watershed is dependent on several factors—slope, soil type, vegetative cover and the amount of precipitation. There are no set criteria for determining whether a watershed is sufficient for a given size pond, but there are some general rules.
Watersheds containing mostly pasture with heavy clay soils may supply 1 acre of water for every 5 acres of land. At the other extreme, timberland on sandy soil may require a ratio of 30 acres of land to 1 acre of water.
Excessively large watersheds can be just as problematic as limited watersheds. Too much water may dilute water amendments such as lime and salt, allow valuable fish to escape during floods, and make it necessary to install expensive flood or diversion devices. Ponds with excessive watersheds also may fill in faster with sediments, requiring frequent and costly renovations.
An undersized watershed may cause pond water to remain shallow, allowing weeds to get a foothold and preventing the use of emergency aeration devices when fish become stressed.
Authors:
Gregory N. Whitis
