The design and construction of ponds is very important if a fish farm is to operate properly. Ponds are earthen impoundments for holding aquatic species and have been used for thousands of years. Ponds can be holes in the ground (sunken pond), a dammed-off valley or stream bed (barrage pond), or constructed above ground (embankment pond).
Before constructing a pond, the questions in Annexure A should be answered. If a farmer can answer positively to the questions relevant to him, he will have a good chance of having a successful fish pond.
A number of factors need to be considered when
designing a successful pond –
- The type of soil;
- A reliable source of good-quality water (ideally gravity-fed and gravity drained);
- The size, type, number and shape of ponds;
- The species to be cultured and the stages of its lifecycle.
How these factors affect the choice of pond construction will be discussed next.
Depending on the desired use, ponds differ in their size, shape and layout. Ponds may be of any size or shape, although embankment
ponds are usually rectangular as they minimize the space between adjacent ponds by having
a common wall. Typically, the length to width ratio is 2-3:1. The advantages of small and large ponds are outlined in the box below.
Fish grow bigger in larger ponds even when the stocking densities are the same as in small ponds and the management of the ponds is identical.
This means that the weight of fish produced per hectare in a 0.5-ha pond may be almost double than that produced in a 0.1-ha pond. The reason for this is that large ponds have a larger surface
area and are more often subjected to wind action, which results in more oxygen entering the water and the water being mixed better.
Although large ponds are preferable, they
are more difficult to fill, drain, harvest and maintain. Therefore, the optimal size and shape of the pond will depend on the practicality and management available to make it large enough to grow fish but small enough to manage properly. The recommended maximum size for ponds for edible fish like tilapia or catfish is 1 ha. Quarter-hectare ponds (50 m x 50 m) are very effective and manageable in small-scale farms. For ornamental fish, ponds can be as small as 5 m x 5 m and only 0.5 m deep.
The design of the walls of the pond should be done with the help of an engineer. The wall design needs to consider the height and the slope of the wall. Because the pond is not filled to the top, the height of the wall must consider the desired depth of water plus the freeboard (the additional height above the water to the top of the wall). Ponds are generally between
0.8 m to 1.8 m deep as this –
- allows for light to penetrate the water thereby allowing the growth of plants and algae;
- reduces temperature fluctuations; and
- reduces the chances of thermal and oxygen layering of the
The penetration of light depends on the clarity of the water. Therefore, ponds with clean water can generally be deeper than those with dirty water. If plants are to be grown on the bottom of the pond, it should be shallow enough to allow for the penetration of light to the bottom.
Stratification (layering) occurs when the water is too deep and mixing cannot occur properly. This results in warm water on the top (heated by the sun) and cold water near the bottom. The levels of oxygen may also be high near the top and low (or even zero) near the bottom. It is obvious that low or zero oxygen levels are not good for the fish in the pond as many species prefer to live near the bottom and so this may result in large fish kills. Another problem that may occur when the oxygen level drops too low is that the bottom of the pond may start to rot. As it rots, it will release hydrogen sulfide (H2S) (which smells like rotten eggs), poisoning the water above it,
thereby killing all the fish. Therefore, it is not a good idea to build ponds deeper than 1.8 m unless sufficient mixing of the water through aerators or uplift pipes is used.
The soil in new ponds will settle by up to 10% depending on the soil type. Therefore, the wall should be built an extra 10% higher to account for soil settlement. The walls of embankment ponds need to be strong enough to hold the water. As it is expensive to move large amounts of earth, the dimensions of the walls should ensure the pond is strong enough without taking up unnecessary space. When building the walls, the dimension ratio should be 1:1 (vertical- horizontal) on the inside pond wall and 2:3 on the outside wall, as illustrated below. Erosion protection should be introduced (such as plants above and stones below the waterline). Grass should be planted on the outside embankment wall to reduce erosion. Ponds that are to be used for growing small fish should have a small wall or plastic (smooth) fence (50 cm high
and 10 cm buried) built all the way around
the pond to prevent predators from entering. Above this should be wire netting to keep out larger predators such as otters and leguaans. If platanna frogs get into the pond they will quickly eat many of the baby fish.
The top width of the wall should be wide enough to allow access along the length of the pond.
Depending on the size of the pond, vehicles may be required to drive around the edge, and equipment may need to be installed. The walls must therefore be wide and strong enough to carry the load.
The bottom of the pond should be cleared of any trees and bushes, which may snag on nets during harvesting. The surface should be smooth and graded at a slope of around 5% (5 cm vertical per 1 m horizontal). Channels can also be dug (30-50 cm wide, 10-20 cm deep) to help drain the pond when emptying it. The water can be channeled and collected in a harvest sump, usually about 10-20 cm deep with an area of around 1% of the pond.
The pond should be built to suit the requirements of the species to be cultured. For example, shallow ponds are better for grass carp as the growth of plants on the bottom will only occur if the light can reach the bottom of the pond. If ponds are used for holding cages of tilapia broodstock, they should also be shallow enough to allow the breeding cages to be easily staked into the soil. Ponds used for the later stages of carp or tilapia grow-out can be deeper (max 1.8 m) as this increases the amount of water available to grow fish, thereby increasing production.
If the pond is to be stocked with high densities of fish, it is important that additional aeration is provided as the dissolved oxygen levels are likely to drop below the minimum level required for the fish to survive. Aeration can be increased using paddlewheels, spray-bars or aerators.
If possible, these should operate all the time to maximize the amount of dissolved oxygen
available to the fish, although the critical time is usually in the early hours of the morning up till sunrise when oxygen levels are at their lowest.
|INFO BOX: HOW TO CALCULATE POND AREA IN HECTARES|
Example: 50 x 40 paces = 2 000 m2.
2 000 divided by 10 000 = 0.2 hectare