Site evaluation is not only undertaken to determine if a site is suitable for shrimp farming. It is also valuable in determining what modifications are needed concerning layout, engineering, and management practices to make shrimp farming possible at a given site. No site will have all the desirable characteristics, so a number of judgements have to be made for every site. First, can shrimp be farmed profitably? Second, what is the most appropriate type of management? Third, how must the pond system be constructed for that type of management in that location? The material presented below is designed to help in the decision-making process.
This is an elusive term which is difficult to define, but it includes all the physico-chemical and microbiological characteristics of the water. Certainly pH is an important aspect, and pH of water on, or adjacent to, the pond site should be within the range of 7.8 to 8.3. Water with a good growth of phytoplankton can usually be considered productive. The sedimentation characteristics of the water are important. If the water carries an excessive amount of sediments a sediment trap may have to be built into the water supply system. The amount of dissolved oxygen present near the bottom of the source of water to be used should be determined.
The normal salinity of water during high tide at different seasons of the year should be known. Especially important for rivers and canals is the subsurface intrusion of salt water under the fresh- water. The depth of the top of the wedge at different tidal stages during normal weather should be ascertained. Also important is whether or not the tidal wedge persists during floods. The frequency of floods should be known. Just as important is the duration of freshwater conditions during flooding.
The tidal characteristics in relation to land elevation at the proposed site should be determined. This is critical to determine if; tidal flow or pumping will be used to fill the ponds, the elevation of the pond bottom, dike height, etc. In general, places where the tidal fluctuation is moderate, between 2 and 3 m, are most suitable for fish farms using tidal flow to fill the ponds. Places where tidal fluctuations are large, over 4 m, are not suitable sites for tidal ponds because very large and expensive dikes would be required to prevent flooding during high tide. Also it would be more difficult to hold water in the ponds during low tides since due to the higher pressure, water loss and erosion from seepage, crab holes, etc. would be greater. Areas with slight tidal fluctuation, 1 m or less, are also unsuitable for tidal ponds, because the ponds could not be filled or drained properly. So, if ponds are to be constructed in areas where the tide is less than 2 m or more than 3 m, the use of pumps should be considered (Jamandre and Rabanal, 1976).
Actual measurements should be made at the pond site to determine high and low tide bench marks. One must keep in mind that tidal fluctuation is much less at certain times of the year than at others. Tide tables should be consulted to determine this.
Highest tides during past floods and storms should be known. Sometimes the only way to acquire this information is from local residents. Wave action during normal tides, storms and monsoons should be known.
A knowledge of currents is important for planning erosion control measures to protect the dikes and main gate as well as to determine the probability of sediment deposition in water control structures. Shifting mud or sand can block water supply canals or sluice gates, making effective water management impossible. As it is seldom practical to conduct surveys, one should ask local people if shifting sand or mud has ever been built up in areas near the pond site. Take into account changing wind and current patterns at different times of the year.
Important factors in the immediate area are maximum daily rainfall and annual distribution. The area of watershed and runoff in relation to the pond site should be looked into.
If evaporation is high, determine if there is an adequate supply of freshwater with which to dilute the pond water to maintain proper salinity.
If the site is near a river, determine if harmful substances are used or released upstream. These would include such things as pesticides for agriculture and malaria control, mining wastes, industrial and urban wastes. Are materials discharged continously or only once in a while? Try and anticipate future pollution problems. Do not locate near a city that is growing rapidly or an area that is designated as a future industrial estate. Consult with local government planning officials to investigate these aspects.
In new areas where ponds are to be constructed for the first time, soil samples should be taken at ten random locations per hectare. Soil core samples should be taken at least to a depth of 0.5 m below the proposed pond bottom. This is because good soil might overlay unsuitable soil and a surface sample would not be sufficient.
In existing ponds, it is recommended that 12 samples be collected from ponds of 1 ha or less, and 25 samples from ponds of 2 to 20 ha. In ponds samples need only be taken from the top 5 cm. A 100 ml portion of each soil sample should be placed in a plastic bucket to give one composite sample per pond. The composite sample should be mixed thoroughly.
The soil samples are then taken to a soil-testing laboratory for analysis.
Many coastal soils are high in peat or sand content and will not hold water. The potential pond soils must have a high enough clay content to assure that the pond will hold water. A good field test to use in determining this is to shape a handful of moist soil into a ball, if the ball remains intact and does not crumble after considerable handling, there is enough clay in the soil to provide a water tight seal (Perry, 1972). Sandy clay or sandy loam is best for dike construction, because it is hard and does not crack when dry. Peaty soil is not a good dike material as it settles too much and may even burn when dried (Denila, 1976).
An excellent discussion of the effect acid soils have on brackishwater ponds is given by Potter (1976), a summary of which follows:
The fact that many newly-constructed ponds are reported to give poor production is generally attributed to low fertility of the soil, but acid soils may be the cause in many cases. Due to the conditions under which some coastal soils are formed, iron pyrites often accumulate. As long as these pyrite-containing soils remain submerged, they are subject to little change. When the land is drained to make fishponds the pyrites become oxidized producing sulfuric acid which cause the soil pH to become extremely low. Low soil pH can result in lowered pH of the pond water either by leaching from the pond bottom or by runoff of rainwater from the dikes during heavy storms.
The sulfuric acid formed when pyrite oxidizes not only affects pH of the pond water, it also affects soil minerals, releasing iron and aluminum which can bind up phosphates and other essential algal nutrients. This lowers the natural productivity of a pond and makes fertilization ineffective. The resulting lack of natural food causes slow growth.
Dikes made from acid sulfate soil develop vegetative cover very slowly, thus they are subject to severe erosion. This requires added maintenance, both to repair the dikes and to remove sediments from the pond. In addition, as the dikes are subject to oxidation, sulfuric acid and active aluminum and iron may be washed into the pond with eroded soil creating water quality problems.
When the pyrite containing soil becomes highly acidic after oxidation it is called an acid sulfate soil. A soil which will become acidic upon oxidation is called a potential acid sulfate soil.
Acid sulfate soils can be identified easily by taking a soil pH. Their pH is 4.0 or less and mottles of the pale yellow mineral jarosite are usually abundant. In drained areas, an acid sulfate soil condition is characterized by a red colouration on the soil surface.
Potential acid sulfate soils are much more difficult to determine, because they do not become acidic until after oxidation. The soil can be acidified by exposure to air, but the extent and rate of the acidification process are regulated by chemotrophic bacteria. Bacterial activity is low in dry soil, so it is best if the soil is kept moist. To do this, a soil sample is made into 1 cm thick cake and sealed in a thin plastic bag. The bag preserves the soil moisture and, if thin, is permeable enough to allow oxidation of the pyrite to proceed rapidly. The pH of the soil should be reduced to below 4.0 within one month if it is potential acid sulfate.
Considering the many problems associated with acid sulfate soils, a detailed soil survey is well advised before construction is started to develop brackishwater ponds. For determination of the amount of lime which will be required to improve an acid sulfate soil, see Section 12.6.
See Section 6.5 for suggestions for methods of construction in acid soil areas and Section 8.2.2 for management procedures.
A knowledge of the rate of percolation of the soil will help in determining the extent of water loss through the pond bottom or dikes and can affect both design and management. If, for instance, a portion of the soil is good clay, it may be better to use this for the puddle trench and/or centre core of the dike. If percolation can occur through the dike, and the dike soil is acid or potentially acid, it would be best to plan on having a positive water head in the pond to prevent acid from being washed into the pond by seepage through the dike.
If the subsoil is unsuitable for the dikes, it may be better to construct the dikes of topsoil, or the poor subsoil can be used for the core of the dikes and the outer surface can be covered with topsoil. If the subsoil is highly acidic, it might be better to leave it undisturbed, reducing the amount of excavation, and filling the pond by pumping instead of tidal flow.
This is especially important if heavy equipment is to be used. It also will help determine the number of pilings required under the gates, and the need for special foundations under dikes.
Determine if fry are available from hatcheries or dealers who obtain stock from the wild. If fry are not to be purchased, the local resource must be assessed to determine the species present and their seasonality of abundance.
The predominant pests vary from area to area and the kind present in a given area may have an effect on management, construction or cost estimates (see Section 9).
Find out if these organisms are a problem in the area, if possible the extent of damage caused. The best way to determine what group causes the damage is to search out and examine old pieces of wood stuck in the ground, or the wooden boats of local people. This information can affect the decision as to what type of material to use for sluice gate construction (See Section 9).
The type of vegetation growing in the area can be an indicator of elevation and soil type. Following is a listing of some types of mangrove and the tidal zone they are usually associated with (Zinke, 1975).
|Medium high tide||-||Avicennia, Sonneratia|
|High daily (normal tide)||-||Rhizophora, Ceriops|
|Spring high tide||-||Lumnitzera, Acrostichum|
|Abnormal high tide||-||Melaleuca, Phoenix|
Mangrove with growths of Avicennia have good soil and fishponds built on them are generally productive. Rhizophora, Bruguiera, Sonneratia acida and most other mangroves with the same type of extensive, above ground, root system usually occur on acid soils which are less suitable for fish-ponds (Padlan, personal communication, 1977). Nipa and other trees with a high tanin content have a long lasting effect on ponds, causing low pH (Jamandre and Rabanal, 1976).
The number of trees and the size of their stumps and root systems is an important factor in the cost of land clearing and excavation.
Land cost should be determined so that economic viability of the project can be evaluated.
Accessibility is important for the transport of both construction equipment and materials, and for daily operations. Costs can increase significantly if materials have to be carried far by hand. If access to the pond site is by water, make sure that travel is possible during the monsoon.
Local labour, meaning residents living adjacent to the pond site, is the cheapest labour which can be obtained. This is because there will be a large saving in housing, transportation, food and other expenses, because if workers are brought in from other areas they will have to be paid for these expenses. It is important to know the customs and tradition of the local people, as this will greatly affect the funding for labour. Identify the months when agriculture activities are greatest. This will help in formulating programmes for repair of dikes and gates, stocking and harvesting. It may be difficult to get enough manpower during the time for rice planting, harvesting or the milling season for sugar (Denila, 1976).
It is important to determine whether or not the supplies and equipment you need are available in the local area or the country. Fine mesh screening material is generally not available. Frequently the variety of inorganic fertilizer is greatly restricted and costs may be higher for non-agricultural use than for agricultural use. Manures or other organic fertilizers might be difficult to obtain, or available only infrequently, requiring storage. If some materials will have to be imported one should determine if there are any restrictions or extra costs involved.
This will have an impact on management. If local buyers pay acceptable prices, the best form of management may be to practice partial harvesting, or to harvest one pond at a time, so that a small market is not flooded. If the shrimp have to be shipped some distance to a market, it might be better to plan to harvest and market large quantities at one time. Sometimes buyers come to the farm and furnish ice. If not, is ice available? Determine if the buyer will accept only whole shrimp or if just tails would be acceptable. If he will take tails, the heads can be removed at the pond and used as supplemental feed. Ask if a higher price will be paid if heads are removed from the shrimp at the pond.
This could include such items as: licensing requirements, land ownership laws, navigation laws, delays in processing applications, regulations against importing certain required materials (i.e. machinery, equipment, etc.).
This can be from government extension services, government or university research stations, or loan granting agencies.
This might be a useful aid in obtaining financial and/or technical assistance.
The uses of nearby land and water should be assessed to determine what impact, if any, they will have on the project. Activities to be included would be such things as navigation, fishing, industry, public utilities, recreation, nursery areas. Problems can arise particularly if the activities of local people are disrupted. Make sure the project does not block a traditional right of way or interfere with work or recreational activities. It is recommended that plans for industrial development include provisions for rural districts as well as industrial districts so that the effects of industrial pollution on both agriculture and aquaculture will be minimized.