The success or failure of shrimp farming depends on the environmental conditions of the location of the farms. The social and environmental impacts like soil and drinking water salinization and nutrient loading which are attributed to shrimp farming, mainly arise due to improper location of the shrimp farms. A vast majority of problems affecting the shrimp culturists as well as the environment could be avoided by proper site selection. The following criteria are recommended for consideration during site selection;
Social and Environmental Considerations:
The following aspects should be kept in mind while deciding on a site for shrimp farming.
- Mangrove forests play a very important role in coastal ecosystem. They are a source of livelihood for the coastal population and it protects the coastal settlements. They also act as habitat and nursery for a variety of marine organisms. Hence, destruction of mangroves for any purpose will have far reaching social and environmental impacts. Further mangrove areas are generally acidic in nature and are not suitable for shrimp farming. In view of these facts, shrimp farms should not be located in mangrove forest area.
- Similarly, shrimp farms should not be located near ecologically sensitive areas like marine parks and sanctuaries to avoid any disturbance to the otherwise, fragile ecosystem.
- Establishment of shrimp farms by converting productive agricultural lands and saltpan will have social consequences since these are essential commodities for human beings and involve the livelihood of many farmers. Use of unproductive agricultural lands located in the tail end of the river systems could be used for setting up of the shrimp farms, but only after getting it reclassified by the concerned Government authority's agencies.
- The nearness of shrimp farms to various other land uses may have some negative impacts due to the seepage of water, which will increase the salinization of land and water resources. To avoid such salinization impacts, buffer zones should be provided in such areas depending on the soil conditions. Sandy andor porous soils should be avoided.
- Locating shrimp farms close to one another prevents access to the traditional users of the water front. Hence it is advisable to leave enough space between the fms for free access to the water front. Smaller farms of 2-5 ha should leave a minimum of 20 m between the farms.
Soil is the most important component in a culture system. The quality of soil should be ascertained for pH, permeability, bearing capacity and heavy metal content. Soil with low pH of below 5 and acid-sulphate soils should be avoided. Similarly soils with high concentrations of heavy metals also should be avoided. Generally clayey loam soils are preferred. Sandy soils are seepage prone and will lead to problems of salinisation of adjoining land and water resources. Further, maintenance of a farm in sandy area needs high capital and operational costs. Hence, sandy areas should be avoided. A best site is the one, which involves lesser capital investment for constructing filly drainable ponds.
Availability of good quality water in required quantities is one of the most important prerequisite for sustainable aquaculture. While locating the farm site, careful study should be made on the source of water, quantity of water available during the different seasons and the quality of water. The optimal levels of various water quality parameters required for the best growth and survival of cultured shrimps are presented below.
Since drying of the pond bottom and proper water exchange form integral part of the technology of shrimp farming, ponds that are drainable by gravity are essential for a successful venture. Hence, the elevation of the site from the lowest low water level of the supplying creek should be given due consideration while selecting the site. A minimum elevation of 0.45 to 0.6 m is essential to ensure proper drainage.
The infrastructure facilities like roads, electricity, proximity to hatcheries, ice plants, processing plants should be considered while choosing the site for a shrimp farm since these play very important roles in the economics of culture operations.
Pond design, Layout and Construction :
The earthwork in the construction of the ponds, canals, levees alone comprises 35-50% of the construction costs. If capital costs can be minimized while still maximizing productivity and reducing the risks, the farming operation will be more profitable. A proper layout of the farm significantly reduces the cost of construction and ensures the smooth and trouble free operator of the various facilities and structures allowing proper management of production activities. Proper designing and construction of farms are essential for their efficient management and for promoting environmental protection. Good site selection and incorporation of mitigate features in the design of the farm are the best ways to avoid problems related to flood levels, storms, erosion, seepage, water intake and discharge points. Proper planning during the construction can prevent or greatly limit the probable environmental impacts. Since site characteristics vary greatly from place to place, a site-specific approach to design and construction is necessary. A well experienced construction team under the supervision of a qualified aquaculture engineer should be employed to ensure proper construction. Earthmoving equipment like bulldozers, scrapers, hydraulic shovel etc. can be used to execute the work expeditiously. In areas where soil productivity is less, the top fertile soil layer should be removed and re-laid after the construction of the ponds. Construction of sluices and supply channel should be done carefully to avoid future problems in water management. The following aspects of the design and lay-out of the farm should be given importance to avoid major problems during culture.
The peripheral dyke of a farm is the most important structure since it protects the farm against flood, tidal thrust and cyclone. The structure of pond dykes depends on the load bearing capacity of the soil. In areas with sandy soil, impervious materials like concrete, clay or sand bags should be used as the core of the dyke.
Wherever the outer side of the dyke faces the water front, it should contain a berm and stone pitching or a retaining wall should be constructed.
The height of the pond dyke should be at least 1.5 m so as to retain a maximum of 1.0 m water in the pond. The height may vary depending on the highest flood level and highest high tide level (Spring tide). A free board of 0.6 to 0.7 m is required above these levels.
The slope of the dyke may range from 1:1 for clayey soil and 3:1 for sandy soil. The top width of the dyke (crest) should be large enough to hold the supply channel and also to be used as a road around the farm.
Water Intake System:
The design of the supply canal mainly depends on the daily water requirement of the farm. Depending on the soil quality, earthen or lined or concrete supply canals are designed. In small farms of 3 to 5 ha, PVC pipelines with valves are used for the supply. The supply inlets can be simple PVC pipes or concrete structures with suitable screens to prevent the entry of pests and predators.
The outlet is generally made up of wood and concrete with provisions for harvest bags, strain nets and wooden shutters. For efficient and best possible water exchange, the outlet should be located diagonally opposite to the inlet. The wooden shutters should be made of small planks so that the draining of water either from surface or bottom could be effected easily. The width of the outlet sluice may vary from 0.3 m to 1.0 m depending on the size of the pond and the daily rate of water exchange. The bed of the drainage canal should be at least 30 cm below the pond bed level with adequate slope (1:2000) towards the main outlet. The size of the drainage canal .depends on the maximum amount of water to be let out in a day. A bottom width of about 1 m will be normally sufficient.
Rectangular or square ponds are appropriate for shrimp culture. Natural aeration through wind action could be maximized by designing the longest axis of the pond parallel to the wind direction. But in places where the wind action is very high and there is a need to reduce the wave action in the ponds, the ponds should be positioned with the longest axis perpendicular to the wind direction. The rearing pond must have a minimum depth of 1 m and a maximum of at least 1.5 m. The pond bottom should have a slope of 1:2000 towards the outlet with an overall drop of 20 to 30 cm for a 1 ha pond. This will facilitate easy draining and drying of the pond bottom.