Since ancient times, fish have been held in various man-made structures. These structures were built using simple methods and readily available materials. The fish or other aquatic crop were cared for and relied upon as an important source of food for their families. The typical fish farming operation usually involved hand digging of a small pond or an even simpler method of trapping tidal flow water from an existing water plain. In many parts of the world, the basic earthen pond is still the principal system relied upon in modern day aquaculture. Naturally, considerable technical advances have been made in the design and engineering of aquaculture systems, but the basic system remains the same in most developing nations. Proper design and construction remains an essential component governing the success of today's aquaculture ventures.
The size of earthen ponds built today can vary anywhere from 20 square meters to 20 hectares (44 acres) and higher. Pond size is determined by the type of species cultured, the intensity of the system, land availability, water availability, the harvesting method, market goals, commercial nature of the project and other variables. For example, a commercially oriented tilapia farming operation typically utilizes 0.1 or 0.2 hectare ponds for nursery phases and 0.3 to 0.5 hectare ponds for grow-out. Semi-intensive shrimp farms generally tend to use 7 to 20 hectare ponds. The more intensive shrimp farms generally use ponds less than 7 hectares. Most ponds are rectangular in shape, but there are also square, circular and irregularly shaped ponds in existence. Most farms build the ponds to maintain a minimum water depth of at least 1 meter. Ponds are also used for many different purposes: spawning, broodstock conditioning, nursery, growout or finishing. Quite often, the expected use of the pond dictates the design.
Good pond construction begins with good site selection. Complete site analysis is required to assure that a site is suitable for aquaculture. Since commercial aquaculture ventures require considerable capital expenditure, it is always a wise investment to hire an experienced professional to conduct the survey and manage the project if it is feasible. Water supply, soil quality and local topography are probably the three most important characteristics of a good site. Of course, many other factors play into the decision of where to site a farm. Water is obviously important because it is the growing medium for the crop. No venture can expect to succeed without a plentiful supply of good quality clean water. Complete chemical testing of the water is required since cultured species have some very specific requirements. Soil quality is important because it impacts the design and engineering of the farm. At least 20% clay content is required for earthen pond construction.. This is important for good water retention. Pond bottom soils are also an important and often misunderstood part of the pond environment. There are very important interactions in the soil-water interface and these interactions must be predicted in advance based on the characteristics of the water and soil. Elevation and slope of the land is also very critical. Poor drainage or high risk flood zones are to be avoided, while land with moderate elevations and no more than a gentle slope are preferred.
After a site has been thoroughly analyzed and found suitable, it must be surveyed. Based on the survey and overall strategic management considerations, plans are drawn by an experienced aquaculture design engineer and the project manager. Ways to design a farm are endless but certain designs are definitely more efficient and effective than others. Farm design is always an exciting period, but it takes a skilled and experienced engineer to put together the best farm for a given site. It may look easy, but it is really a very involved and complicated process.
Earth movement is normally done using scrapers or bulldozers. The scraper can
give a perfect slope and high rate of compaction, but this kind of equipment
is very expensive. The slope of a pond is always less than 1% and usually closer
to 0.1%, particularly when the pond is large. Cut and fill volumes are determined
from the topographic survey of the area and equipment operators are guided by
stakes set on-site.
Construction of the dikes is one of the most important tasks in the process.
Slopes are normally set at 2:1. Compaction and sufficient clay content are very
important. Note the clean and continuous slope throughout the length of the
pond. The top of the dike is usually made wide enough to facilitate truck movement.
Wherever possible, dikes are shared between ponds to reduce earth movement costs.
After the pond bottom and dikes are completed, the inlet and outlet structures
are constructed. These structures are often referred to as monks. The monks
are used to control the amount of water coming in and going out of the pond.
In modern aquaculture, monks are usually constructed of poured concrete. There
are many different styles and installation points. Our preference is to locate
outlet monks in the center of the short side and build them directly into the
dike for easy access.
Reinforcing steel is used to frame the structural dimensions of the monk. The
floor is poured and leveled to the correct slope. Boards are used to shape and
level the concrete during the pour.
Note the grooves created in the floor of the monk. The grooves will be fitted
later with boards that will be used to control the flow of water in and out
of the pond. Some of the vertical space is also fitted with filters to prevent
the entry of predators and the escape of the cultured crop.
Concrete pipe of the proper diameter is laid on the floor. The entire monk box
is framed around this pipe. The size of these structures varies with the size
of the pond and with the amount of water that needs to be moved in or out of
the pond.. In some cases, these structures are used to concentrate the animals
for harvest. This is especially true in shrimp farming. A recessed area in the
monk box is constructed to give a point where mechanized harvests can take place.
This particular pond is capable of holding approximately 5,000 cubic meters
of water. Obtaining the proper grade and slope is extremely important. If done
properly, the ponds will drain completely and can be harvested and prepared
in between crops with much greater ease and efficiency. In areas that have changes
in elevation, some pond bottoms may require considerable cutting while others
may require very little bottom cut. The overall goal is to develop an entire
farming system that is capable of filling and draining using the natural pull
of gravitational force.
The end result of good site selection, good engineering and lots of hard work
is a fully stocked pond!