Introduction
Shrimp ponds have been abandoned in many parts of the world due to diseases, poor management, and environmental degradation. Tilapia production, supplemented with low densities of shrimp, in abandoned shrimp ponds may provide an opportunity to develop a sustainable aquaculture system that will support local inhabitants who have not benefited from the shrimp boom in many parts of the world. Polyculture, or crop rotation of shrimp and tilapia, may be the modern equivalent to the Chinese polyculture of carp. Tilapia production in former shrimp ponds (with and without shrimp) has increased rapidly in many countries including Thailand, the Philippines, Honduras, Mexico, Peru, and the inland desert of Arizona, USA.
Farmers in several locations around the world appear to have demonstrated that tilapia and shrimp can in fact be grown together. There are several technical questions to be answered regarding optimal salinities, stocking densities, feeding rates, disease and parasite infestations, cost-benefits, and environmental impacts of tilapia-shrimp polyculture. These would be the first replicated trials and technical evaluation of tilapia-shrimp polyculture.
Shrimp aquaculture has been devastated in many countries due to a mix of disease outbreaks and decreasing yields.
The progression of shrimp aquaculture has followed a familiar pattern throughout the tropics. Initially, farms are constructed in the most appropriate areas. These locations are characterized by good soils with proper pH, appropriate levels of clay, silt, and sand, proper elevation, good access to clean water, and convenient disposal of wastewaters to a location that keeps wastes from being cycled back into the farm. These farms, if managed well, tend to be very profitable. This early success leads others to imitate the process as best they can. This has led to a "gold rush" attitude where excessive numbers of farms are built, often in ecologically fragile areas, especially mangrove forests. From a practical point, mangrove forests, in general, are poor sites for shrimp farms. They do not have the proper soils, there is usually poor access to water and inadequate drainage due to low elevation, and they are especially susceptible to storm damage.
A related phenomenon is overstocking of an existing farm. After the initial success of a farm, the managers often assume they can increase yields and profits by stocking more shrimp and feeding more heavily. This may work for one or two crops, encouraging even more stocking. But inevitably the producer overshoots and a disease outbreak occurs because the animals have been overstocked and are stressed under the available environmental conditions in the pond.
In most cases the farm managers react by increasing water flow through the farm or adding mechanical aeration. These do in fact address the problem but also increase operating expenses and environmental impacts. Added to this situation is the fact that the ponds must be properly maintained and the pond soils managed between crops. Many farms do not properly maintain their infrastructure or their pond environments. When multiple farms in one area reach this stage, there tends to be an environmental overload. The effluent from one farm becomes the supply water for another. The receiving environment cannot process the nutrient rich effluents, leading to eutrophication, and diseases are spread by water transfer, birds, and other vectors. Excessive pumping of water can lead to saltwater intrusion and depletion of freshwater aquifers. Farms that had been wildly profitable with little management, suddenly require more investment and sophisticated management for lower levels of profit. Some farms make the investment to operate in a more sophisticated and sustainable manner, while many others just abandon the farm. In many countries the governmental oversight, environmental regulation, and protection have been inadequate to avoid this serious ecological damage.
A related problem has been one of land tenure. In many instances investment groups have come in and gained control of coastal lands and hired local inhabitants. These people are usually happy to have the employment and appreciate the infrastructure (roads and electrification) that often accompanies the farm. However, when these farms fail, the local inhabitants are often left with no jobs and environmental damage that impairs their ability to return to artisanal fishing or small-scale agriculture. Common environmental damages include salinization of soils, saltwater intrusion, loss of breeding areas for marine species, eutrophication, and changes in the water flow through estuaries.
One technique that has been tested to utilize abandoned shrimp ponds is to convert the pond to tilapia production. There have been several variations of tilapia production including rearing in seawater, brackish water, and fresh water. Some have attempted polyculture with shrimp and some are using a crop rotation of tilapia and shrimp (Fitzsimmons, 2001).
In nature tilapias are omnivores. Young tilapia graze on algal and bacteria films scraping most hard surfaces with tongue and teeth. As they grow they also become effective filter feeders of phytoplankton and predators of zooplankton. Larger tilapia are less effective filter feeders but begin to graze heavily on macrophytic algae and aquatic plants. In extensive farming situations, tilapia filter feed on algae, prey on zooplankton, and scrape films from any hard surfaces in the pond. In intensive farming, most nutrition is derived from pelleted feeds, although fish will continue to