The majority of the disease outbreaks have been observed during the summer rainy season (July Ð October).
The rains cause large environmental fluctuations in salinity, temperature and turbidity. It is suspected that these environmental fluctuations stress the shrimps and trigger disease outbreaks. In the past 1-2 years, a number of farmers operated for just one cycle, stocking at low densities after the short winter rainy season (December-February) and harvesting before the start of the summer rainy season (July-October). This is a longer production cycle and larger shrimps were harvested giving good yields per hectare. Although this system enables these farms to continue operating, they are not fully utilizing the shrimp ponds, which are being abandoned for a good part of the year. This results in a seasonal job market for many of the local people or even the people operating the social cooperative farms. It is considered that tilapia would be well suited to culture during this part of the year when ponds are not being used. Lower salinities associated with the rain would favor tilapia culture. Tilapia and low densities of shrimp could be stocked at the start of the summer rainy season in lower salinities and cultured through to December for Christmas markets. The combination of one shrimp cycle and one cycle of tilapia-shrimp polyculture using the culture system developed in collaboration between researchers and industry in the CRSP project would help the social cooperatives operate throughout the year giving a higher financial return from the infrastructure and providing fuller employment for the cooperatives and other local people.
The project also reviewed anecdotal reports of tilapia-shrimp polycultures in Honduras. On further investigation we were unable to find any actual instances where farms had implemented any commercial applications or even experiments. Apparently, all farmers are waiting for published reports and recommendations before they attempt such a change in their culture systems.
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 even be the modern equivalent of the Chinese polyculture of carp. Tilapia production in former shrimp ponds (with and without shrimp) has increased rapidly in many of the Aquaculture CRSP locations including Thailand, the Philippines, Mexico, Peru, and the inland desert of Arizona.
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 waste waters to
a location which 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, 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 transfers, 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, 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) which often accompany the farm. However, when these farms fail, the local inhabitants are often left with no jobs and environmental damages that impair their abilities to return to artisanal fishing or small-scale agriculture. Common environmental damages include salinization of soils, salt-water intrusion, loss of breeding areas for marine species, eutrophication,