to clean water, and convenient disposal of waste waters 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, and hence a "gold rush" attitude exists where excessive numbers of farms are built, often in ecologically fragile areas, particularly mangrove forests. From a practical standpoint, mangrove forests are usually poor sites for shrimp farms. They do not have the proper soils, there is poor access to water and inadequate drainage due to low elevation, and they are especially susceptible to storm damage.
A related phenomenon that causes farm failure is the 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 beyond the buffering capacity of environmental conditions in the pond.
In most cases the farm managers react by increasing water flow through the farm or by adding mechanical aeration. These do in fact address the problem, but they 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, and the receiving environment cannot process the nutrient-rich effluents, leading to eutrophication. Diseases are then 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 highly profitable with little management can 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 and environmental regulation and protection have been inadequate to avoid this serious ecological damage.
A related problem has been one of land tenure. Investment groups frequently come in and gain control of coastal lands and hired local inhabitants. The workers are usually happy to have the employment, and they appreciate the infrastructure (roads and electrification) that often accompanies the farm. However when these farms fail, the local inhabitants are left unemployed and with environmental damages that
impair their abilities to return to artisanal fishing or small-scale agriculture. Common environmental damage includes salinization of soils, salt-water 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 fish in seawater, brackishwater, and freshwater. Some have attempted polyculture with shrimp, and some are using a crop rotation of tilapia and shrimp (Fitzsimmons, 2001).
In nature, tilapia are omnivores. Young tilapia graze on algal and bacterial films scraping most hard surfaces with their tongues 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 graze heavily on macrophytic algae and aquatic plants. In extensive farming situations, tilapia feed on algae, prey on zooplankton, and scrape films from any hard surfaces in the pond. In intensive farms most nutrition is derived from pelleted feeds, although fish will continue to spend time scraping algal and bacterial films from all surfaces.
In nature, larval shrimp feed first on phytoplankton and then zooplankton. As juveniles and adults they are omnivores and detritivores. Their natural behavior is to search the bottom substrates for decaying plant and animal material. They also constantly pick up sand grains and pieces of organic matter, graze the algae and bacteria, then drop the grain or particle and go onto the next item. In farm settings, shrimps feed on pellets and products of natural processes in the pond. Research by Samocha et al. (1998) has demonstrated that shrimp can be reared in systems with little water exchange, taking advantage of the natural abilities of shrimp to thrive in conditions with high bacterial loading so long as dissolved oxygen levels and other water quality factors are maintained.
There are several variations of tilapia-shrimp polyculture: simultaneous, sequential, and crop rotation. In the simultaneous method, the fish and shrimp are grown together in a pond or raceway. In the sequential case, the water is moved from one growing unit to another, and crop rotation alternates between tilapia and shrimp. There appear to be distinct advantages to each of these systems.
In a polyculture setting, tilapia and shrimp can utilize different niches. In an extensive farm, tilapia can feed on phytoplankton and zooplankton in the upper water column. Shrimp spend most of the time in the pond bottom, where they graze on bacterial films on the substrate and on the