RECIRCULATING POND SYSTEMS: MODELS FOR SHRIMP FARMING IN AFRICA
Limitations on the availability of land, water or both, and the desire for increased control over the production environment have led some commercial shrimp producers (stocking density 10-15/m2) in Latin America to develop recirculating pond systems. Although water exchange continues to be a common practice, it can result in sedimentation of water supply canals and ponds, poor biosecurity, poor pond water quality, difficulty regulating salinity, and difficulty establishing mature and stable microbial and algal communities. Water exchange also displaces the waste treatment function from inside the farm to the surrounding environment.
Properly designed treatment systems include basins for sedimentation and oxidation and one or more circulation pumps Significant treatment occurs in drainage canals, operating as oxidation ditches. In the main reservoir, finger-like projections of embankment direct water in a serpentine flow, improving treatment capacity. All water introduced to the farm from outside should be treated in sedimentation ponds before adding to recirculated water.
At the farm level, the main benefits of recirculation are reduced sediment accumulation, reduced water pumping requirements, and improved biosecurity. Ponds can be managed to stock and harvest at different times, so carrying capacity can be "shared" among ponds in a block. A secondary benefit to recirculation is the development of "mature" water that can be effective as a method to control or limit EMS. The main costs of establishing a recirculating are associated with a reduction in productive area by 10-15% and the additional investment and construction costs.
At the pond level, the main benefit to recirculation is likely related to water movements. Relative to static ponds, water movement, even in hypereutrophic conditions, increases the supply rate of dissolved oxygen to cultured animals, including benthic-dwelling shrimp. Movement can be accomplished by mixing individual ponds, but it is more efficient and practical to apply energy to circulate water with one main recirculating pump station. To maximize the benefit of water movement, long and narrow ponds are best.
If maximum feeding rate (=carrying capacity) is restricted to 30-35 kg/ha per d, then processes internal to ponds are sufficient to maintain good water quality, and only a small reservoir (<10% of area) is needed. As the intensity of pond production increases, the proportion of the farm area in reservoir increases, up to 80% for intensive shrimp production. The configuration of most recirculating pond systems is based much more on practical experience than on application of rational waste treatment design approaches or a clear understanding of pond ecology. Nonetheless, these systems enable good crop production (10-15 t/ha) without widespread use of paddlewheel aeration.