Abstract

Greenwater tank culture of tilapia is an appropriate method for producing commercial levels of tilapia in locations that have environmental constraints such as limited land and water (e.g., U.S. Virgin Islands) or sub-optimal temperatures, where a greenhouse would be used 10 control temperature. High densities and feeding rates can be maintained through continual aeration and solids removal. Ammonia is removed by phytoplankton uptake and through nitrification on suspended organic particles within the water column. These four treatment processes maintain good water quality, reduce the need for water exchange and maximize water use efficiency. Phytoplankton and other organisms within the water column are grazed on by tilapia, thereby recycling waste nutrients and lowering feed conversion ratios. A greenwater tank system for tilapia culture was developed in Sto Croix. It consists of a 31.O-m3 rearing tank (culture volume) and a 1.4-m3 clarifier from which solids are removed twice daily. The rearing tank is continually aerated with 13 air stones and a 1/20-hp vertical lift pump. An experiment with Nile tilapia (Oreochromis niloticus) stocked at 26 fishlm3 and fed 32% protein feed for 24 weeks attained a final biomass of 13.4 kg/m3, a feed conversion ratio of 1.41 and a survival rate of 99.3% while exchanging only 0.23% of the rearing tank volume per day. From another experiment, solid waste was used 10 irrigate and fertilize a field of pak choi (Brassica rapa), achieving a production level comparable to that obtained with inorganic fertilizers. Land application of sludge increased the organic content of the soil as well as provided nitrogen and phosphorus. Greenwater tank culture of tilapia is an intermediate technology that is easy 10 learn and manage. Some preliminary economic calculations indicate that this system has profit potential due 10 its high productivity and reasonable capital and operational costs, which makes greenwater tank culture appropriate for widespread application.

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