THE GREENWATER SYSTEM OF FRESHWATER PRAWN CULTURE

Many modifications of the ‘clearwater’ system exist. As stated earlier, no two hatcheries are alike. Some operators claim higher productivity for sophisticated alternative clearwater systems but they are more difficult to operate and they are therefore not detailed in this manual. The sections of the manual provided above have a general application to all clearwater freshwater prawn hatcheries, whether ‘flow-through’ or ‘recirculation’.

A more common alternative to the ‘clearwater’ system for flow-through hatcheries is known as the ‘greenwater’ system. In the original type of greenwater system, which was developed in Hawaii, a mixed phytoplankton culture in which Chlorella spp. was dominant was maintained in separate tanks. Its cell density was about 750 000-1 500 000 cells/ml. A fertilizer solution in tap water was added to the tanks at least once per week to maintain the culture. This solution provided a mixture of 4 parts of urea to 1 part of NPK (15:15:15) garden fertilizer, applied at the rate of 185 g per 10 m3 tank. Tilapia (Sarotherodon mossambicus) were held in the tanks at the rate of about 1 per 400 L to graze on and control filamentous algae. Copper sulphate, at the rate of 0.6 ppm was added to the greenwater tanks once per week to control rotifers. The tilapia also helped to fertilize the culture.

The sodium salt of EDTA (ethylene diaminetetraacetic acid) was also sometimes included in the greenwater culture at 10 ppm as a chelator. The greenwater was prepared at the same salinity as the larval rearing water (Note: greenwater does not thrive at more than 12 ppt salinity) and was used as replacement water during exchange procedures instead of plain brackishwater. The greenwater culture was never used for larvae if the culture was more than three days old. Part had to be discarded or used for filling larval tanks and the rest diluted regularly to avoid phytoplankton ‘crashes’ (with the ensuing problems with low dissolved oxygen) occurring in the larval tanks.
Various local variations on the original greenwater system have been used since the Hawaiian system was created, particularly in Malaysia and Mauritius. Elsewhere, however, this technique has had limited success. Although the greenwater system may have some advantages (for example, it is claimed to act as a buffer against ammonia build up) it is difficult to manage successfully and adds more complications to the hatchery process. For this reason, most commercial freshwater prawn hatcheries now use clearwater systems of management, whether they are flow-through or recirculation. The greenwater system is not recommended in this manual.