Partial Change (or Clear Water) Method
 This practice involves utilizing continuous or batch exchanges
of clear water in the larval tanks. With this technique, 75% or
more of the waste is exchanged daily, either by continuous
flushing or by a daily rapid purge, and the accumulation of toxic
metabolites in the larval tanks is prevented without the necessity
of culturing phytoplankton. However, the water usually requires
pretreatment or conditioning and the larval tanks must be shaded
from direct sunlight.
 A major disadvantage of the open system is the necessity of an
adequate supply of both fresh and saline water. Thus, care must
be taken to insure that the water sources will remain uncontami-
nated. If water is not of suitable quality, considerable expense to
treat the water for hatchery use may incur. Pumping costs may
also be high. Another disadvantage of an open system is that the
hatchery might be located far from the growout facility.


Closed Systems
 Closed system hatcheries can be situated far from the coast,
since the addition of both fresh and saline water is minimal.
Closed systems are useful in areas with freshwater and energy
shortages (New and Singholka, 1982). Such systems usually in-
volve filters and other water quality improvement apparatus to
improve the quality, so construction expense may be higher.
 Seawater is usually transported to the hatchery site. This opera-
tion is costly, so water conservation is essential in the system. Ar-
tifical seawater has been used but this practice is usually expen-
sive (New and Singholka, 1982). Most closed systems, however,


still need a 25% exchange of their water each week. This is
because of the buildup of nitrate, which is the least toxic end pro-
duct of nitrification, but which can still affect larvae when in high
concentrations.
 An innovative use of brine water from salt ponds has been
reported by Tunsutapanish (1980a; as reported by New and
Singholka).The use of brine reduces the amount of water that has
to be transported to the hatchery. The brine can be diluted at the
hatchery. The water from salt ponds contains few or no
pathogenic organisms. However, more research is needed before
this source is utilized extensively. Dunaliella salina, a green algae,
is frequently present in these salt ponds. The effect of this algae
on prawn larvae is unknown.

SUMMARY
 Open and closed systems have both advantages and disadvan-
tages. Open systems are safer, since the contaminants can be
washed out. Closed systems are also designed to remove con-
taminants, but the use of filters and other water quality ap-
paratus becomes necessary. The location of either system should
take into consideration the effects from hurricanes or other
tropical storms prevalent in the Caribbean.

References
 1. Aquatic Farms, Ltd. 1979. Multiple-use options for a prawn hatchery on
 Guam. Prepared for the Government of Guam, Department of Commerce, Project
 Number 07-6-02028.
 2. New, M.B. and S. Singholka. 1982. Freshwater prawn farming. A manual
 for the culture ofMacrobrachium rosenbergii. Food and Agriculture Organization of
 the United Nations, Fisheries Technical Paper Number 225.


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