Fish Culture and Hydroponics in The Virgin Islands


 By Ayyappan Nair
 Assistant Aquaculturist
 C.V.I. Agricultural Experiment Station

 Until recently, fish culture has not been possible in areas
of the world where freshwater is scarce. Now there is a method
that has the potential of producing fish as well as vegetables in
arid regions by using only limited amounts of freshwater. The
fish are cultured in a recirculating system where the water is
purified and reused hundreds of times. The purification process
includes the culture of vegetables to remove the nutrients that
accumulate as part of the fish waste. The vegetables are grown
without soil, a method known as "hydroponics." In the Virgin
Islands, where inadequate freshwater is a factor limiting the growth
of agriculture, fish culture-hydroponic systems may play an impor-
tant part in future fish and vegetable production.

 At the College of the Virgin Islands Agricultural Experiment
Station, research dealing with fish culture-hydroponic systems has
been encouraging. A unit was developed that demonstrated the
feasibility of producing fish and vegetables in a backyard. (See
article entitled "New Approach to Backyard Fish and Tomato
Production" by Barnaby J. Watten in the 1980 Agriculture and
Food Fair booklet.)

 Nearly 140 pounds of fish and 170 pounds of tomatoes
were grown in 6 months in an area occupying 1/200 acre. The
integrated fish and tomato production unit consisted of a fish
culture pool, two waste removal drums and two hydroponic
tomato beds. Cost of the building material amounted to approxi-
mately $459.00 in January 1979. The system appeared to be
economically viable for small scale production of fish and vege-
tables for home use. These favorable results have prompted scien-
tists at the Agriculture Experiment Station on St. Croix to expand
their research program. A facility is being established that will
include 34 pools and 6 recirculating systems. The results of future
experiments may encourage the development of commercial fish
culture-hydroponic systems in the Virgin Islands.

 Tilapia, a tropical freshwater fish, has been selected for
culture in recirculating systems because it is a rapid growing fish
that is easy to breed, resistant to diseases and tolerant to handling
and poor water quality. A recirculating system is an unnatural
environment for tilapia, and therefore several factors must be
controlled by the fish culturist to achieve maximum fish produc-
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 For rapid growth, tilapia should be given feed containing
approximately 32% protein. Fish feed is available at local feed
shops. The optimum feeding rate will depend on the water quality
and the size of the fish. To avoid overfeeding, tilapia should be
given small quantities of feed several times a day. The best method
is to allow tilapia to feed themselves by using a "demand feeder."
A demand feeder consists of a feed storage container and a trigger
mechanism that hangs in the water. Tilapia quickly learn that by
bumping the trigger, feed pellets will.be released and fall into the
water.
 Maintaining adequate levels of dissolved oxygen is extremely
important in recirculating systems because the fish are concen-
trated in a small volume of water and they may quickly deplete the
oxygen supply. Levels of dissolved oxygen should average 5 or 6
mg/liter to support the healthy growth of fish. Natural mechanisms
for adding oxygen (aerating) to water are not sufficient in a recir-
culating system, and therefore mechanical devices aeratorss) must
be used. Aerators splash water into the air or mix air into the
water so that oxygen from the air passes into the water by a pro-
cess called diffusion. Dissolved oxygen is also important to the
growth of vegetables and the bacteria that remove ammonia from
the water.
 Fish can alter the chemistry of the water in a recirculating
system. They cause a deterioration of water quality resulting from
the accumulation of toxic waste from the uneaten food, feces and
excretions collectively termed metabolitess."

 Uneaten food and fish feces are solid pollutants that must be
removed from the system. In the fish pool, the solids are suspended
by currents produced by the fish and aeration equipment. The first
stage of the wastewater treatment process consists of a settling
basin clarifierr) where water allows the solids to settle by gravity
and accumulate on the bottom. Unless the solids are frequently
removed by draining or siphoning, they will decompose, a process
which adds ammonia to the water and consumes dissolved oxygen.

 Ammonia is a metabolite of fish that must be controlled.
Concentrations as low as 0.2 ppm have been shown to reduce the
growth of fish and lower their resistance to disease. Higher concen-
trations of ammonia can be toxic to fish if they are allowed to
remain in the culture water. Removing ammonia on a continuous
basis is an essential part of water quality maintenance.

 Ammonia can be removed by a biological filter, which




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