speed up, requiring more oxygen, and the water contains less
oxygen at saturation. When fish feed, their respiration rate in-
creases and large amounts of oxygen are required as the feed is
digested. It is therefore better to feed when dissolved oxygen
levels are high in the morning and afternoon.

 A fish culturist must remain vigilant about the condition of
his fish by observing them frequently and noting changes in weather
conditions and the appearance of the pond. When fish become
sluggish, breathe at the surface and do not feed well, these are signs
that oxygen levels are low. Measuring dissolved oxygen concentra-
tions on a regular basis is helpful in alerting the culturist to un-
favourable changes. Dissolved oxygen concentrations can be
measured in five easy steps with a test kit manufactured by Hach
Chemical Company (Loveland, Colorado).

 When an oxygen depletion occurs, the culturist can save the
fish by acting immediately to replenish the oxygen supply. There
are three ways of doing this. Some of the water in the pond can be
replaced with fresh water if it is available, which is frequently not
the case in the Virgin Islands. Another method is to "aerate" the
pond water (add oxygen to the water) by spraying it into the air
whereby oxygen from the air will pass into the small droplets
through a process called diffusion. A number of devices aeratorss)
are used to aerate ponds, but paddlewheel aerators have been
found to be the most effective. In the third method, emergency
aeration is accomplished by pumping air into the pond from a
blower or air compressor. The resultant stream of bubbles adds
oxygen to the water through diffusion. This method is commonly
employed in home aquariums.

 Maintaining adequate levels of dissolved oxygen is a greater


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problem when tilapia are cultured in cages. Large numbers of fish
are concentrated in very small areas and are exposed to only a
small portion of pond water. It is therefore important to obtain
as much circulation as possible between the cages and the surroun-
ding pond. Maximum circulation can be achieved by using cage
mesh size that is one half inch or larger, locating the cages in areas
of the pond that are exposed to the greatest wind-induced currents
and positioning the cages far apart. Research is presently being
conducted to determine stocking rates for cages that will give
high yields while being within safe limits against oxygen depletions.

 Fish culture is probably the most challenging type of animal
husbandry. Instead of the proverbial green thumb,a "slimy thumb"
is required to know what will make fish happy, healthy, and fast
growing. This is not an easy task because fish are under water and
completely out of sight most of the time except for brief periods
when they come to the surface for feed or oxygen. Hopefully,
their visits to the surface for oxygen will be kept to a minimum.


Surface water is
supersaturated with
dissolved oxygen.


Below 3 feet there is no
oxygen or light and the
water is cold and stagnant.


0 4 8 12 16 21
 Dissolved Oxygen (mg/liter)
 Figure 3. Dissolved Oxygen Levels at Different Depths in a Pond
 With Excessive Phytnplankton Densities.