phytoplankton, which sometimes reproduce to such a great extent,
forming a "bloom," that the water becomes green in color. Like
most organisms, phytoplankton require oxygen to live, but when
they are exposed to sunlight, they produce oxygen in excess of
their needs through a process called photosynthesis. As the sun
sets, photosynthesis ceases and the phytoplankton begin to absorb
dissolved oxygen from the water which leads to a decline in oxygen
levels during the nighttime.

 As the source of most of the dissolved oxygen in fish ponds,
phytoplankton are very important. Where phytoplankton are
scarce, water is generally clear and dissolved oxygen levels may be
low. When this occurs, fish culturists sometimes add fertilizer to
the water to stimulate the growth of phytoplankton. A rough
estimate of phytoplankton density can be obtained by measuring
the distance under water at which a white disk, 20 cm in diameter,
disappears from sight. A distance of 40 to 80 cm is considered
desirable for fish culture. If the disk can be seen at greater depths,
applications of fertilizer (40 pounds per acre of 20-20-5) are
recommended at monthly intervals until the desired phytoplankton
density is obtained.

 Fertilization is not required in ponds where the fish are
given large amounts of feed. Of all the nutrients in feed, fish
generally incorporate one third into their body and excrete the
rest into the water. After some time, nutrient levels increase in the
pond and high phytoplankton densities develop. When the white
disk cannot be seen at a depth of 20 cm, phytoplankton are too
abundant and may cause dissolved oxygen problems.

 There are three ways in which excessive phytoplankton den-
sities contribute to the depletion of oxygen that cause fish kills.

 16 r


14


 12-






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as 10

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E

 8
 X
 0
-0r
> 6
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over


clear


During cloudy weather, smaller amounts of oxygen are produced
by phytoplankton in the daytime, but they continue to consume
the same amount of oxygen at night (Figure 2). After two or three
consecutively overcast days, dissolved oxygen may completely
disappear from the pond during the night. Oxygen depletions
also occur when, for unknown reasons, all the phytoplankton die,
causing the water to change from green to brown in color. Without
phytoplankton to produce oxygen, bacteria quickly consume the
remaining dissolved oxygen as they decompose the dead phyto-
plankton. Excessive phytoplankton shade the lower layers of the
pond and prevent sunlight from warming them. This colder bottom
water becomes too dense to be mixed by wind. Dissolved oxygen
gradually becomes depleted in the stagnant, dark lower layers of
the pond (Figure 3). A cold rain can "turn the pond over," causing
surface and bottom waters to mix together. The net result can be
a rapid decline in dissolved oxygen to levels that are lethal to fish.


 Knowledge of the cause of oxygen depletions enables fish
 culturists to follow practices that help prevent their occurrence.
 One very important practice is to avoid overfeeding. It is better to
 feed tilapia small amounts several times during the day rather than
 a large amount once a day. The use of floating fish feed pellets,
 which are available at local feed shops, allows the culturist to
 visually determine feeding response and to adjust the feeding rate
 when necessary. At each feeding, tilapia should consume all the
 pellets-within 30 minutes. Uneaten feed decomposes, which uses
 up oxygen and stimualates excessive phytoplankton growth. The
 maximum feeding rate for tilapia should not exceed 50 pounds
 per pond acre per day. Natural water purification mechanisms
 can handle the waste generated by this much feed. At higher
 feeding rates, phytoplankton will become too dense and early
 morning dissolved oxygen levels will become dangerously low.
 luring cloudy or extremely hot weather, it is wise to reduce the
 feeding rate or even stop feeding because biological processes

cast

 overcast



















 nighttime
 oxygen
 depletion
 (fish die)

 I I


6 a.m.


6 a.m. 6 a.m. 6 a.m.

 Figure 2. Dissolved Oxygen Cycle for Three Consecutive Days.
 Oxygen depletion occurs after second overcast day.