FOR RURAL DEVELOPMENT
INTRODUCTION
A fish pond is a unique environment created by
man. It must be managed properly to achieve good fish production. For centuries
fish farmers have increased fish yields in ponds by using inorganic or chemical
fertilizers and organic fertilizers or "manures."
WHY FERTILIZE PONDS?
Microscopic green plants called algae or "phytoplankton" form the base of the
food chain for fish. All green plants need light, proper temperature and
nutrients for growth. If sufficient light and proper temperature are present,
the nutrients in chemical fertilizers (nitrogen, phosphorous and potassium) are
readily assimilated by phytoplankton and their abundance increases. Manure
contains the same nutrients. They are released and become available to
phytoplankton during and after decomposition. As phytoplankton assimilate
fertilizer nutrients and reproduce to form dense communities pond water turns a
greenish or brownish color. This is called a phytoplankton bloom.
Figure 2: Viewing phytoplankton in a drop of pond water under a
microscope.
As phytoplankton multiply they are eaten directly by some
fish or by other mostly microscopic aquatic animals called "zooplankton."
Figure 3: Viewing zooplankton in a jar of pond water through a magnifying
glass.
Phytoplankton and zooplankton (collectively called "plankton")
also serve as food for larger aquatic organisms.
Figure 4: Larger aquatic organisms consumed by fish.
Through a complex
chain of interactions fertilizers increase production of natural food organisms
eaten by fish. Different fish may have different food preferences. Some can
filter plankton, others eat aquatic insects and others may feed on decomposing
material. The following diagram illustrates this.
Figure 5: Fertilization increases the abundance of natural fish
food
MEASURING THE EFFECT OF FERTILIZATION
Response to
fertilization can be measured by the abundance of phytoplankton. When
phytoplankton is abundant, it makes water a turbid green or brownish color. If
the pond water is not very muddy, the turbidity caused by phytoplankton can
serve as a measure of phytoplankton abundance.
A Secchi disk is a
standard way to measure visibility in water. The disk measures 20 cm in diameter
and is painted black and white in opposing quarters as shown below. A simple
disk can be made from a round can lid. The disk is attached to a wooden stick or
a rope marked off in centimeters. Measure plankton density by lowering the disk
into the water with your back to the sun while viewing the disk from directly
above. The depth at which the disk just disappears from sight is the Secchi disk
reading.
Figure 6: Making and using a Secchi disk.
It is often easier for a
farmer to use his arm and hand instead of a Secchi disk. The principle is the
same. The person's arm becomes a meter stick and the upturned palm of the hand
becomes the disk as illustrated below. Rules on how to interpret the results of
either Secchi disk or arm and hand and what management actions to take depend on
what fish is being cultured and on what fertilizer is being used.
Figure 7: Measuring phytoplankton abundance by arm and
hand.
DIFFERENCES BETWEEN CHEMICAL FERTILIZERS AND
MANURES
Chemical fertilizers are concentrated nutrients for green
plants. The qualities that have made them popular in modern agriculture are: 1)
they can be stored for a long time, and 2) relatively little is needed since the
nutrients are in a concentrated form. These are important advantages over
manures if labor and transportation are costly. Two disadvantages of chemical
fertilizers, especially for isolated farms operated on a limited budget, are
that they are usually expensive and available only from commercial
suppliers.
Another important consideration about chemical fertilizers is
their potential for being wasted. Adding chemical fertilizer to a pond initially
stimulates phytoplankton growth. However, if too much is added plankton can
become so dense that sunlight penetration through the water is restricted. When
this happens algae cells may have more than enough nitrogen and phosphorus
available in the water, but they do not receive sufficient sunlight. No
additional plankton will then be produced. Keeping phytoplankton abundance
within the limits suggested for Secchi disk or arm measurement helps ensure that
excess fertilizer is not applied.
Chemical fertilizer may not be eaten
directly by fish. Manure, however, can serve several roles. It releases
nutrients for phytoplankton through decomposition; certain fish can digest
specific components of manure; fish may digest the bacteria, fungi and other
organisms contained in manure even though the manure itself may have no
nutritional value.
Large quantities of manure are needed to fertilize
ponds. This is its main disadvantage. There is a danger in adding too much
manure to a pond at one time. Decomposition may deplete oxygen in the water or
cause harmful substances to accumulate. Fish may die as a result. However, with
proper management this problem can be avoided or corrected and where manures are
available they are often the fertilizer of choice.
FOOD
CHAINS
Nutrients in chemical fertilizers are "food" for green plants,
and have no direct food value to fish. When chemical fertilizers are added to a
pond phytoplankton become more abundant. They may then be eaten directly by fish
or by zooplankton and insects which are subsequently eaten by fish. This
step-by-step process is called a food chain.
A step in the food chain can
be eliminated by adding manure instead of chemical fertilizer to a pond because
many fish will consume manure directly. Manure may also be eaten by zooplankton
or insects which are later eaten by fish or it may be decomposed by bacteria and
other organisms. Decomposition releases nutrients for assimilation by
phytoplankton. A simplified food chain illustrating direct and indirect
consumption of fertilizer nutrients by fish follows.
Figure 8: Simplified food chain showing pathways through which fertilizer
nutrients areturned into fish flesh.
CONCLUSION
Both
chemical fertilizers and manures are used to increase fish yields. Different
results may be obtained under different conditions. The choice of which
fertilizer to use will be influenced by local availability, cost and other
factors. Read Chemical Fertilizers for Fish Ponds and Organic
Fertilizers for Fish Ponds for more detailed information on fertilizer
use.
GLOSSARY OF TERMS
assimilate - to take in and
appropriate as nourishment.
chemical/inorganic fertilizers -
manufactured fertilizers containing nitrogen, phosphorous and potassium in
varying proportions.
decomposition - the decay or breakdown of
organic materials into simple compounds available for assimilation by
phytoplankton.
fertilizer - a substance added to water to increase
the production of natural fish food organisms.
food chain - the
pathways through which nutrients added to a pond are converted into fish
flesh.
manure/organic fertilizer - animal or plant matter used as
fertilizer in ponds.
microscopic - invisible to the eye without
the aid of a microscope or magnifying glass.
natural fish food
organisms - plankton, insects and other aquatic organisms that fish
eat.
organic fertilizers/manure - fertilizers composed of animal
or plant materials which must be decomposed to release their minerals and
nutrients.
oxygen depletion/low oxygen - a condition, normally
occurring at night, in which oxygen dissolved in pond water has been depleted
mainly because of the decomposition of organic matter and respiration of
organisms in the pond.
phytoplankton - the plant component of
plankton.
phytoplankton bloom - an increase in phytoplankton
abundance resulting from fertilization.
plankton - the various,
mostly microscopic, aquatic organisms (plants and animals) that serve as food
for larger aquatic animals and fish.
Secchi disk - a circular disk
measuring approximately 20 cm in diameter which is used to measure the abundance
of plankton in water.
turbidity - an opaque or unclear appearance
imparted to water by the presence of suspended foreign particles (soil,
plankton, etc.)
zooplankton - the animal component of
plankton.
Funding for this series was provided by the United States
Agency for International Development.Communications regarding this and other
technical brochures on water harvesting and aquaculture should be sent to:
Information contained in this manual is available to all persons regardless
of race, color, sex, or national origin.