INTRODUCTION
Fish grow fast and stay healthy if they have
enough nutritious food to eat. Living organisms are natural fish foods and are
produced in the water where the fish live. Phytoplankton (microscopic plants),
zooplankton (microscopic animals), insects and certain other plants are all
examples of natural foods (Figure 1). Fertilization increases their
abundance.
| Phytoplankton | Zooplankton | ||||||||||||||||||||||||||||||||||||||
When natural foods are not available in sufficient quantity to provide
adequate nutrition for fish growth, feeds that are manufactured or grown outside
of the fish pond may be fed at regular intervals (daily, weekly, etc.). These
feeds supplement natural foods. They are not nutritionally complete, and will
not adequately support fish growth in the absence of natural foods. Natural food
organisms in the water will provide essential nutrients. Some examples of
supplementary fish feeds are commercially produced rations for chickens and
pigs, rice bran, manioc leaves, kitchen refuse, oil seed cakes, or other
agricultural products and by-products.
In the absence of natural foods,
nutritionally complete manufactured feeds that contain all essential nutrients
and vitamins must be fed to fish. These feeds are used in high technology,
intensive culture systems that are normally inappropriate for rural development
applications and will not be discussed here.
PREPARATION OF
SUPPLEMENTARY FEEDS FOR FISH
Manufactured supplementary fish feeds
may be available in some developing countries. The local economy determines if
it is profitable to use them. Other less costly feeds may be used by farmers.
Table 1 provides a list of supplementary feed ingredients that can be used alone
or in combination. Several guidelines for using items listed in Table 1 should
be followed (Figure 2).
1) Use ground ingredients when possible. Leaf meals
should be sun or oven-dried before
grinding.
2) Measured ingredients
should be mixed thoroughly in desired proportions.
3) Dry rations, such as
rice bran, ground corn and leaf meals, may be stored in a cool, dry
place for
several weeks. Portions may be taken as needed to feed fish.
4) Moist rations
are prepared daily by adding about 350 ml of water per kg of ingredients
to
form a dough-like mixture. This ration may be stored in plastic bags or
containers and
divided for morning and afternoon feedings. The mix is broken
apart and small pieces
are dropped into the water for the fish.
The conversion ratio in Table 1 represents the dry
weight of feed needed to produce one unit wet weight of fish. A low conversion
ratio means that fish will convert the feed into flesh more efficiently. High
ratios indicate less efficient conversion. For example: it takes about 4 to 6 kg
of ground corn, but 10 to 20 kg of fresh cassava leaves to produce 1 kg of fish
flesh.
CHOOSING FEEDSTUFFS
Single feed ingredients may
be fed to fish to supplement available natural food in a pond. Better quality
supplemental feed may be made by combining ingredients. Fish should grow well on
a feed containing 20 to 30% crude protein, of which 7 to 10 % of the protein is
from animal sources. When natural food is abundant and fish are stocked at low
densities, a 20 to 25% protein content is suitable. A 30% or higher crude
protein content is more suitable for commercial operations where fish are
stocked at higher densities.
Choose ingredients from Table 1 when
preparing a supplemental feed, so that a feed mixture having the desired crude
protein content is obtained. Inquire from poultry and livestock dealers and
farmers what ingredients are locally available. Two simple methods can be used
to determine how much of a selected ingredient should be used for making a fish
feed with a desired crude protein content. The trial and error and Pearson's
square methods are described below.
Method 1: Trial and error
method
1) Choose a combination of ingredients from Table 1 that will
provide a feed containing
25 to 30% crude protein.
This feed would contain 27.3 kg of crude protein if
100 kg of the listed ingredients were combined as indicated. This would make a
feed containing approximately 27 % crude protein by weight since (27.3 kg / 100)
x 100 kg = 27.3 % protein content.
Method 2: Pearson's Square
This method may be used for two or more feed ingredients and is
preferable to the trial and error method. Examples of feed formulations with two
and more ingredients are shown.
Example 1: - two ingredients
Find
the proportions of rice bran and fish meal required to make a feed containing 30
% crude protein.
1) Draw a square (See diagram on the following
page).
2) Place the desired protein level at the center of the square. In
this case, 30 %.
3) Place the two ingredients on the two left corners of
the square along with the protein
content of each.
4) Calculate the
difference in crude protein content of the two ingredients (65 and 11)
and
record this number (54) near the lower left corner of the square.
5)
Subtract the desired protein level (30 %) of the feed from the protein content
of each
ingredient and place the answer in the corner diagonally opposite
from each ingredient.
Ignore positive or negative signs. The difference
between percentages of protein in rice
bran and in the feed (19) represents
the amount of fish meal needed. The difference
between fish meal and the feed
(35) represents the amount of rice bran needed.
6) Add the differences
obtained at the right corners of the square (19 and 35) and record
their sum
(54) near the bottom right corner. The sum in the right corner should
equal
the difference in crude protein content recorded near the lower left
corner of the square.
7) Divide the sum obtained in step 6, which was 54,
into each difference obtained in step 6,
which were 19 and 35. Then multiply
each by 100 to obtain the percentage of each
ingredient needed for the
feed.
Thus, 35 kg of fish meal and 65 kg of rice bran are combined to
make 100 kg of fish feed containing 30% crude protein. The feed can also be
described as being composed of 35% fish meal and 65% rice bran.
In this example, find the proportions of soybean meal cake, fish meal, ground
corn and cassava flour needed to make a fish feed with a 30% crude protein
content.
1) Draw a square and place the desired protein level (30%) at
the center of the square. (See diagram on the following page).
2) Group
the ingredients into energy sources (crude protein less than 20%) and protein
supplements (crude protein greater than 20%).
3) Calculate an average for
the crude protein (CP) contents of each group of ingredients (Table 1).
4) Place the averages obtained above at the left corners of the
square.
5) Calculate the difference in crude protein content between the
protein supplements and energy sources and record this near the lower left
corner of the square. The answer in this case is 49.5.
6) Subtract the
desired protein level (30 %) of the feed from the combined protein content of
the protein supplements and energy sources and place the difference in the
opposite diagonal corners. Ignore positive or negative signs. Results are 25 and
24.5, respectively for the protein supplements and energy sources.
7) Add
these differences and record the sum near the lower right corner of the square.
In this case, the answer is 49.5 on the left side and 49.5 on the right
side.
8) Divide the left side sum (49.5) into each difference obtained in
step 6 (24.5 and 25) and multiply by 100 to calculate the percentage of protein
supplement and energy source needed for the feed. The respective answers are
49.5 % and 50.5 %.
9) One half of the protein supplement (24.75 %) is
provided by fish meal and one half is provided by soybean oil cake. One half of
the energy source (25.25 %) is provided by ground corn and one half is provided
by cassava meal.
Thus, to make 100 kg of fish feed containing 30% crude
protein from a combination of ingredients including fish meal, soybean meal
cake, ground corn and cassava meal the following proportions would be mixed.
| fish meal | 24.75 kg | |
| soybean meal cake | 25.25 kg | |
| ground corn | 25.25 kg | |
| cassava flour | 25.25kg | |
HOW TO FEED YOUR FISH
Feeds are provided to increase fish
yields, and are especially beneficial: 1) when fertilization is not practiced;
2) when a pond does not respond well to fertilization; 3) when fish are stocked
at high density in a pond; 4) when fish are confined in a cage, pen or other
structure; 5) when fish are held in tanks. The following general rules should be
followed when providing supplemental feed.
RULE 1: Always feed
fish at the same time and place
Fish become trained and learn when and
where they are fed. Two feedings per day are normal (Figure 3). Half of the
daily amount is given about mid-morning and half in the early
afternoon.
| Morning Feeding | Afternoon Feeding | ||||||||||||||||||||||||||||||||||||||
RULE 2: Never overfeed the fish
Give only as much feed as
the fish will consume in 20 minutes or less. Uneaten feed will pollute the water
and increase the cost of raising your fish. When accumulated uneaten feed
decomposes water may become low in dissolved oxygen. In such ponds, fish will
usually be seen at the pond surface in the predawn hours gulping air. They will
die if the oxygen concentration in the water becomes too low (Figure 4). Steps
must be taken to exchange or aerate the pond water. Feeding should be
temporarily suspended when dissolved oxygen is low to improve water quality.
Signs of overfeeding are listed below.
1) An obvious inability of the
fish to consume all of the feed offered within a 20-minute period. A farmer can
walk into the feeding area of his pond 20 minutes after feeding and feel the
pond bottom. If a large amount of feed is stirred up, too much feed is being
offered. Feed may also be placed on trays or platforms which are lowered into
the water. These platforms can then be pulled up after feeding to determine the
quantity of feed consumed.
2) A foul smell in the water when the bottom
sediments are disturbed. A handful of bottom sediment picked up from the feeding
area should not be black and foul smelling.
3) An overabundance of
phytoplankton. Overfeeding can make plankton become so abundant that a submerged
object can only be seen at depths less than 25 cm. This should be a warning sign
to farmers that indicates possible overfeeding.
RULE 3: Do not feed on harvest day
Stop feeding fish 24 to
48 hours before they are harvested (Figure 5). This allows them to clean their
intestines and makes them better able to survive the stress of handling and
transportation. It will also save feed.
HOW MUCH TO FEED
Feeding rates may be adjusted on a
monthly basis by estimating the fish biomass in the pond. There are several ways
to determine the fish biomass. The most accurate is to catch a small number of
fish and weigh them. The average weight of fish in the sample is multiplied by
the number of fish stocked to calculate fish biomass in the pond. This method
requires a farmer to have accurate scales and to maintain good records of fish
weight and number.
Another way to adjust feeding rates is to assume a
growth rate for the fish based on previous experience. Growth rates will differ
depending on the amount of natural food in the pond, the type and amount of
supplementary feed given, fish species and size, the stocking rate and the water
temperature.
Table 2 presents a chart showing what percentage of body
weight should be fed to tilapia of a certain size. This chart may be used as a
rough guide to determine feeding rates. The following example illustrates
this.
Example of feeding rate determination using Table
2:
A farmer samples his fish pond with a net and determines the
average length of his fish to be about 15.5 cm. The average individual weight
would be approximately 85 g (from Table 2). If the farmer has 350 fish in his
pond, and feeds them at 4.0% of their body weight per day, he would feed about
1.2 kg of feed.
Calculation:
(350 fish X 85 g per fish)
X 0.04 of body weight/day = 1190 g of feed or about 1.2 kg/day.
GLOSSARY
fish biomass - total
weight of fish in a pond.
crude protein - total amount of nitrogen
present in the feed.
conversion ratio - dry weight of feed
required to produce one unit wet weight of fish.
complete feed - a
feed which completely satisfies all the nutrient requirements of the
fish.
supplemental feeds - feeds which supplement natural food
available in the pond by adding to available nutrients, but which alone do not
completely satisfy nutritional requirements of the fish.
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 addressed to:
Information contained herein is available to all persons regardless of race,
color, sex or national origin.