|Better Farming Series 38 - Better Freshwater Fish Farming: Raising Fish in Pens and Cages (FAO, 1990, 83 p.)|
33. You can build a pen in the shallow water of a stream, a river, a lake or a reservoir.
Stream or river
34. The water in a pen should be no more than 1.5 metres in the deepest part.
35. You must also be sure that the water in a pen will never be less than 1 metre even during the dry season.
36. A pen should be put in a place where the flow of water is gentle, not fast.
37. It should be protected from wind so that the surface of the water will be calm, not rough.
38. The water should be clean. You should not build a pen too
near an inlet where the water may be muddy or full of rubbish.
39. Choose a place in the stream or lake where the bottom is firm. It is not so easy to build a pen if the bottom is too soft and where the water may become muddy when you have to work in the pen.
40. In addition, try to find a place where you can build a pen using very few materials, such as the corner of a reservoir or an angle in a stream.
Corner of reservoir
41. A pen can be small or large depending on how many fish you want to raise.
42. However, when you first begin, start by building a small pen. You will need fewer materials. It will be easier to take care of.
43. You can begin with a pen that has about 100 square metres (10 by 10 metres). Later, when you have learned more, you can build bigger ones.
44. A pen is closed by a fence. So, start by driving a row of posts. Posts can be made either of bamboo or wood.
45. If you are using wood, choose wood that does not rot quickly when it is in the water.
46. The posts should be long enough to be driven firmly into the bottom. They should also be long enough to reach at least 50 centimetres above the highest water level.
47. Posts should be 1 to 2 metres apart depending on the material that you use to close the pen.
48. The best material for the fence of the pen is ordinary fish net. You can also use plastic mesh, or woven material made from split bamboo or wooden strips.
49. If you are using fish net to close your pen, the posts can be further apart. If you use woven material or plastic mesh, the posts may have to be closer together.
50. The openings in the material that you use to fence in a pen must be small enough to keep the smaller fish from getting out.
51. However, if the openings are too small they may become filled with dirt or with the very small plants that grow in the water.
52. If the openings in the material become filled with dirt, fresh water will not flow through to clean out the pen and the fish may not have enough air.
53. So, be sure to use material with openings of 1.5 by 1.5 centimetres. This is the right size to keep in the smaller fish and also to let fresh water flow through.
54. Whatever material you use for the fence, it should be high enough to be buried in the pen bottom at least 30 centimetres deep.
55. It should also reach to the top of the posts which, you were told in Item 46, should be at least 50 centimetres above the highest water level.
56. This will keep your fish from getting out of the pen along the bottom or jumping out over the top.
57. Now you will be shown how to build two kinds of pens, how to attach the fence material and how to bury the material in the bottom.
Put a pen along a bank
Put a pen away a bank
mark the place where you will put the fence using wooden stakes or poles and a piece of heavy line
Put a fence
Put a fence
check the bottom along the line with a long pole to see how far you will have to drive in the posts so that they will stand firmly
Check the bottom
measure the depth of the water every 2 metres along the line
Measure the depth
remember, all posts must be at least 50 centimetres above the
highest water level
so, each post must be long enough to reach 50 centimetres above the water, plus the depth of the water, plus the depth that each post must be driven into the bottom
now you can cut all of the posts to the proper length
Cut the posts
drive in the posts along the line 1 to 2 metres apart, depending on the kind of fence material that you are going to use (see Items 47 to 49)
be sure to use material that is high enough to be buried at least 30 centimetres in the pen bottom and that can also reach the top of the posts
if you use fish net for the fence, tie the edge- line of the net to the top of the posts using strong cord
Use strong cord
if you use woven material for the fence, tie it to the posts with strong cord or you can use strong vines
Use strong vines
if you use plastic mesh for the fence, fasten it to the posts with wire
Use plastic mesh
when the fence material is well fastened to the posts, you are ready to bury the material in the pen bottom
you can bury fence material by pushing it into the pen bottom and putting a row of stones on either side to keep it in place
Use fence material
you can also bury fence material by fastening it to straight poles and pushing the poles into the pen bottom with wooden stakes to keep it in place
Use wooden stakes
note: to bury fence material you may have to work under the water
58. When your pen is finished, cut the weeds and grass on the banks clean out most of the plants in the water and take away any rubbish that you find.
59. Then, remove the unwanted fish, crabs or other water life in the pen. You can do this using a fine- mesh net.
Use fine- mesh net
60. When you have done this, you are ready to stock your pen with baby fish.
61. Small fish can be hurt easily. So, you must be very careful when you handle them or carry them from place to place.
62. If you collect small fish using a seine net Carry them in a container such as
· clay pot with a
· a bucket with a lid
· a plastic bag.
Use a container
63. When you carry small fish you must be careful to
· use only clean water
· keep the fish out of the sun or cover the container with a well cloth 10 keep it cool.
Keep the fish out of sun
64. If you carry small fish in plastic bags, carry the bags in a box so that they will not break. Cover the box and the plastic bags with a wet cloth to keep them cool
Use plastic bags
65. When you are carrying small fish, if you see that they are gasping for air pull some air into the water by
· splashing the water gently
with your hand
· forcing air into the water with a tire pump.
Put air into the water
66. If you are raising Tilapia nilotica you will need to put in two baby fish for each square metre of pen. So, if you have built a small pen of 100 square metres you will need 200 baby fish.
Use tilapia nilotica
67. The baby fish that you use should be at least 8 to 10 centimetres long or weigh about 15 to 20 grams each.
Note If you need any help or want to raise a different fish, your extension agent or fishery officer will be able to give you good advice.
68. Just before you put the baby fish in your pen, be sure that the water they are in is not hotter or colder than the water in the pen.
69. You can find this out by putting one of your hands in the water with the fish and the other in the pen.
70. If one is hotter or colder, slowly put water from the pen in the water with the fish until they are both the same.
71. If you do not do this the fish may die when you put them into the pen.
72. Do not pour the fish Into the pen. Gently put the container into the water, tip it a little, and let them swim out by themselves.
Put fish into the water
Put fish into the water
73. You have already been told that there may be very little natural food in a pen. So, you must be sure to give your fish very rich foods.
74. Here are some rich foods that fish like to eat. You can give them
· grain mill sweepings
· rice bran
· beer wastes
· cottonseed or groundnut cake.
Foods for fish
75. Prepare all of the food for one day's feeding at one time. Give the fish part of this food at two or three different times during the day.
76. Always try to feed your fish at the same time each day.
Feed the fish at the same time
77. The bigger your fish grow, the more food they will need.
For each 100 square metres of pen feed them
· 150 to 200 grams of food every
day during the first month
· 200 to 275 grams of food every day during the second month
· 275 to 350 grams of food every day during the third and fourth months
· 350 to 450 grams of food every day from the fifth month until the time you are ready to harvest.
100 grams of rice bran is about one handful. Use a small tin to measure this amount.
Measure rice bran
78. It is not easy to know exactly how much to feed your fish. You must watch them carefully to learn how much food they need.
79. Feed your fish in the shallow part of the pen so that you
can see them eat.
80. If you see that the fish do not eat all of their food, give them a little less.
81. If you see that the fish eat all of their food quickly, give them a little more.
82. If you feed your fish in the same places each day you will be able to see whether they are eating well. If you give them too much, the food that is not eaten will stay on the bottom.
83. Too much uneaten food on the bottom of the pen will make me water bad for the fish.
84. To make it easier to see if your fish are eating well, choose several places in your pen and always feed them there.
85. Mark each place by making a square or ring of bamboo or light wood. Drive a pole into the pen bottom and attach the square or ring. The drawings at the top of the next page will show you how.
86. When you feed the fish, put the food inside the square farming. That way you can easily see uneaten food collecting on the bottom and you will know that you are giving your fish too much.
87. You must always watch you fish to see that they are healthy and swimming strongly.
88. If the fish in your pen are not well, they will stop eating.
89. It this happens, stop feeding your fish for a day or two.
90. When you begin to feed them again, at first give them small amounts of food and then slowly more and more. When your fish begin to eat as before you will know that they are well again.
91. If you find any dead fish floating in the pen, take them out right away.
Take away dead fish
92. Then, stop feeding your fish and watch them very carefully during the next days.
93. If no more fish die you can begin to feed them after a day or two.
94. If fish continue to die take them out as soon as you can and ask your extension agent or fishery officer for help.
95. You have already been told that a pen can be easily damaged. If this happens your fish may get out.
96. So, check the pen fence every day to be sure that there are no holes. At the same time, you should check that the fence is well buried in the pen bottom.
97. If you find any places where your fish can get out, repair them right away.
Repair places where fish can get out
98. In addition, you must be sure that the openings in the fence are free of dirt and plants so that fresh water can flow into the pen.
99. You can clean the fence of a pen using a brush or a broom. However, do it very gently so that you do not damage the fence material.
Clean the fence material
100. If you have used baby fish 8 to 10 centimetres long and have fed them well, they will be ready for harvest in about six to seven months. At this time the fish will weigh 100 to 150 grams each.
101. If you want to harvest only some of your fish you can use a seine net with a large mesh.
102. A seine net with a mesh size of 3 to 3.5 centimetres will catch only the biggest fish and allow the medium and smaller fish to escape.
103. To harvest with a seine net you will need several people. Start at the deep end of the pen and pull the seine net slowly toward the shallow end. The drawing below shows you how.
Harvest with seine net
104. If you want to harvest only part of your fish, you can also use a cast net or a lift net.
105. If you use a cast net or lift net you will catch both big and small fish. Keep the fish that you want and put the others back in the pen.
106. With a cast net or a lift net it is easier to catch the fish while they are eating and they are all together. So, feed them some food first.
Give fish some food
107. If you want to harvest all of the fish you can use a seine net with a fine mesh of less than 1 centimetre.
Now if you do not need all of the fish harvest only what you need and leave the rest in the pen. Fish can be left in a pen as long as you continue to feed them.
108. After you have harvested, clean and repair your pen as quickly as you can so that you can begin again. You must
· cut the weeds and grass on the
· clear out most of the plants in the water and take away all rubbish
· clean and repair the fence and make sure that it is still buried in the pen bottom
· remove unwanted fish, crabs and any other water life using a fine- mesh net.
109. If you have harvested using a large- mesh seine net there will be many baby fish left in your pen.
110. So, to begin again you will not need to put in more baby fish. However, you will have to be careful not to let the baby fish escape when you clean and repair the pen.
111. If you have harvested using a fine- mesh seine net you will catch all of the baby fish.
112. You can hold the baby fish that you catch in a pen or in a cage for small fish (see Item 114 and Item 184) until you have finished cleaning and repairing the pen.
Hold the baby fish in a pen or cage
The faster you can do all of these things, the sooner you will be able to raise another crop of fish.
113. When you are ready, put in two baby fish 8 to 10 centimetres long for each square metre of pen. However, be sure to choose your biggest and best baby fish to put back into your pen.
114. You can build a small pen in a corner of your big pen and raise your own baby fish there. You can also use a small pen like this to keep the baby fish that you catch when you harvest.
Use small pens
115. The small pen should be about 2 by 2 metres and at least 50 centimetres deep. Build the small pen just as you built your large pen.
116. However, the material that you use for the fence of a small pen should have smaller openings to keep in the baby fish. So, be sure to use material with openings of 0.5 by 0.5 centimetres.
117. Material with such small openings will get dirty more quickly. You must be sure to clean it often enough so that fresh water can flow into the small pen.
118. You can put up to 500 baby fish 1 to 2 centimetres long in a small pen this size. However, do not put in more than you need to use or sell.
If you have more baby fish that you can use or sell you can dry them in the sun. After you have dried the baby fish, grind or pound them and mix them with crushed cottonseed, groundnut cake or rice bran to feed to your big fish, your poultry or your pigs. You can also use the baby fish to fertilize your farm garden.
119. Feed your baby fish two or three times each day just as you do with your big fish. You can give them
· rice bran
· cottonseed or groundnut cake.
Food for baby fish
120. Feed your baby fish carefully. Break their food into very small pieces and sprinkle it little by little on the surface of the water. Watch very carefully that they eat well and stop giving them food as soon as they stop eating.
121. With a small pen, it is important to give the baby fish only what they can eat and no more. Uneaten food on the bottom of a small pen is very bad for baby fish.
122. In 30 to 40 days the baby fish will grow to 8 to 10 centimetres long and you can let them into the big pen. To do this open a part of the fence. However, do not take out the posts.
Let out the baby fish
123. The baby fish can then swim into the big pen through the open fence. Later, when you need to raise more baby fish or keep small fish you have caught, you can use the small pen again.