15. Producing fish in pens

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Locating fish pens
How large should your pen be?
Building a pen
Putting baby fish into your pen
Feeding fish in pens
Taking care of your fish in a pen
Taking care of your fish pen
Harvesting fish in pens
Starting again

Locating fish pens

368 You can build a pen in the shallow water of a stream, a river, a lake or a reservoir.

Figure 184

369 The water in a pen should be no more than 1.5 metres in the deepest part.

370 You must also be sure that the water in a pen will never be less than 1 metre even during the dry season.

371 A pen should be put in a place where the flow of water is gentle, not fast.

372 It should be protected from wind so that the surface of the water will be calm, not rough.

373 Choose a place in the stream or lake where the bottom is firm. It is not 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.

Figure 185

374 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.

Figure 186

How large should your pen be?

375 A pen can be small or large depending on how many fish you want to raise.

376 However, when you first begin, start by building a small pen. You will need fewer materials, and it will be easier to take care of.

377 A good size pen to begin with is 10 by 10 metres This is 100 square metres Later, when you have learned more, you can build bigger ones.

Figure 187

Building a pen

378 A pen is closed by a fence. You start to build this by driving in a row of posts. Posts can be made of either bamboo or wood.

379 If you are using wood, choose wood that does not rot quickly when it is in the water.

380 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.

Figure 188

381 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.

382 Mark the place where you will put the fence using wooden stakes or poles and a piece of heavy line.

Figure 189

Figure 190

383 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.

Figure 191

Figure 192

384 Measure the depth of the water every 2 metres along the line.

385 All posts must reach at least 50 centimetres above the highest water level. So the length of each post must equal the depth of the water, plus the depth that each post must be driven into the bottom, plus 50 centimetres for the segment above the water.

Figure 193

Figure 194

386 Now you can cut all the posts to the proper length.

387 If you use woven material made from split bamboo, for example, or plastic mesh, drive in the posts 1 metre apart.

388 If you use a fishing net drive in the posts 2 metres apart.

Figure 195

Figure 196

389 If you use fishing net for the fence tie the edge-line of the net to the top of the posts using strong cord.

390 If you use woven material for the fence, tie it to the posts with strong cord. You can also use strong vines instead.

Figure 197

Figure 198

391 If you use plastic mesh for the fence, fasten it to the post with wire.

392 When the fence material is well fastened to the posts, you are ready to bury the material in the pen bottom.

393 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.

Figure 199

Figure 200

394 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.

395 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.

396 Then remove the unwanted fish, crabs or other water life in the pen. You can do this using a fine-mesh net.

Figure 201

Figure 202

397 Now you are ready to stock your pen with baby fish.

Putting baby fish into your pen

398 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.

399 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 kind of fish, your extension agent or fishery officer will be able to give you good advice.

400 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.

Figure 203

401 If the water the fish are in is hotter or colder, slowly add water from the pen to the water with the fish until it reaches the same temperature.

Feeding fish in pens

402 As you know, with a pond you fertilize the water. You add compost, animal manure or plant materials so that there will be natural food for the fish to eat.

403 In addition, fish in ponds must be given other kinds of food each day to make them grow quickly.

404 With well-fertilized water and enough additional food, the fish can swim about and find food anywhere in the pond.

405 However, with a pen built in a stream, lake or reservoir, there may be little natural food and the movement of water may carry the food that you give the fish outside the pen where the fish cannot get it.

Figure 204

406 Be sure to feed your fish well. You may use cottonseed or groundout cakes, wheat or rice bran, mill sweepings or beer wastes.

407 Follow the recommendations given for ponds (see items 284 to 297):

Taking care of your fish in a pen

408 You must always watch your fish to see that they are healthy and swimming strongly.

409 If the fish in your pen are not well, they will stop eating.

410 If this happens, stop feeding your fish for a day or two.

Figure 205

411 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.

412 If you find any dead fish floating in the pen, take them out right away.

413 Then stop feeding your fish and watch them very carefully during the next days.

414 If no more fish die you can begin to feed them after a day or two.

415 If fish continue to die take them out as soon as you can and ask your extension agent or fishery officer for help.

Taking care of your fish pen

416 A pen can be easily damaged. If this happens your fish may get out.

417 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.

Figure 206

418 If you find any places where your fish can get out, repair them right away.

419 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.

420 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.

Figure 207

Figure 208

Harvesting fish in pens

421 If you have used baby fish 8 to 10 centimetres long and have fed them well, they will be ready for harvesting in about six to seven months. At this time the fish will weigh 100 to 150 grams each.

422 If you want to harvest only some of your fish you can use a seine net with a large mesh.

423 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.

424 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.

Figure 209

Figure 210

425 If you want to harvest only part of your fish, you can also use a cast net or a lift net.

426 If you use a cast net or lift you will catch both big and small fish. Keep the fish that you want and put the others back in the pen.

Figure 211

Figure 212

427 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.

428 If you want to harvest all of the fish you can use a seine net with a fine mesh of less than 1 centimetre.

Note: 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.

Figure 213

Figure 214

Starting again

429 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 banks 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.

430 If you have harvested using a large-mesh seine net there will be many baby fish left in your pen.

431 In this case 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.

432 If you have harvested using a fine-mesh seine net you will have caught all of the baby fish.

433 You can hold the baby fish that you caught in a smaller pond or in a cage for small fish until you have finished cleaning and repairing the pen.

Note: The faster you can do all of these things, the sooner you will be able to raise another crop of fish.

434 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.



To locate your fish pens best, look for

To begin, prefer a small pen such as 10 x 10 m

Build your pen with cheap, locally available materials:

Prepare your pen properly:

Obtain baby fish and put them in your pen:

Feed your fish well with local by-products and wastes:

Harvest the fish after 6 to 7 months:

Keep the baby fish alive as you have learned earlier

Prepare your pen for another production season

Select the strongest baby fish and begin again

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