1. On a fish farm, live fish have to be handled on many occasions, for example during routine monitoring of their growth and health, transfer from one pond to another and final harvesting. This handling usually involves the use of various nets and other small pieces of equipment.
2. In the following sections you will learn more about nets and netting materials, how to make simple nets, how to choose others from specialized stores and how best to use them (see Section 8.1 to 8.4).
3. You will then learn how to estimate numbers of small fishes and how to measure the length and weight of live fish (see Sections 8.5 and 8.6).
4. Finally, you will learn how to handle live fish more easily by using certain chemicals to tranquillize them for short periods (see Section 8.7).
1. Most kinds of netting are made with netting twine, which nowadays is usually made with synthetic fibres. Single fibres are assembled into single yarns. Several such yarns are then either twisted or braided together to produce the netting twine.
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2. There are several kinds of synthetic fibres, with varying chemical compositions. The chart below gives the main trade names under which various kinds of netting twine are sold.
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3. The resistance of netting to sunlight, rain, wind and chemicals varies mainly according to:
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Relative resistance of undyed synthetic netting
to sun rays
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4. The strength of netting material depends on the yarn and thread thickness. Many conventional systems are used to designate the thickness of the individual yarns, of which the most common ones are the following.
(a) The tex system measures the weight (in g) of 1000 m of yarn. This is the international standard system.
Example
An individual yarn designated 23 tex weighs 23 g per 1000 m; a thicker yarn of 40 tex weighs 40 g per 1000 m.
(b) International titre in deniers is the weight (in g) of 9 000 m of yarn.
To convert it into the tex system, multiply the value by 0.111.
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a deniers x 0.111 = b tex
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Example
A 210-deniers yarn weighs 210 g per 9 000 m. The same length of a thinner 125- deniers yarn weighs only 125 g. The latter is equivalent to 125 deniers x 0.111 = 14 tex.
5. To designate the total thickness of the netting twine, two conventional systems are commonly used:
(a) R tex designates the weight (in g) of 1 000 m of netting twine.
(b) Runnage designates the length of netting twine per unit weight:
Note: equivalences are respectively:
Example
6. In addition to overall twine size specifications, the number and thickness of individual threads in the twine are sometimes also specified.
Example
If a netting twine R 280 tex is made of 12 threads of 23-tex size each, it is specified as a 23/12, R 280 tex twine. It can also be specified as a 210/12 twine made of 12 yarns of 210 deniers each.
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Note: if the netting twine is made of one yarn only, it is called monofilament. Its thickness is designated by its diameter (in mm). You can easily find the size of a monofilament by tightly winding a length of it around a pencil for example, so that each turn touches the other; measure the length (in mm) of 10 turns and divide it by 10 to find the thickness of the monofilament.
7. The size of diamond mesh can be measured in two ways, depending on convention:
Note: stretched size = bar size x 2.
Example
A 25 x 25 mm mesh may be designated either as a 25-mm mesh (bar size) or a 50-mm mesh (stretched size).
8. If hexagonal mesh is used, side is given:
9. When buying netting material to make nets yourself or when buying ready-made fishing nets, you should clearly specify the following.
(a) Identify the characteristics of the netting twine: kind of synthetic
fibre and thickness of the netting twine.
(b) Choose knotted or knotless: knotless material is smoother than
knotted.
(c) Indicate size of mesh: see above (bar or stretched).
(d) Indicate size of netting:
Example
The netting should be 80 m long (stretched); or the netting should be made of 2 000 meshes of 40 mm (stretched mesh) each equal to 80 000 mm = 80 m long.
Example
Netting depth should be 35 meshes; or netting depth should be 1.40 m, 70 bars of 20 mm (or 35 stretched meshes of 40 mm).
(e) Specify additional treatments, for example, dyed black or brown to increase resistance to sunlight or treated to include anti-oxidant and ultra- violet protectors.
10. To be useful for catching or holding fish, a piece of netting should be mounted on a support such as a rope or a metal frame. The netting can be mounted or hung in different ways so that the mesh of the netting will remain more or less open while the net is being used.
11. The way in which the netting is mounted on a support is defined by the hanging ratio E, which is the ratio of the length of the support to the length of stretched netting that hangs from it. The hanging ratio may be expressed either as a decimal number or as a percentage.
Example
12. The hanging ratio of a net may also be calculated by:
Example
From your net you measure d = 28 mm and s
= 20 mm. The hanging ratio of this net is
E = 28 mm � (2 x 20 mm) = 28 mm � 40 mm = 0.70 or 70 percent.
13. Once the net is mounted and stretched out normally from top to bottom, the actual shape of the mesh will vary with the hanging ratio, as shown in the figure below.
(a) a flat and wide mesh corresponds to a high value of E;
(b) a narrow mesh corresponds to a low value of E;
(c) a wide open square mesh corresponds to a value of E around 0.70.
14. The hanging ratio should be selected according to the type of fishing net to be made and to the body shape of the fish to be caught.
Note: if the net is not stretched out vertically, the netting will be more loosely set and the meshes will tend to be flatter. This is sometimes done deliberately, for example with a "tangle" net, but is usually avoided by using a weighted bottom rope.
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15. If you choose to mount a piece of netting with a hanging ratio E, the depth D (in cm) of the finished net when normally stretched may be estimated as D = (N x 2 x s) x [�(1 - E2)]
where N = number of mesh in the depth of the net;
s =
length of the side of one mesh, in cm.
Example
If N = 50 meshes, s = 1.9 cm and E
= 0.71, the estimated depth of the net will be:
(50 x 2 x 1.9 cm) x [�(1- (0.71)2)] = 190 cm x 0.70 =
133 cm.
1. A seine net is the most common type of net used on fish farms to harvest fish. It is a long net with ropes at each end and is pulled along the pond to collect the fish and then drawn into a circle to trap them.
2. A seine net consists of one or more pieces of netting material mounted:
3. These ropes are normally extended beyond the netting to form pulling ropes.
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4. The dimensions of a seine mainly depend on the size and depth of the ponds where it will be used. For best results, try the following:
Example
To seine a pond 5 m wide and 1 m deep in its deepest end, use a seine:
5. Do not buy large seines unless you have mechanical means to pull them through the water. A seine that can be used easily and manually by a few people should not be:
6. Remember that:
7. Mesh size essentially depends on the size of fish to be seined. You should use the largest mesh size possible: the smaller the mesh, the easier the nets become clogged with debris or plants, and the more difficult it becomes to pull the seine properly. Ideally the mesh should be small enough to hold the fish while keeping them from pushing their heads through and trapping their gills. If this "gilling" happens, they will quickly die.
8. Check the guidelines given in the chart below to select the mesh size best adapted to the size of fish you want to harvest from your pond.
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Mesh sizes to suit fish sizes
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9. The type of netting material will vary with the type of fish to be seined. Thus, for harvesting very small fish from nursery ponds, seines made from nylon cheesecloth or mosquito netting are usually preferable. For larger fish, one of the synthetic nettings described earlier may be used, with or without knots (knotless).
10. There are several kinds of seine. The two following designs are most commonly used.
(a) The seine is made similarly over all its length. It consists of a single rectangular netting panel.
(b) The seine is made of three parts:
11. You will learn how to make these two types of seine in paragraphs 20 to 27.
12. Remember that it is useful to standardize the seine nets on your farm, according to your ponds and the fish you have to handle.
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13. In addition to the netting material, you will need: rope, floats, sinkers, netting twine and a net needle.
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14. Rope can be made either of natural fibre (hemp, manila, sisal) or synthetic fibre (polyamide, polyethylene or polypropylene, see Section 8.1). Synthetic fibres are stronger and more resistant. Rope can be either twisted or braided. You will use rope for:
Note: before using a new rope for the first time, you should soak it in water and then, using a tree for example, stretch it well to take out all kinks.
15. Floats can be made of several materials such as:
16. Floats are usually either cylindrical or round, with a hole pierced through their centre. They are characterized by their floatability, the maximum weight (in g) they can support without sinking underwater, and the size of the axial hole, which should be slightly larger than the diameter of the rope on which floats will be attached (see the chart below).
Note: you can also make simple floats from plastic bottles or plastic bags filled with scrap polystyrene. Floatability (in g) is approximately 80 to 90 percent of the volume in ml.
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Characteristics of some commercial floats
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17. Sinkers are usually made either from baked earth or lead. In this last case, they are available as thin lead sheets or in the form of olives of various individual weights. You will need a total weight of sinkers equal to 1 to 1.5 times the total floatability of the floats. You can also use small stones, but they may break more easily.
18. Netting twine is needed to mount the netting on the various ropes. Use a synthetic twine slightly thicker than the twine of the netting.
19. A net needle is usually made of wood or plastic. You can buy it cheaply from a specialized store.
20. You can make a simple seine, taking the following characteristics as an example:
Note: if you need different characteristics, you can use the same method, adapting the calculations shown below.
21. First, make a series of calculations to design your seine net more precisely. Use the following procedure.
(a) Calculate how many meshes need to be mounted to make the net:
(b) Calculate how long the mounting ropes should be:
(c) Define the mounting interval M at which the netting will be attached to the ropes. Depending on the mesh size and the length and position of the floats, select M between 10 cm and 25 cm. In this particular case, let us take M =12 cm. The length of floats and sinkers will have to be less than this.
(d) Calculate how many meshes to mount within each mounting interval of 12 cm:
(e) Calculate how many meshes (N) you need in the vertical depth of your net so that its final depth D = 130 cm.
You learned above that D = N x 2 x s (in cm) x [�(1 - E2)] and therefore
N = D � [2 x s x �(1 - E2)] = 130 cm � [2 x 0.95 cm x �(1 - (0.71)2)] = 130 cm � [1.90 cm x 0.70] = 130 cm � 1.33 cm = 97.7 or 98 meshes.
(f) Calculate how many floats (for example for the 4 x 7 cm egg model in PVC with 63 g floatability) you will need, using one float every four mounting intervals: 58 intervals � 4 = 14.5 = 15 floats; total floatability is 15 x 63 g = 945 g. Select a float hole size slightly bigger than the rope diameter.
(g) Calculate how many sinkers you will need, using one sinker every three mounting intervals: 58 intervals � 3 = 19.3 = 20 sinkers.
(h) Total weight of the sinkers should be at least equal to the total floatability or 945 g. Use lead olives of about 945 g � 20 = 47 g each, for example 3.2 cm long with a 0.8 cm hole (50 g each).
22. Now, you are ready to make the first part of your seine net. The drawings below will show you how.
(a) Tie one end of an 11-m rope (6 mm diameter) to a tree or a strong post, about 1.5 m above ground level.
(b) From the other end, string all 15 floats on to the rope.
(c) Attach the end of the rope to another tree or post, about 1.5 m above ground level.
(d) Starting at 2 m from one of its ends, mark the rope every 12 cm for 58 intervals (i.e. 59 marks), using for example a 12-cm long piece of wood.
(e) Obtain netting twine slightly thicker than the netting material. Wind a good length of it on a net needle.
(f) Tie the end of this netting twine on to the rope at the first mark, making a special non-slip knot (see drawing) with the net needle.
(g) Slip one float along the rope next to this knot.
(h) Pass the needle through the first eight meshes of the netting material.
(i) Loop the netting twine fairly tightly past the float and tie it on to the rope at the second mark, making a normal knot with the net needle, as shown above.
(j) Pass the needle through the next eight meshes and tie the netting twine on to the rope as above, at the next mounting point.
(k) Repeat this process until you reach the last mark on the rope and the last mesh of the netting, remembering that:
23. When you have mounted the netting on the head line of your seine net, proceed in the same way to mount it on the foot line.
(a) From the same trees or posts, attach the two ends of an 11-m rope at 1.30 m (the depth of your seine net) below the head line.
(b) Starting at 2 m from one end, mark the rope at 12-cm intervals for 58 intervals.
(c) Tie the end of the netting twine on to the rope at the first mark, with a non-slip knot.
(d) Mount the bottom meshes of the netting on the rope exactly as you did above for the head rope, placing eight or nine meshes in each interval and attaching the twine to the rope at each mark.
(e) When the netting is mounted to the rope, add the sinkers:
Remember: the head and foot ropes have the same length and are mounted with the same hanging coefficient.
24. To finish your seine net, all you need to do is to place a 1.30-m side rope at each end, vertically.
(a) Attach the top end to the head rope next to the first knot.
(b) Pass the side rope from top to bottom through all side meshes.
(c) Attach the bottom end to the foot rope next to the first knot.
(d) Repeat this process for the other end of the netting.
25. Your seine net is now completed and ready to use.
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26. If you want to have a better seine for fishing nursery or fattening ponds, you can use a seine with a central bag. This kind of seine consists of three parts, a central section and two lateral sections called wings. The hanging ratio of the central section is much smaller than the hanging ratio of the wings. In addition, the depth of the netting used for the central section is greater than the depth of the wings. This forms a bag shape in the central section of the seine when the three pieces are assembled.
27. To make a seine with a central bag, assemble the three sections as shown below. Then, complete the seine as you were shown above. Specifications for three different sizes of this kind of seine are given in Table 21. You can also adapt these specifications to your own needs and to the materials available to you.
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Parts of a 23-metre central-bag seine net for nursery
ponds
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28. If the bottom of the pond is muddy, a standard foot rope will easily sink in, making it more difficult to pull the seine and harvest the fish.
29. In this case, it is best to replace the standard foot rope with a mud rope, which will tend to skid on the mud. There are several easy ways to make a mud rope.
(a) Replace the foot rope with a thick hemp rope, 25 to 30 mm in diameter.
(b) Alternatively, replace the foot rope with an assembly of many strands of standard rope bound together.
(c) You can also bind together a bunch of 15 to 25 lengths of synthetic twine to form a thick bundle; tie it along the standard foot rope.
(d) Another method is to cut a series of plastic or sacking (e.g. fertilizer bag) strips about 20 cm wide and 40 cm long. Fold them over the bottom rope so that they extend about 20 cm behind it while the seine is being pulled.
30. For smaller seine nets, you can use the net by itself. For larger ones it is better to attach a wooden pole slightly longer than the depth of the seine net to each of its side ropes.
(a) Use strong but not too heavy wood, about 5 to 7 cm diameter.
(b) Attach the foot rope of the seine to a notch, as near as possible to the bottom of the poles.
(c) Attach the head rope of the seine 20 to 30 cm below the top of the poles.
(d) Attach the side ropes of the seine to the pole at regular intervals, using strong netting twine.
(e) Join the ends of the head and foot ropes together with a strong knot. If necessary, attach an additional pulling rope to this knot.
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Large seine nets can be handled
more easily by attaching a wooden pole to the side ropes at each end  |
31. Usually a seine net is pulled by hand from both sides of the pond either from the banks or by harvesters who wade into the water along the banks. In deeper and larger ponds, a seine may also be pulled from boats or by using mechanical power onshore such as winches and tractors.
32. A small seine may be handled by as few as two people, one at each end of the net, who hold the net vertical with the wooden poles. If no poles are used, take care to keep the bottom rope slightly ahead of the top rope. With a muddy pond bottom or with a larger and heavier seine, additional strength may be needed. In this case, one person pulls at each end pole of the net while others assist by pulling at the extended end ropes. It is useful to have an additional person standing near the middle of the seine while it is being handled to help whenever necessary, for example when it gets stuck on some underwater obstacle.
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33. In small to medium ponds, you may harvest in the following way.
(a) Stretch the seine along the width of the pond at its deeper end.
(b) Enter the water at both sides at the same time.
(c) Hold the seine vertically in its hauling/fishing position.
(d) Pull the seine through the pond, its end poles close to the banks and the foot rope closely following the bottom.
(e) When you reach the shallow end of the pond, drop the poles and pull the seine by hand, keeping the foot rope on the bottom at all times. Pull the seine to collect the fish into a smaller space.
(f) There are then several ways to get the fish out of the net:
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34. Larger ponds can sometimes be seined only partially.
(a) Enter the water with the seine at one point of the bank. Stretch the seine along this bank in one direction. Then start seining by pulling this end of the net out from the shore in a circular path, to return to the bank on the other side of the starting point at which the net entered the water. Wading along the bank, bring back the end of the seine to this point.
(b) Alternatively, enter the water with the seine at some point along the bank. Stretch the seine across the pond width, to the opposite bank. Pull the seine along both banks, preferably towards the shallow end of the pond. After reaching the shore, move the two ends together to the harvesting point.
Note: to stretch a seine across water as fast as possible, use a floating stretcher made of a wooden frame fixed on two to four floats or inflated car tyre inner tubes.
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35. To work more efficiently, remember the following points.
(a) Do not disturb the area just before seining.
(b) If you plan to seine only part of a pond, attract the fish to this area by feeding them there for one or two days. Start seining about 30 minutes after the last food distribution.
(c) While wading and pulling the seine, try to keep your feet and legs in front of the foot of the poles.
(d) Keep the foot of the poles, and thus the foot rope, well down against the pond bottom, so that fish cannot escape under it.
(e) Keep the foot rope as taut as possible by pulling the side poles away from the centre of the seine.
(f) It is often better to pull the seine as fast as possible rather than to haul it too slowly.
(g) If the fish have a tendency to jump over the floating head rope, it may be necessary to keep the top of the net above the water. You can do this by:
Note: it is important that the two ends of the seine be pulled at the same speed, especially if there is a central bag. The latter should always be in a central position.To identify the exact position of the centre of the seine, mark it with one or two floats of a different colour or of a bigger size than the regular floats.
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36. Seines can be rather expensive pieces of equipment. To keep them in good condition you should take good care of them. Watch especially for the following.
(a) Protect them from direct sunlight and dry them in the shade.
(b) After seining, clean and rinse them well, removing all debris and fish slime, scales, etc.
(c) Protect them in a cool, dry place such as an open shed.
(d) Protect them from rats and mice, for example by hanging them on horizontal bars above ground level.
(e) Repair them regularly. Replace a section of netting it necessary.
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1. In addition to the seine net, several other kinds of fishing gear may be used on the farm. Some of the most common are discussed and shown here.
2. One of the most widely used nets in freshwater capture fisheries is the gill net, which may also be useful on a farm for selective harvesting of larger fish for marketing.
3. A gill net is very similar in overall shape and design to a seine net. Major differences are the following.
(a) The netting twine is thinner and usually made of synthetic monofilament such as polyamide monofilament with a diameter from 0.12 to 0.25 mm, depending on the opening of the mesh.
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Diameter of monofilament according to stretched
mesh size
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Fish caught in a gillnet
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(b) Mesh size is determined by the size of fish to be harvested. Fish should be able to pass through the extended mesh just beyond their gill covers but not further. When they feel caught and try to back out of the mesh, their gill covers should be caught by the mesh sides (thus the name gill net). Such nets are highly selective. The mesh size is calculated by measuring the body perimeter, or girth, of a few fish of the size you wish to harvest. Your gill net should have a stretched mesh size about a quarter smaller than the fish girth.
(c) The hanging ratio of a gill net should be around 0.50.
(d) When mounting the netting on the foot rope, make the mounting interval M slightly longer than for the head rope, for example M = 11 cm instead of M = 10 cm.
4. To fish with a gill net, you should set it in the pond, stretching it between two fixed points such as wooden posts. A gill net should be set either floating near the water surface, or midwater, or just above the bottom of the pond. It is usually best to set it perpendicular to one of the dikes during the night. As gill-netted fish will eventually die in the net, it is important to check and remove the gilled fish at least once a day.
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Choosing a mesh size for a gillnet
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5. To catch any size of fish, you may use a lift net, usually made of a square piece of netting, held horizontally, which bags slightly at the centre. It may be any size from 1 x 1 m up to 4 x 4 m. Smaller nets are sometimes made more circular in shape. Mesh size is usually small, about 10 to 20 mm bar size according to the fish to be harvested. The netting is mounted (E = about 0.50) along its four sides either on rope or on a stiffer material such as bamboo, heavy wire or a steel frame. The four corners are joined together above the centre of the net with twine, rope or bamboo poles, and a rope is attached. The other end of this rope is tied to a pole from which the net can be raised or lowered in the water. For larger nets, this pole may be mounted on a pivot stand with a counterweight (e.g. a stone or concrete block) to simplify lifting. Smaller nets are sometimes operated directly by hand, without a pole.
6. To catch fish, lower the net in the pond until it rests near the bottom. Throw some feed over the net to attract fish. While the fish are actively feeding, lift the net up quickly and smoothly. Lift nets normally work best in rather shallow water.
7. A team of people with small lift nets can also harvest fish by lining up across the pond and walking through to one end of the pond. Larger lift nets can be useful for keeping harvested fish alive either in an adjacent pond or in a water feeder canal (see Section 13.1).
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8. Where fish density is relatively high, such as at a feeding place in a pond or near a water inlet, it is relatively easy to harvest with a cast net.
9. A cast net consists of a flat circular piece of small-mesh netting heavily weighted along its periphery with sinkers. Usually a series of strings run from the outer edge through a central ring to join into a single pulling rope. As it is not very easy to make, you can buy your cast net from a specialized store.
10. Skill is required in the handling of a cast net: it should be thrown well opened and horizontally on to the water surface. It sinks rapidly to the bottom, and is closed by pulling on the central rope, entrapping the fish inside the net.
11. A cast net can be used either from the banks, in the water or from a boat.
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12. There are many different kinds of traps commonly used when fishing in lakes and rivers in the wild. Certain kinds may be useful for simple and regular harvest of food fish without disturbing the rest of the pond stock.
13. These traps are usually made with wood, plastic pipe, bamboo or wire frames, with netting, bamboo slats or wire mesh surfaces.
14. There are two main types:
15. You can either buy the traps in local fishing areas, or try some of the manufactured ones from specialized suppliers. You may also make your own, using similar net-making techniques to those described earlier.
16. You should make certain to follow these guidelines.
(a) Use an entrance size suitable for the size of the fish to be trapped.
(b) Choose a chamber size sufficient to hold the expected catch quantities.
(c) Check and, if necessary, rebait the traps. If the traps are not emptied regularly, the captured fish will start to escape.
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Split bamboo pot traps with funnel-shaped entrance
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Wooden frame and mesh chamber trap with V-shaped
entrance
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17. You may also harvest fish with a simple barrier made, for example, of a woven fibre mesh or split bamboo screen. Use screens with a slightly open weave or spacing to allow them to be moved through the water easily.
18. The barrier is slowly moved through the pond, as with the seine net, by a team of people holding it at regular intervals. Fish are concentrated in the shallow end of the pond from where they can be easily scooped out with baskets, nets or lift nets.
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19. A simple bag net can be used for capturing young fish safely and efficiently from your small fry and fingerling ponds. A simple design is given here.
(a) Obtain some smooth, synthetic, very fine mesh netting, such as mosquito netting, cheesecloth or curtain material, according to the size of fish you wish to handle. You need two pieces of netting 2.5 to 3 m long and 0.9 to 1 m wide (parts A and B) and one piece 1 x 1 m (part C).
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Front and bottom (A, B) 2.5 - 3 m x
1 m
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(b) Stitch parts A and B together, lengthwise, using strong twine.
(c) Cut two corner pieces from part C, as shown, and stitch them to each end of piece AB, forming a long V-shaped bag.
(d) Attach a 6-mm rope along the top and the bottom of the bag mouth, with a strong loop at each end.
(e) Fix a similar rope along the two sides of the bag mouth.
(f) Fix a 2-m wooden pole alongside each side rope and through the bottom and top loops, extending upward to form a handle.
20. To use the bag net, two people slowly push it in front of them, along the pond bottom, while keeping the mouth open with the handles inclined backwards at a 45� angle.
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1. Dip nets are commonly used on fish farms for handling and transferring small quantities of fish. They can be bought complete, assembled from ready-made parts or you can make the nets yourself.
2. A dip net is made of three basic parts:
3. The size and shape of dip nets vary greatly. It is important to keep the following guidelines in mind.
(a) Handle live fish using dip nets with relatively shallow bags. Their depth should not exceed 25 to 35 cm.
(b) Select a size suitable for the size of fish to be handled.
(c) To harvest fish from tanks or containers with square corners, use dip nets with a straight edge.
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4. You can easily make your own dip nets as explained below. Carefully select your netting material (see Section 8.1) such as:
1 Adapted from Woynarovich, E. The artificial propagation
of warmwater finfishes. Fisheries Technical Paper, 201, 1984, p. 132
5. You wish to make a round dip net with the following dimensions:
6. You will need the following materials:
7. Proceed as follows:
(a) Make the frame by bending the length of wire into a circular shape of diameter d. It will be easier if you use a series of bending guides such as strong nails set in a thick piece of board, as shown.
(b) Make the handle by tying together the two straight ends of the frame with some extra galvanized wire or a strip of rubber inner tube.
(c) Make the bag:
(d) Mount the conical bag on the frame with strong net twine. Place the stitched fold in front of the handle.
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2 Adapted from Woynarovich, E. The artificial propagation of warmwater finfishes. FAO Fisheries Technical Paper, 201, 1984, p. 133.
8. You can make somewhat larger, shovel-shaped dip nets using synthetic netting material and 6 to 10 mm galvanized wire. Check how much of each you will require in the chart below, where a is the width of the front edge, b the length, h the depth of the bag and I the length of the handle attachment.
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Composition of larger, shovel-shaped dip nets
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9. Proceed as follows
(a) Make the frame by bending the wire into the given shape and dimensions, using bending guides for greater accuracy.
(b) Make the bag:
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Netting dimensions
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(c) Mount the bag on the frame with strong synthetic net twine such as 23/18.
(d) Make the handle:
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10. To lengthen the life of your dip nets, particularly those that come into contact with bricks or concrete, you may:
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11. To handle large individual fish such as breeders, it is easiest to use a very deep circular dip net which is open at the bottom; the frame diameter usually varies from 20 to 30 cm, according to the shape of the fish to be handled. The netting is 60 to 80 cm deep, and the handle is kept very short.
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12. To use the open dip net:
Note: you can use a similar dip net for rapid handling of batches of smaller fish.
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1. Live fingerlings and larger fish are relatively easy to count quickly, in small batches, without excessive handling damage. But small fish such as larvae and fry are much more susceptible to damage, and it is best to count them using the following procedure.
(a) Fill a small tea strainer with the fish to be counted. Count them one by one to obtain count A.
(b) Obtain a large tea strainer, and determine B, the number of small tea strainers filled with fish needed to fill it.
(c) Estimate the number of fish C contained in the large strainer by multiplying A and B. This is your standard measure.
(d) Estimate the total number of fish present by counting the number of standard measures filled and multiplying it by C.
Note: to increase the accuracy of your total estimate, count several batches of fish in the small tea strainer and use the average of these counts for your estimates. A little practice will show you how many batches to check to obtain an average count that is more or less constant.
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Example
You have to count a particular group of carp fry before their transport. Fill a 5-cm diameter tea strainer with a sample. Count the fish one by one to obtain A = 175. Count how many of them you require to fill a 14-cm tea strainer: B = 9.5. This large tea strainer holds about 175 x 9.5 = 1662 carp fry. If you need to fill this 35 times with the fish to be counted, their total number is about 35 x 1662 = 58 170.
2. For larger fish such as fingerlings, adapt the above method in the following way.
(a) Take a large strainer and fill it with fish.
(b) Count the number of fish present in the strainer.
(c) Repeat such a count several times to obtain a good average count.
(d) On this basis, estimate the total number of fish present by multiplying the average count by the number of strainers filled.
1. For proper management, you will need to know on regular occasions how big your fish are and how fast they are growing (see Section 16.0). For this you will either have to weigh the fish themselves or measure their length and estimate their weight from it. The latter is particularly useful for medium to large fish.
2. Usually, you will measure the total weight of a certain number of fish. For live fish it is always best to weigh them in clean water, as quickly as possible.
3. There are several kinds of scales or balances commonly used to weigh fish:
4. Commercial weighing balances are better than spring balances, but they are also more expensive. Keep your balances in good condition, particularly your spring balances: do not apply heavy weight on them suddenly as this will wear the springs and give unreliable results.
5. It is best to hang a spring balance or a Roman-scale balance from a fixed and stable support such as a simple wooden frame fixed in the ground or a movable wooden or metal frame (a movable wooden frame is shown in paragraph 8). Commercial weighing balances should be used on a flat surface. Before use, check carefully that their base is horizontal.
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6. For best results, proceed as follows.
(a) Hang from or place on the balance a watertight container such as a bucket or a plastic bin.
(b) Fill this container with water up to a well-defined level mark, such as a groove or a fine painted line, typically one-third to one-half full.
(c) Weigh it and note this tare weight, W1 = container + water.
(d) Scoop up a batch of fish with a net. Let excess water drain off and place the fish in the container. Be careful not to put in too many fish and not to spill water.
(e) Note the weight, W2 = container + water + fish = gross weight.
(f) Net the fish out of the container to count them and quickly transfer them to a good water supply.
(g) Bring the water level in the container back up to the reference level, to have tare = W1 again.
(h) You are ready to weigh and count another batch of fish.
7. Calculate the net weight of each batch of fish as W3 = W2 - W1. If the number of fish in each batch is known, obtain the average weight of individual fish by dividing W3 by this number.
Example
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Calculating the weight of a batch of fish
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 W1 = weight of container + water |
 W2 = weight of container + water + fish W2 - W1 = W3 (weight of batch) |
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8. To measure the live weight of relatively large fish such as breeders, simply use a satchel or stretcher made for example of canvas slung from two wooden bars.
(a) Note first the tare weight of the wet satchel, W1, by hanging it from a spring balance or a Roman-scale balance.
(b) Carefully place the fish breeder in the satchel and obtain W2. Take down the satchel and place the fish back into water.
(c) Calculate the breeder's weight W3 = W2 - W1.
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A movable wooden frame with a Roman-scale balance
and a canvas sling for weighing larger fish  |
Calculating the weight of a larger fish
W1 = weight of wet sling   W2 = weight of wet sling + fish W2 - W1 = W3 (weight of fish) |
9. Length measurements are particularly quick and useful for medium to large fish and can be done with far less stress or damage to the fish. Two length measures are widely used on fish farms.
(a) Total length: from tip of snout to the furthest point of the tail fin: extend the fin by hand and crowd all the fin rays together. This method is generally used for fish reared in earthen ponds.
(b) Fork length: from tip of snout to the notch in the tail fin of fork-tailed fishes such as trout and carp. This method is useful for fish reared in concrete tanks where the tail end is generally damaged.
Beware: before using recorded length measurements, find out which kind of length is used.
10. The easiest way to measure fish length is to use a fish measuring board. You can make one simply of wood. Fix a flat ruler graduated in millimetres and centimetres on top of the horizontal board. Make sure the board is smooth. A coat of good waterproof varnish is useful.
11. To measure the length of a fish, place it flat on the horizontal board, with the tip of its snout against the front vertical board. Extend its tail fin well and measure the fork length or the total length on the graduated scale.
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12. Length and weight of fish can be related mathematically, and so weight can be estimated from length measurements. This relationship varies with the species and its environment. You may therefore establish this relationship for the various fish stocks reared on your farm under similar conditions.
13. To establish the length-weight relationship of a particular stock, proceed as follows.
(a) Obtain a sample of fish ranging in size from the smallest to the largest.
(b) For each individual fish, measure total or fork length and live weight. Note these values in a two column table.
(c) Using mm-square graph paper, draw two scales:
(d) In this graph, enter the data from your table above.
(e) Trace a regular curved line passing as close as possible to all the graphed points. This line represents the length-weight relationship for this particular species reared under a given set of conditions.
Example
You are farming Nile tilapia in medium ponds and feeding them regularly with good supplementary feed. Establish the length-weight relationship as follows:
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Plotting the length-weight relationship curve
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14. If it is difficult to weigh fish on your farm, use the length-weight relationship curve to estimate the average individual weight. Simply proceed as follows.
(a) Obtain a sample of fish, for example 50 individuals.
(b) Measure and note down the total (or fork) length of each fish (see paragraph 9). Be sure to use the same type of fish length as the one shown in your length-weight relationship curve.
(c) Note the shortest and longest of these, and define the length range as the difference between the two.
(d) Divide this length range into at least four classes of length, spanning not more than 3 cm each.
(e) In a table enter successively, as shown in the example:
(f) Calculate the average length Li for each length class by adding together the individual lengths observed and dividing the sum by the number of fish N within each class. Enter this average into the next column of the table.
(g) Using these average lengths, determine from your length-weight relationship curve the corresponding average weight Wi for each length class.
(h) Multiply each of these average weights Wi by the number of fish N present in its length class.
(i) Add these results together at the bottom of the column.
(j) Divide this sum by the total number of fish measured to obtain an estimate of the average weight of the fish sampled.
Example
From a particular pond you obtain a sample of 50 fish. After measuring their total length one by one, you observe that these lengths range from 14.4 to 21.6 cm. Proceeding as described, you progressively make the following table, using four length-classes of 2 cm each:
 Estimate the average weight of the fish living in the pond as W = 5903 � 50 = 118 g. |
1. Handling fish such as large breeders particularly when they reach a certain size often requires the use of specific chemicals (anaesthetics) which have a sedative effect (anaesthesia).
2. Several such chemicals may be obtained from specialized suppliers. Four of them are relatively common.
(a) MS 222 (tricaine methane-suIphonate) is a very mild and safe anaesthetic from which fish easily recover. Now thought possibly to cause cancer in people it should be handled with care. It is very expensive for general use.
(b) Quinaldine (2-4-methylquinolin) is a smelly colourless liquid. It is a toxic chemical which should be handled with great care and stored safely. It is insoluble in water but soluble in acetone and other organic solvents (see benzocaine below). Recent studies indicate possible long-term human health effects. Although it is quite effective and has been widely used, it should be avoided if possible.
(c) 2-phenoxyethanol is milder and less effective than MS 222 but is far cheaper. It also helps to control bacteria and fungi, and so may reduce the after-effects of handling. For dosage calculation, remember that 1 ml/I (by volume) of this chemical is equivalent to 770 mg/I (by weight).
(d) Benzocaine (ethyl-4-aminobenzoate) is a white crystalline powder similar chemically to MS 222. Insoluble in water, it first needs to be dissolved in acetone or ethanol (ethyl alcohol) before being mixed in water. You can make a stock solution of say 100 g/I which can be mixed by volume. Benzocaine is similar in most respects to MS 222 but is usually much cheaper.
Example
Benzocaine is used at 20 to 30 mg/I for short handling (see chart). First prepare a stock solution containing 100 g/I by dissolving 100 g of benzocaine powder in 1 litre of acetone or ethanol. Keep the solution in a dark bottle.
To prepare a 30 mg/I benzocaine anaesthetic solution in a tank containing 200 l water, you require 200 l x 30 mg/l = 6 000 mg or 6 g of benzocaine.
if 1 litre of your stock solution contains 100 g benzocaine, to obtain these 6 g you need to mix (1 litre x 6 g) � 100 g = 0.060 l or 60 ml of stock solution with 200 l water to obtain the 30 mg/I benzocaine solution.
3. Always use these chemicals with caution, not endangering the life of the treated fish.
(a) Make preliminary tests (see Section 15.3) to determine the safe and effective dosage of the particular chemical to be used. This varies greatly with many factors such as fish species, fish size, water temperature and water chemical composition.
(b) Keep a watchful eye on your sedated fish at all times. Their opercules should keep moving, even if more slowly than usual.
(c) As soon as the opercules of a particular fish stop moving, quickly transfer it to fresh, well-oxygenated water for recovery.
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Indicative dosages of chemical tranquillizers
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