NATURAL PROPAGATION
1. Several methods exist for the breeding or propagation of cultivated fish. Choosing among them depends on the reproduction biology of the species, on local environmental conditions and on the facilities available. These methods may be grouped into three categories: natural propagation; semi-natural propagation; artificial propagation.
2. For natural propagation, males and females are placed together in a breeding area such as a small pond or an enclosure where they spawn naturally. This method is usually used, for example, to produce tilapias cheaply.
3. The successful breeding of certain species may require some environmental manipulation such as the inflow of new water and a sudden rise of the pond water level for the African catfish Clarias, the presence of grassy vegetation as egg collectors nests for common carp, or the presence of artificial nests for American and European catfishes, Ictalurus and Silurus.
4, For semi-natural propagation, the fish (usually the females only) are first given one injection of chemicals, such as a pituitary gland* extract, which will trigger spawning. Males and females are then placed together in a specially prepared breeding area such as a small grassy pond or an enclosure where spawning takes place. The fertilized eggs are usually collected and reared under improved conditions, either natural or artificial.
5. For artificial propagation, the females are given one or more injections of chemicals which regulate the final ripening of dormant eggs in the ovaries. As soon as the eggs are ripe, they are stripped from the females. The males are usually also injected. Eggs are artificially fertilized with sperm obtained from the males and reared under controlled conditions.
6. Each of these propagation methods is controlled by a series of environmental factors, as shown in the chart below for common carp, for example.
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NATURAL PROPAGATION
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SEMI-NATURAL PROPAGATION
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ARTIFICIAL PROPAGATION
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Relative importance of environmental factors on
common carp propagation
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7. The technology for the propagation of fish has been described in detail in several manuals such as for example:
8. In the following sections, you will learn more about some of the major practical aspects of fish propagation by natural and semi-natural methods:
1. To propagate your fish successfully, you need healthy and sexually mature individuals of both sexes. These will be your broodstock.
2. There are two ways to obtain such broodstock.
(a) Fish may be captured from natural waters with fishing gear (see Sections 8.2 and 8.3) and transported alive to your fish farm (see Chapter 13). There they are stocked either in broodstock ponds until they become sexually mature, or if they have been captured during their spawning season and are already mature, in storage ponds.
(b) Fish breeders can be raised on the farm itself, which makes it easy to improve your stock progressively through careful management.
3. The farm ponds where you plan to raise your broodstock should be well adapted to the fish species. Well-regulated temperatures and well- oxygenated water (see Sections 2.4 and 2.5) will contribute to successful rearing. There should be an ample supply of natural food available (see Section 10.1), well adapted to the particular feeding regime of the fish breeders. During the maturation period, protein-rich food ingredients (see Section 10.2) should be supplemented if necessary. Adapt the stocking density of the breeders to the food supply available but keep it, in all cases, relatively low. Select your broodstock ponds so that they are easily accessible but secure from poaching.
4. It is best to keep a relatively young broodstock made of small to medium- sized fish, depending on the species. Small breeders of common carp and Chinese and Indian carps weigh from 0.5 to 2 kg, while medium breeders weigh from 2 to 5 kg. Small breeders of tilapias weigh from 0.15 to 0.25 kg, while medium breeders weigh from 0.25 to 0.40 kg. Such fish produce more eggs of a better quality, with a better food utilization efficiency.
5. When the breeding season comes, broodstock should be carefully selected. Only fish that are ready to spawn should be used. Select fish with the following characteristics.
(a) Males should release a few drops of milt when the abdomen is slightly pressed.
(b) Females should have a swollen and protruding genital opening, reddish/rose in colour, and a well-rounded and soft abdomen, showing that the gonads are developed up to the dormant stage.
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6. When there is risk of male aggression (e.g. in the case of catfish) or uncontrolled spawning (e.g. tilapias and common carp), the two sexes should be kept in separate ponds after selecting them.
7. If you wish to propagate your fish semi-artificially or artificially, you need a supply of the chemicals (or hormones*) which play a decisive role in ovulation, the final ripening of the dormant eggs. These chemicals, the gonadotropins*, are produced, accumulated and stored in the pituitary gland of the fish, also called hypophysis*, while they become sexually mature.
8. This small pituitary gland can be found in the upper part of the fish head, on the ventral side of the brain. It is quite easy to collect such glands from mature fish, store them for later use if necessary and extract the gonadotropin hormones from them, as you will learn in the next paragraphs.
9. It is very important to collect pituitary glands from suitable fish, to be certain that these glands contain enough gonadotropins to be effective. Select fish with the following characteristics:
Note: most pituitary glands collected commercially belong to large fish such as common carp and salmon. They may be used for the propagation of other species also.
10. The pituitary gland can be collected from a freshly killed fish in two ways: by cutting open the head or by removing the pituitary gland with a drill. It is easier to work on the head of a fish if you have a wooden frame to hold it firmly in place when cutting or drilling.
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11. To cut open the head, proceed as follows.
| (a) Remove the top part of the skull with a saw or a strong sharp knife. | (b) Locate the pituitary gland in the brain mass. (c) Remove it carefully with forceps. |
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12. It is often easier to use a drill, preferably an electric one, and a special drilling head which you can have made in a local workshop. Proceed as follows.
| (a) Select or make a drilling head that is the correct size
for you (see note in para. 13). (b) Locate the drilling point on the top point of the skull, as shown. |
(c) At the drilling point, press a wooden guide piece (with a hole drilled) against the skull. | |
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| (d) Drill through the top of the skull, the brains and the base of the skull, down to the mouth cavity. |
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(e) Take out the drilling head together with the small plug of bone and tissue it contains.
(f) Remove this plug of material from the drilling head.
(g) Cut off the upper half of this plug and carefully separate the brain tissue lying at the base of the skull from the lower half.
(h) Pick out the pituitary gland from this with forceps.
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13. You may either use this gland immediately or store it for later use (see paragraph 14 on).
Note: to ensure that the gland comes with the drilled core, select the diameter of the drilling head according to the size of the fish: appropriate diameters are 2.5 cm for fish up to 1 kg, 4 cm for fish up to 3 to 4 kg, and 5 to 6 cm for larger fish.
14. When you plan to keep the pituitary glands for a certain time, a simple way to treat them is as follows.
| (a) As you collect the fresh gland from the fish, place it in a small bottle containing acetone. This chemical will start removing water and fat from the gland. It will harden and preserve the gland and the hormones it contains. | (b) Collect together, in the same bottle, all glands obtained on the same day. | |
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(c) About every eight hours replace this acetone with new acetone, over a total period of 24 hours. Then, drain out all acetone.
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(d) Dry the hardened glands on blotting paper.
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(e) Put the dry glands in small glass containers and press them down with a ball of fine cotton wool. Cork the container tightly and seal it with wax or other sealing material such as paraffin. Label it, indicating the origin of the glands and the date of their collection.
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(f) Keep these sealed containers either in a plastic bag or in a desiccator or air tight storage jar, in the presence of a desiccating chemical such as silica gel or calcium chloride.
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15. Acetone-dried pituitary glands can be safely stored this way for several years, without the need for refrigeration, as long as they are kept free from moisture. You may also keep fresh glands in a freezer.
16. The gonadotropic hormones which are injected to induce ovulation and/or spawning are extracted from the pituitary glands, either immediately on collection or after a certain storage period. Proceed as follows.
(a) Obtain the number of glands required on the basis of the dose of hormones to be used according to specialized manuals.
Example
You are planning to use dried pituitary glands with 34 females (average weight 2 kg; two injections each) and 17 males (average weight 1.5 kg; one injection each); you will require the following quantities of dried glands:
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34 fish x 2 kg x 0.3 mg/kg =
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20.4 mg |
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34 fish x 2 kg x 3.5 mg/kg =
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238.0 mg | ||
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17 fish x 1.5 kg x 2.0 mg/kg =
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51.0 mg | ||
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309.4 mg | |||
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31.0 mg | |||
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340.0 mg |
If the average weight of one gland is 2.5 mg, you will require 340 mg � 2.5 mg = 136 pituitary glands.
(b) Prepare a 0.65 percent salt solution (saline): dissolve 6.5 g of common kitchen salt in 1 litre of clean water. You may use either boiled and filtered water or distilled water. Use clean glassware and mix well. Keep this solution in a corked glass bottle.
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(c) Calculate how much salt solution you require, according to the manuals referred to above.
Example
For the above example, you may require:
Add 10 percent to these calculated volumes of salt solution to compensate for losses.
(d) Grind the required number of glands into a mash or fine powder in a mortar.
Example
For the first series of injections, you require 20.4 mg + 10 percent =
20.4 mg + 2.1 mg = 22.5 mg of glands,
or about 22.5 mg � 2.5 mg = 9 glands to be finely ground.
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(e) Measure the required volume of 0.65 percent salt solution and pour it in the mortar, over the gland mash/powder. It is best to use a syringe to measure such small volumes.
Example
For the first series of injections, measure 34 ml + 10 percent = 34 ml + 3.4 ml = 37.4 ml of the prepared salt solution.
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(f) Mix the saline and gland material well to extract the gonadotropic hormones from the gland tissue into the liquid.
(g) Let it settle, or better still, use a small hand centrifuge to separate the upper (supernatant) liquid from the pieces of gland material.
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17. Selected breeders should be prepared for hormonal injection. Use the following procedure.
(a) Early in the morning, select from the storage ponds the male and female breeders to be injected that day. Check carefully that they are ready to spawn, as described in paragraph 5.
(b) Bring them close to the spawning area or into the hatchery. Store them in a net or a tank, in a good water supply, keeping the sexes separated.
(c) Prepare all the necessary materials: suitable quantites of hormone extract, clean working area, recovery tank, pond or net for the broodstock. Once these are ready, prepare the fish.
(d) If possible, anaesthetize the fish to be injected in 50 to 100 l of a suitable chemical solution (see Section 8.7); if the individual size of your fish is not uniform, treat them successively in batches of similar sized fish.
(e) At all times, watch the sedated fish closely and, if necessary, transfer them back to well-aerated water quickly.
18. Then proceed with the injection of the hormonal extract.
(a) According to the live weight of each breeder, fill a syringe with the exact dose of pituitary gland extract necessary.
(b) Take the breeder out of the water with a dip net (see Section 8.4). Place the fish gently over a soft smooth surface such as a piece of rubber foam and immobilize it inside the net.
(c) Slowly give the injection at a 45� angle as follows:
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19. Immediately after the injection, replace the fish in well-aerated water. Depending on the propagation method used, the fish may be:
Example
Proceed as follows for the following types of fish:
20. The time required for the initially developed eggs to become fully ripe (also called the ovulation period) is usually expressed as the number of hours required at prevalent water temperatures. This is measured in degree-hours (dh).
21. During the ovulation period, you should measure the water temperature (in �C) at the end of every hour and add these values together progressively. When you approach the number of degree-hours required (see following example), your fish are getting ready to spawn.
Example
You give the decisive Injection at 20.00 h to common carp broodstock. You measure the water temperature of the holding tank every hour as follows.
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By the next morning at 6.00 h, you reach a total of 239.6 degree-hours, very close to the amount required in this particular case, 240 to 260. You should start watching your fish carefully.
22. The number of degree-hours required varies with:
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The number of degree-hours required varies with:
 
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23. Experience should help you to improve your estimates of the number of degree-hours required in each particular case.
Example
The number of degree-hours required under various circumstances to obtain full ripening of eggs within the optimum temperature (Table 24), may vary as follows:
1. Induced spawning may take place in a variety of breeding enclosures such as:
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Various breeding enclosures
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2. It is always best to use more males (M) than females (F) in
the breeding enclosure, for example (1 F/2 or 3M) or
(2 F/3 M) or (3 F/4 M), to ensure successful propagation.
3. The total number of breeders to be stocked in the breeding enclosure depends on the size of the enclosure and on the size of the breeders.
Example
In a 2-m2 breeding enclosure, you may stock common carp as:
4. As soon as spawning has occurred, the breeders are usually taken out of the breeding enclosure and stored in a pond for later use.
5. The method for the collection of fertilized eggs after spawning depends on the type of eggs produced by the fish.
(a) Non-sticky eggs such as the floating eggs of Chinese carps and the semifloating eggs of Indian carps can be easily collected either within the breeding enclosure if it is not too large, using a fine-mesh dip net for example, or outside it by filtering the draining water through some fine netting material.
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(b) Sticky eggs such as those from common carp should be collected using egg collectors as discussed in paragraphs 6 to 8. After spawning, these collectors with the adhesive eggs are usually transferred to another enclosure where incubation and hatching take place (see Section 9.3).
6. For collecting sticky eggs, you can simply use fresh, washed, submerged aquatic weeds, such as Elodea, Hydrilla or Najas or the roots of floating plants such as water hyacinth (Eichornia crassipes). Introduce into the spawning area a quantity of weeds roughly double the weight of the female fish to be stocked or enough to cover loosely the water surface.
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Collecting sticky eggs on the roots
of floating plants
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7. You can also make for yourself a type of egg collector (known in Indonesia as a kakaban), inexpensively, using local materials. Proceed as follows.
(a) Obtain fibrous vegetal material (about 40 cm long) such as palm tree leaves, thin branches of pine trees or long dry grass.
(b) Obtain bamboo poles, 4 to 5 cm in diameter and about 1.2 m long. Split them in two lengthwise; if you do not find bamboo, you may use wooden poles instead.
(c) Place the vegetal material within the split bamboo. Fix it well by tying the bamboo halves tightly back together.
8. Such egg collectors are joined together in a sort of raft which can be fixed slightly off the pond bottom by using two long poles tied in place with cord and stakes. Egg collectors can also be supported on a light frame of poles 20 to 30 centimetres above the pond bottom. You will need about 5 m2 of egg collectors per kilogram weight of female breeder.
Making a simple collector for sticky eggs |
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A Prepare 40 to 50 cm lengths of fibrous
vegetal material
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B Bamboo 1.2 to 1.5 long and
4 to 5 cm in diameter split lengthwise
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C Place the vegetal material between two
pieces of split bamboo and tie them together  |
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1. As soon as fertilization has taken place and the eggs come in contact with water, their development starts. It proceeds by stages until the fish larvae hatch. This is the incubation period.
Note: the stickiness of certain fish eggs such as common carp, African catfish and substrate-spawning tilapias, also develops as soon as these eggs come into contact with water. It becomes strongest 30 to 60 seconds thereafter.
2. Incubation encompasses three main phases of egg development.
(a) Egg swelling phase (see 1, 2 and 3 below): the dry fertilized eggs take up water to become hydrated, and the perivitelline space develops. In the egg kernel, the animal pole appears on top of the yolk. The eggs increase in volume (swollen eggs), and become larger than the dry eggs (see Table 22).
(b) Cleavage and germ development phase (see 4, 5, 6 and 7): the one-cell animal pole subdivides into 2, 4, 8, 16 and 32 cells successively, all arranged in one layer. Further cell divisions take place to produce a multilayered blastoderm (see 5) at the end of the morula stage. This stage is then followed by:
(c) Embryonic development phase (see 8): the fish embryo develops around the yolk. Head and tail are formed. Eyes become visible. Movements increase until the egg shell is broken and hatching occurs.
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Notes: when a ripe egg falls into the water, it becomes round (1)
and within a short time begins to swell (2). Water seeps in between the shell
and the cell kernel (animal pole and yolk mass), creating the perivitelline
space. If it is fertilized, the egg soon starts to develop.
By the time the swelling Is complete (3), the animal pole of the kernel rises
as a small hillock on the yolk mass. It divides (4) and redivides,successively
reaching the morula (5), the blastula (6) and the gastrula (7) stages.
The embryo finally appears with tail, head and eyes (8). It develops into a
larva, breaks out of the shell and hatches (see various larvae
stages).
3. Successful egg development requires a favourable environment for the species concerned:
4. You should be able to distinguish bad eggs from healthy eggs (see the chart below). If possible, separate and remove the dead eggs, as they may become a source of fungal and bacterial infection in the live eggs (see Section 15.2).
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Characteristics of good and bad eggs
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Note: fish eggs are particularly sensitive to disturbances during the cleavage of the animal pole and until the end of the morula stage. It is best to start incubating your eggs as soon as possible after fertilization. Do not transport eggs while they are sensitive.
5. The time required for the fertilized egg to develop into a fish larva varies mainly with fish species, water temperature and dissolved oxygen content of the water. It is commonly expressed in degree-days, the sum of the daily average water temperatures over the incubation period, similar to the method applied for determining the ripening of the eggs in female breeders (see Section 9.1).
6. You can estimate the length of the incubation period at temperatures close to the optimum range from Table 23. Remember that the higher the water temperature within this range, the shorter the incubation.
Note: try to avoid temperatures which are too high as they may cause deformities and poor egg survival; it is usually better to incubate more slowly for better quality.
7. Several types of incubators are available. Select the type best adapted to your needs according to:
8. Table 24 should help you to select the most appropriate type of incubator. You will learn more about each of these incubators in the next paragraphs.
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9. Single enclosures of cloth or screened boxes can be used to incubate sticky eggs collected on egg collectors or floating plants (see Section 9.2).
(a) Cloth incubators (hapas) are preferably made of synthetic material, with an overall size of about 2 m x 1 m x 1 m deep. The mesh size should be fine enough (about 0.5 mm) to retain the hatching larvae inside. The top of the cloth bag is mounted on thin ropes. Top and bottom corners are attached to bamboo or wooden poles well fixed into the bottom of a shallow body of water.
(b) Screened boxes consist of a strong wooden frame to which metal or plastic mosquito netting is attached to form a square or rectangular box. Supports are provided on the two sides to hold one or more egg collectors horizontally; such boxes are placed in shallow standing water during incubation.
10. To reduce predation from birds and frogs, you should place a cover over the enclosure.
11. It is best to transfer the egg collectors with sticking eggs on them into these simple enclosures during the first sunset period after spawning. If spawning takes place at night, such transfer may take place eight to ten hours later, at dawn. Transfer distances should be short and the sticky eggs should be covered with wet cloth.
12. After the larvae hatch, remove the egg collectors from the enclosures, clean them thoroughly and dry them. You may reuse them several times.
Note: the incubator below can easily be moved from place to place since the wooden frame joins the poles into a single unit. However, a series of poles can also be driven into a pond bottom without a wooden frame (see paragraph 14) and used in this manner.
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Screened box incubator for two egg collectors
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13. You can easily make yourself a double-walled cloth incubator by adding an inner bag to hold the eggs.
(a) For the outer bag use very fine mesh (0.5 mm), cotton or synthetic cloth to retain the hatched larvae. Good dimensions are 2 m x 1 m x 1 m deep. Attach this bag to four corner-poles fixed in the bottom of a standing water body.
(b) For the inner bag use larger mesh (2 to 2.5 mm), such as round-meshed nylon mosquito netting: good dimensions are 1.5 m x 0.8 m x 0.5 m deep.
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14. Fix this cloth incubator in shallow water. Spread the fertilized eggs uniformly over the bottom of the inner bag. When hatching occurs, the larvae fall into the outer bag, leaving behind the egg shells and dead eggs in the inner one. Remove the latter promptly, as soon as the hatching is complete. A cover helps reduce predation from birds and frogs.
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Double cloth incubator
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15. A simple trough 1 to 3 m long, 0.3 to 0.5 m wide and about 0.3 m deep, made of wood, fibreglass or metal, may be used to incubate various types of eggs. The water enters at one end and flows out at the other. Flow should be sufficient to supply oxygen and carry away wastes, but should not be too strong. Position eggs as follows.
(a) Sticky eggs may be spread in one layer over the lower, outflow half of the trough bottom.
(b) Sticky eggs can also be incubated on egg collectors placed in the trough.
(c) Non-sticky eggs, such as trout eggs, should be heavy enough not to be swept away by the flowing water. Ensure that the current is spread evenly across the trough and is not too strong. Spread the eggs away from the inflowing water, in one layer.
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16. The simple trough incubator shown above can be greatly improved by adding one or more inner trays to hold fertilized eggs. An inner tray should be built so that:
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17. You can easily make this type of incubator from a wide-mouthed container such as an earthen jar or a plastic bucket.
(a) About 5 cm above the bottom, place a screen to retain non-sticky eggs within the container.
(b) Inside the top of the container, fix a fine-mesh sieve to retain the hatched larvae inside the container.
(c) In the middle of the container, fix a vertical tube (about 1 cm inside diameter). It should extend a little below the bottom screen. Connect it at the top to the water supply.
18. The fertilized eggs are introduced into the container. The water flow is regulated so that the eggs are kept suspended and turn very gently during their first two phases of development. As they reach the eyed embryo stage, the water flow is slightly increased.
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Diagram of an incubator made using a 10-litre bucket
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19. In many places it is possible to get 1- to 2-l light plastic bottles, often used for soft drinks. These can be easily adapted as incubators.
20. Clean the bottle thoroughly and cut off the base. Cut a notch in the top edge for draining water, if required.
21. Set up the bottle as follows:
22. The bottle can also be used without a bottom inflow pipe.
(a) Keep the screw cap in place, and run the incoming water supply through a narrow tube from the top of the bottle, placed so the bottom of the tube lies near the bottom of the bottle. A bottom screen is not usually needed.
(b) Set the bottle up in a suitable frame, as before. In this case, the bottom of the bottle can rest directly on the base of the frame, as it does not have any pipes below it.
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Diagram of incubators made using 2-litre plastic
bottles
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23. These incubators are arranged so that water enters through the bottom, flows upward and then out at the top. The flowing water keeps the egg mass suspended and in a slow continuous motion, through nearly the entire water column.
24. There are two basic shapes of funnel incubators:
25. Funnel incubators are the most commonly used devices for nonsticky eggs, and for eggs from which the sticky layer has been removed. They can be made of various materials including:
Their water capacity varies from 6 to 10 l to more than 100 l.
26. These incubators should be kept vertical:
Note: In some instances you may wish to provide for draining water away from funnel incubators. This can be done by using a draining gutter such as a plastic pipe or a piece of bamboo cut in half lengthwise.
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27. Commercial funnel incubators, complete with water supply and draining systems and a holding frame, can be purchased from specialized fish farm suppliers. However, these may be too costly. To save money you can make your own funnel incubators using locally available materials, as shown below.
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Commercially made cylindrical/conical funnel
incubators
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1 Adapted from a design by E. Woynarovich, in Elementary guide to fish culture in Nepal, p. 80-81. Rome, FAO, 1975.
28. You can make a simple funnel incubator using strong plastic sleeving or sheeting. A list of materials is given below for an incubator of 8- to 10-l (15 to 16 cm diameter) capacity.
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29. Proceed in the following way.
(a) Cut the plastic sleeving as shown to produce two cones.
(b) If sleeving is not available, use flat plastic sheet (for example, building-grade PVC sheet) cut as shown.
(c) Sew the edges of each cone with fine (for example 3 mm) stitches, leaving an edge of 1 cm. You can also use staples, but they will not last so well.
(d) Tuck the two funnels inside each other so that the joined edges are on opposite sides.
(e) Hold the wire ring tightly between the two pairs of plastic sheets at the upper part of the funnel. Stitch it on by hand, with a strong thread.
(f) Fold the double stitching at the top of the funnel over the wire ring to the outside, and make four holes through the plastic under the ring, using for example a hot nail. Attach two loops of string from them.
(g) Place the shower head inside the funnel, at the bottom of it. Fix it there with the clamp. Alternatively, stretch a piece of coarse cloth over the end of the inflow pipe.
(h) Push the end of the inflow pipe on to the stem of the shower head.
(i) Tightly attach with string the bottom of the funnel around the pipe. Hang the funnel with string or rope from a support, using the two loops. Your incubator is now ready to be used.
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A Cutting material for a funnel
from plastic sleeving
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B Cutting material for a funnel
from plastic sheeting
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C Roll the plastic into a cone
and sew
the edges together  |
D Insert one cone into
the other with
the seams on opposite sides  |
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E Insert the wire ring between
the inner and
outer cones and sew the top edges together  |
F Shower head and clamp to be
used
in the bottom of the completed funnel  |
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G Diagram of completed funnel
incubator
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1 Adapted from a design by E. Woynarovich, in Elementary guide to fish culture in Nepal, p. 82-83. Rome, FAO, 1975.
30. You can make a better quality funnel incubator using plastic and cloth. When finished, this type of incubator is suspended vertically in water. A list of materials is given here for both 35-litre (31 cm diameter) and 81-litre (47 cm diameter) capacities.
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31. Proceed in the following way.
(a) Prepare the conical bottom part of the incubator: cut out either a small or a larger piece from the plastic sheet, using a 45-cm radius as shown. Sew together with 3-mm stitches the two straight sides of the plastic piece, leaving a 1-cm edge.
(b) Prepare the cylindrical part of the incubator:
(c) Attach the top cylindrical part to the bottom conical part: sew them together with several rows of 3-mm stitches. Take care that the seam is placed on the outside.
(d) Fix the top ring to the linen cloth: turn the upper edge of the cylinder outward and around one of the wire rings. Stitch it together strongly by hand.
(e) Fix two pairs of loops around the ring to hang the funnel vertically.
(f) Stitch 15-cm long pieces of ribbon around the middle part of the incubator, where the conical and cylindrical parts meet. Attach the second wire ring around the incubator with these ribbons.
(g) Make the water flow regulator:
Note: instead of a double-funnel water regulator, you may also use a shower head as described for the previous incubator.
(h) Add the outlet for collecting larvae (particularly important for floating eggs):
(i) Your incubator is now ready to be used. Hang it vertically within a water container from an overhead support with the two string loops. Attach a weight to the bottom. Connect the water inflow pipe to the water supply, through a flow-regulating valve. Regulate the water level within the container so that it constantly remains a little below the top ring of the incubator.
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A Cutting the plastic cone for a small size incubator
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B Cutting the plastic cone for a medium size incubator
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C Diagram of assembled plastic and
cloth funnel incubator
 Note: the dimensions shown on the illustration of the funnel incubator above may vary slightly when actually assembled. |
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D Attach one wire ring at the
top of the incubator and
the second wire ring at the top of the plastic cone  |
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E Assemble and install the water
flow regulator
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F For a water flow regulator
you can also use a shower head and clamp
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G Install a rubber patch and plastic
tube larvae collecting outlet
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H Operating funnel incubators hung
vertically in a water filled container
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32. Earlier in this section, you learned how to distinguish good fertilized eggs from bad eggs. As soon as egg development reaches the stage when the blastopore is closed (see paragraph 2 above), you should if possible remove the bad eggs from your incubator. At later stages, you may also wish to remove rubbish such as egg shells as the eggs hatch.
33. This is relatively easy to do with non-sticky eggs reared in a trough or transparent funnel incubator by using a siphon tube. Proceed as follows.
(a) Obtain a piece of tubing (1 to 1.5 cm in diameter) at least twice as long as the height of the incubator.
(b) Introduce one end of this tubing into the top layer of water of the incubator.
(c) Keeping the other end of the tubing well below the level of the firsf end, suck water out of the tube to start the siphon, delivering water into a bucket.
(d) Siphon the bad eggs out one by one by directing the top end of the siphon tube at them in the incubator.
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34. The number of eggs that can be reared in an incubator depends on the individual size of the swollen eggs (see Table 22). It is also related to the surface area available for spreading the eggs over one layer as well as to the usable volume of the incubator.
(a) In the inner bag of a double cloth incubator (1.5 x 0.8 = 1.2 m2), 50 000 to 100 000 eggs of Indian carp can be uniformly spread.
(b) In trays of a trough incubator, you can spread 400 to 600 trout eggs (5 to 4 mm diameter) per 100 cm2 of area or about 50 000 per square metre of water area.
(c) In a 10-l bucket incubator, you can incubate about 100 000 trout eggs.
(d) In a funnel incubator, it is best to fill the conical portion only to 30 to 50 percent of its capacity, especially when rearing more delicate floating eggs.
35. During incubation, you should regulate the water flow according to the development phases of the fish eggs (see paragraph 2) as follows.
(a) During egg swelling, the water flow should be minimum. In funnel incubators, it should be just sufficient to keep the egg mass moving very slowly through the bottom part of the incubator.
(b) From initial cleavage to the end of the morula stage, the water flow should be slightly increased. If a funnel is used, the egg mass should be kept moving slowly through the bottom part of the incubator.
(c) From the blastula stage until the eyes of the embryos become visible, increase the water flow slightly again. In a funnel, keep the egg mass moving a little faster and in a slightly larger volume of the incubator.
(d) From the eyed-embryo stage until hatching, the water flow should be further increased, to ensure that the oxygen requirements of the growing embryos are met. In a funnel incubator, keep the egg mass moving faster and within about half the volume of the incubator.
Example
Consider these water flow requirements for various kinds of incubators.
(a) To incubate trout eggs in a trough:
(b) To incubate Chinese carp eggs in a sieve-cloth incubator, you will need:
(c) To incubate common carp eggs in a 7-l conical/cylindrical jar, you will need about:
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Water flow in 7-litre conical/cylindrical jar
incubators for carp eggs in various stages of development
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9.4 Larval rearing |
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1. Larval rearing takes place from hatching until the time when the larva:
The larva then becomes what is usually called early fry. How do fish larvae develop2. During this period, fish larvae mostly develop their alimentary and respiratory organs. The yolk sac provides the material for growth and development. Its size slowly decreases until complete reabsorption, soon after the larva has started to ingest external food. The length of this period therefore greatly depends on the initial size of the yolk sac, which varies from species to species, and on the rate of larval development, which mainly varies with water temperature. For each species, there is an optimal thermal range, similar to the one defined for egg incubation (see Table 23).The length of the larval rearing period is similarly defined in degree-days (dd), as is the incubation period (see Section 9.3). This in general corresponds to three to four days for most warmwater fish, but is rather longer for cooler-water fish. Example Length of the larval rearing period at optimum temperature (a) Cold water:
(b) Warm water: |
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3. For successful rearing, fish larvae require the following:
4. Fish larvae also need a rearing device adapted to their specific behaviour:
Note: larval behaviour may change during development, and you may have to change the rearing conditions accordingly.
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Larval behaviour
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5. There are several types of devices for rearing fish larvae under different conditions.
(a) Enclosures in standing water should preferably be used for actively swimming larvae. Select their position in the water so as to take advantage of wind-induced currents (see Section 2.5) which favour a good oxygenation of the water in the enclosures. Their wall mesh should be regularly cleaned to maintain a good water exchange. Such enclosures described earlier (see Section 9.3) are, for example:
(b) Enclosures in running water are also better for actively swimming larvae because of the danger of dissolved oxygen deficiency within certain parts of the enclosures. You can easily make one as follows:
(c) Troughs with running water, also better for active larvae, are similar to those described earlier (see Section 9.3). If used during incubation, the inner trays are removed after hatching is complete, and the larvae are reared in the troughs; the water flow should then be maintained at about 3 to 5 I/min.
(d) Small tanks can also be used for active larvae. These are usually a circular or a rounded-square shape of glass-reinforced plastic (GRP), corrugated metal or frame-and-liner type (a wood or metal frame with a reinforced PVC, or butyl liner). Typically 1 to 4 m in diameter, 80 cm deep, with a working depth of 10 to 50 cm, the tanks have a flow rate of about 1 to 2 I/min per kg of larvae. Up to 5 kg of larvae can be held per cubic metre.
(e) Funnel rearing devices are best for passive larvae, which either lie down or only move in a limited way. These devices are available from specialized firms or can be made locally from various materials, such as plastic/cloth or fibreglass (see paragraph 7). Watch especially for two points of special importance for larval rearing:
6. Such funnels function as described earlier for egg incubation (see Section 93), but usually their individual size is larger.
7. You can make a larger funnel for rearing larvae similar to those shown in Section 9.3. When finished, this kind of funnel is also suspended vertically in water. A list of materials is given below for a funnel of 200-litre (60 cm diameter) capacity.
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Note: with the exception of the conical bottom part, which you are shown how to cut and make in the adjoining illustrations, this rearing device is otherwise made in the same way as the better quality plastic and cloth funnel incubator (see from para. 30, Section 9.3).
  Note: you will need a water flow of about 12 to 15 l/min circulating through this size of rearing funnel. |
8. A freestanding larval rearing funnel for up to 500 000 larvae may be easily made, from fibreglass, using a mould. For a 200-l model, use the dimensions shown. At the top of the funnel, place a light frame supporting a filtering ring made of synthetic sieve material with 0.2 to 0.4 mm mesh. Glue the lower part of this netting to the funnel wall, using water-resistant cement, about 10 cm below the rim. Support the funnel on a strong tripod made of welded iron bars, protected from rusting by several coats of paint. Make sure the top of the funnel is level, allowing water to run out all around the rim.
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A fibreglass larval rearing device
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9. When using this large rearing funnel, it is essential to have a constant water inflow of 12 to 15 I/min and to clean the filtering ring regularly:
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Dimensions for making a 200-litre fibreglass funnel
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Clean the filtering ring regularly
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10. With some larvae, such as Chinese carps, it is possible to transfer the freshly hatched larvae directly into a rearing device, by the swimming out technique. For this purpose, you may use three plastic/cloth funnels (see Section 9.3 and above), kept in a water container. When hatching starts, proceed as follows.
(a) Lower the water level to just below the sieve-cloth ring of the funnels, so that their larval outlets reach just under the water surface of the container.
(b) Place a larger larval rearing funnel at a lower level, between the two incubation funnels.
(c) Connect the larval outlet of each of the incubation funnels to an inlet built in the rearing funnel, above the sieve-cloth ring.
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Two incubators in place with water at the normal level  |
Lower water level to just below the sieve-cloth,
place a larval rearing device between the two incubators and connect the tube outlets as shown  |
11. If you do not use such a method, you will have to remove the larvae from the incubation device in another way.
(a) When hatching is complete, siphon the larvae gently out of the funnel incubator. Be careful to place the bottom end of the siphoning tube under water in the receiving containers (see Section 9.3, para. 33).
(b) At the end of the larval rearing period, either similarly siphon the early fry out of the rearing device or use a fine-mesh dip net.
1. Now that you have seen the basic units, the jars, troughs, tanks, etc. used in the hatchery, you can consider how to plan the hatchery itself to accommodate the production you require. For your hatchery you will need:
2. Before deciding on the full details of the design and layout of your hatchery, there are several points you should consider further.
(a) Examine the site or possible sites available. Check on the area available and its overall topography (see Topography), the potential water supplies (see Water 4) and consider the possible construction features you might need (see Construction, 20).
(b) Assess your overall hatchery requirements to determine the type, size and layout of the hatchery, considering the following factors:
(c) Put together a stock and production plan based on the type of information given in the earlier parts of this section: broodstock, egg and fry numbers and sizes, types of equipment, their size and holding capacity, their water requirements, etc. See for example Table 25a to estimate, according to the species involved and the type of equipment to be used:
(d) Consider this stock and production plan further, using information on broodstock availability, spawning time, hatching time, and first feeding and early rearing period, to make an outline schedule of operation (see two examples below). Each sequence of spawning, hatching, fry rearing, etc. can be defined as a batch or cycle.
(e) Look at the timing of such a batch or cycle, and consider whether you will operate with a single batch each season or year, or with several batches (possibly of a different species). Thus once a tank or incubator has finished one batch, it may be cleaned out and used for the next one. In this way, you will get more production from the facilities you have set up. Prepare your final schedule of operation.
(f) Check approximately to make sure you will have enough space and water supply for the intended production. The chart below can be used for guidance. Modify your plans if necessary, either by changing the target production or by changing the number of cycles. Thus you may get the intended production with smaller facilites but a greater number of cycles.
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SCHEDULE OF OPERATION
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SCHEDULE OF OPERATION
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3. Once you have estimated the number and size of hatchery units involved, you can plan the position and layout of the hatchery. To do so you should consider the following.
(a) You can also use the chart on this page to estimate the overall internal areas required for holding and spawning tanks, hatching equipment, water supplies, access space, storage and work areas and possibly office space. This will be the main hatchery unit. In most cases it is housed in a single building, although in larger and more complex systems several buildings may be used, such as a broodstock unit, a hatching unit and a service/storage unit.
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Space and water flow requirements for hatchery
facilities
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Note: space required includes piping, stands, etc.; remember to allow for access - for hatchery staff, nets, bins, etc., as well as storage space and access to pipe valves for maintenance.
(b) Estimate the external areas required for broodstock ponds, conditioning or spawning facilities, early rearing facilities and access roads. Identify any areas that should be near the main hatchery unit, and which may possibly be partially sheltered and/or enclosed.
(c) Review the site(s) available, and identify a suitable and convenient position, sufficient to accommodate the areas required but reasonably compact in layout, and permitting the easy arrangement of water supplies, access and security. Identify the location of the hatchery building itself.
(d) Sketch out the actual layouts within this chosen site area; in particular sketch out the Internal layout of the hatchery building, bearing in mind:
Note: see below for a plan and a cross-section of a simple carp hatchery of 30 m2.
Example
Typical hatchery dimensions for the production of common carp early fry (see previous example in table 25b) or tilapia fingerlings are summarized in the following chart.
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CROSS-SECTION AA
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4. The arrangement of the hatchery water supply is particularly important for its successful operation. Some points to consider are listed here.
(a) You may use gravity-fed river or stream supplies, pumped water from rivers, ponds, lakes, etc. or groundwater sources.
(b) Make sure there is sufficient water of the quality you require during the periods of hatchery use. Check water quality (Chapter 2).
(c) It may be necessary to clean the water supply with screens and/or fifters (see Section 2.9). For ground/well water supplies, you may need to aerate the water to make sure it has sufficient oxygen (see Section 2.7).
(d) If water supplies are only available at certain times, you may need to provide storage. Check the daily use of water by the hatchery and allow for the number of hours or days of storage required. Concrete tanks or earthen ponds can be used (see Chapter 4,Water, 4). If it is found on land higher than the hatchery, the water could be supplied by gravity; otherwise it could be pumped.
Example
A hatchery uses 10 I/min of water for hatching, 10 m3/day for
exchanging water in broodstock tanks and 5 m3/day for washing, cleaning,
etc. The total daily water use is: (10 I/min x 60 min x 24 h/1 000) + 10 m3/day
+ 5 m3/day = 14.4 m3/day + 10 m3/day + 5 m3/day
= 29.4 m3/day. If storage for 10 days' operation is required, this
will be equivalent to 29.4 x 10 = 294 m3; this amount could for example
be provided with a pond of approximately 300 m2 x 1 m average water
depth.
(e) Water quality requirements may be different for broodstock, their final ripening and fry rearing. You should normally supply the best quality water to the ripening tanks and egg hatchery areas. However, you may be able to save on water treatment, or use a different water supply for the other parts of the system. If necessary, you may also be able to reuse hatchery water in fry or broodstock ponds.
(f) You may need another small domestic water supply for washing, cleaning, etc. Wastes from this should not drain into ponds, as they may contain detergents, chemicals, etc.
5. Water supplies and drainage for the external ponds can be arranged as for normal farm ponds, using canals, pipes, and sluices (see Chapter 8, Construction, 20).For external tanks and internal areas of the hatchery, a piped water system is usually used. Its main features are as follows.
(a) Water is fed by gravity or pumped either directly to the main supply pipe, or more commonly, to a header tank, which provides short term storage and regulates flow to the other tanks. It is usually large enough for at least 10 minutes of flow. You need a 1 m3 tank for a flow of 100 I/min. In some cases a storage tank can be used as a header tank (see hatchery design above).
(b) The header tank is usually set with its base at least 1 m above the hatchery tanks. It is typically 0.5 to 1 m deep; it may be set up on a wall, on a timber stand, or on the roof of the hatchery. The tank can be made of concrete, timber (with a polythene or butyl liner), fibreglass or plastic. In some cases, domestic water supply tanks can conveniently be used.
(c) The main supply pipe normally runs from the header tank to the hatchery tanks. From this are run the secondary supply pipes, serving small groups of tanks, and individual supply pipes to the tanks themselves. These pipes are usually made of PVC or ABS. They are sized according to the flow rate required and the head available from the header tank (see Section 3.8, Construction, 20). Table 26 shows some typical dimensions which you may use for guidance.
6. Once you have reviewed the layout and water supply details, you can proceed to the construction and development of the hatchery. Some points to be considered are listed here.
(a) Select a suitable type of building for the hatchery itself. This could be a simple shade structure, a prefabricated building, a conventional local structure or a heavier brick/concrete building. Make sure foundations are suitable.
(b) Check on the overall costs of the site development (for example see Section 12.8, Construction, 20), external areas, hatchery building, hatching facilities and water supplies. Amend quantities and/or specifications if costs do not match your budget. Check that costs per fry or fingerling produced are reasonable by local standards.
(c) When you decide to go ahead, prepare for construction. Proceed according to the guidance given in Chapter 12, Construction, 20.
(d) Remember to plan the timing of construction to allow for factors such as availability of local labour, wet or dry seasons, the supply of broodstock and the timing of the breeding season.
(e) Finish off the hatchery area with suitable access roads, security fences, drainage, and other services if required.
(f) Drainage from the hatching units and tanks is normally either by channel, usually brick or concrete, or by drainage pipe, set in or on the hatchery floor; see Section 3.8 and Section 8.2,Construction, 20, for details on sizing. Remember to allow for tanks or incubators being emptied. Make sure the drainage areas can be easily cleaned and disinfected.
