The production of marketable fish begins with the stocking of fry or juveniles into a rearing environment that assures optimum and rapid growth to allow harvest in the shortest possible time. The fish farmer has to obtain adequate number of young fish to meet his production goals. These fish can come from wild capture. However, there is little or no guarantee that adequate numbers can be captured and stocked in the time corresponding to optimum production conditions. The fish farmer then naturally turns to other means of obtaining his stock. By simulation of the conditions necessary for the reproduction of his fish, the farmer can spawn the fish in captivity. Successful spawning is only the beginning, however, the eggs must hatch, and these reared successfully to fry stage. These stages – spawning, hatching, and early rearing are like a steeple chase which the farmer must win. The race course is well filled with obstacles, for example physico-chemical quality of water such as available dissolved oxygen, feed of the proper nutritive composition and particle size, low resistance to diseases, and so on. A good appreciation of all these factors is needed for successful production of fish.
The ultimate goal of the fish farmer is to produce fish that meet both his needs and the market demand. Through artificial propagation, the farmer can select for desirable characteristics such as fast growth, resistance to disease, etc. By hybridization and selection, these goals can be achieved if the farmer dedicates enough time and patience.
As stated in the introduction, fish farming begins with the stocking of fry, and these can come from the wild or be produced on the farm. Whatever their origin, they are indispensable and the means of obtaining them influences directly farm production. If supplies are erratic, there will be interruptions in other farm activities; if the supplies are regular, farm production may be maximized. The cost of the fry can vary considerably and may be an important factor in overall production costs. It any event; a good supply of fry is essential for successful fish farming.
If one looks at production of eggs, larvae, and fry that is carried out on the farm itself, the major problems are obtaining a sufficient number of eggs, a good hatching rate of these eggs, and good survival and growth of the larvae obtained. In nature, there is very high mortality at these stages, and a lot of attention and effort is needed to overcome these difficulties.
To practice reproduction and fry production, a certain investment in equipment, infrastructure (ponds, tanks, water supply), and trained labour is needed. These costs can be considered a part of overall production costs of marketable fish.
The proportion of total cost in producing saleable fish that is met by seed production should be kept always in mind, and efforts made to find new methods of seed production that are less expensive, hopefully increasing the profitability of the whole farm.
Definition of terms:
Larvae: hatchlings with yolk sack until first feeding.
Fry: free swimming fish – from first feeding until complete development of somatic organs.
Juveniles: sometimes referred to as “fingerlings”, but generally fish of a small size but adequate for stocking, not sexually mature (Coche and Bianchi, 1979).
For certain farmed fish, it is not yet possible to understand and control all the stages of reproduction, and farmers depend on natural supply. In most cases, these fish are brackish water species and their biological cycle includes migrations into a different environment, either moving to the sea, or into fresh water. Thus to simulate the conditions needed for natural reproduction, this would mean a complete change in the environment. In general, fish can only be reproduced on the farm if conditions that correspond to natural spawning can be closely approximated. The cost of creating a completely marine habitat or freshwater habitat in the midst of a brackishwater situation would in most cases be prohibitive.
Collecting eggs or fry from the wild was the first method used in obtaining stocking material. This is still occuring for species for which the spawning behavior is not controlled or not well understood, or for which the costs of artificial propagation are too high or where fry in large quantities is easily obtainable. Means of collection differ with species. Three important species of fish will be examined.
Throughout West Africa, the catfish (Chrysichthys spp) is very popular and in Ivory Coast represents an important species in brackishwater lagoon aquaculture. As it is relatively difficult to reproduce this fish in captivity, eggs are collected from the wild by placing bamboo traps in areas where this fish (C. walkeri) spawns. The traps are placed on rocky bottoms, in deep water (4 – 6 m), by fishermen and the catfish spawns inside the bamboo tube. This method allowed the collection of up to 2000,000 eggs even outside of the peak spawning periods. The eggs are transported to the hatchery, and placed in artisanal Zoug jars made from empty plastic bottles. Two or three days after hatching, the larvae are transferred into tanks made of plywood and covered with polyester resin measuring 2.4 × 0.4 × 0.3 m at a. density of 5,000 to 10,000 per tank.
After about 10 days, the yolk sac is resorbed. After two and half months, they reach an average weight of 750 to 800 mg. During this time, they are fed on a mixture of cattle brain and crab eggs, finely ground which forms a colloid suspension in the water easily accessible as food for all fish. After the first month, the feed is modified and composed of ground crabs, wheat flour (15% of the weight of crab) wheat bran (5%) and fish meal (20%). The fry are then transferred after 2½ months to circular concrete tanks with a running water system. The concrete tanks are 3 m in diameter with a central drain. After the 4th month, the fish are fed with a mixture richer in flour: crabs, fish meal 30%, wheat bran 20%, wheat flour 20%, copra cake 10%. Percentages are in relation to weight of crab. This feed is again changed during the 5th or 6th month to a feed composed of ⅓ crab, and ⅔ agricultural by-products (fish meal 35%, wheat bran 20%, wheat flour 25%, palm kernel cake 10%, copra cake 10%. The fish are then transferred to enclosures when they achieve an average weight of 4.5 grams. One fourth will have reached already a weight of 8 g, when half of the population will weight 4 grams, and ¼ 3 g. The fry are reared in enclosures until achieving a size of 50 g, which takes a further 6 months.
The obvious cost and time necessary for procurring catfish juveniles through this method is considerable, and further experimental work is necessary (Ledoux, 1979; Bailly, 1981).
Larger fish are sometimes captured from the wild. In Nigeria, the catfish are captured using hook and line with soap as bait. The catfish are injured, and care must be taken to avoid mortalities. It is easy to capture large numbers of catfish using traps to collect fish attracted to tilapia cages (Campbell, comm. pers.).
The milkfish (Chanos chanos) is found in warm waters of the Red Sea, and the Indian and Pacific oceans and is one of the most important brackish water species cultured particularly in S. E. Asia. In Africa, it is only found on the east coast. The fish reproduces once or twice a year in coastal marine waters of 25 m in depth. Each female spawns 1.5 to 1.7 million eggs, and sometimes up to 5 million. The larvae drift to the coastal waters near the estuaries with a temperature of about 23°C, the eggs taking about 24 hours to hatch. Salinity isn’t too important (10 to 32 ppt) and the fish are captured where phytoplankton is abondant. When the fry have achieved 1.3 m in length, the yolk sac is completely absorbed and they begin looking for food. At this life stage, their diet is a mixture of blue-green algaes and the associated organisms (bacteria, protozoans). The diet will change later on. Artificial reproduction has been achieved in the laboratory, but not yet on a large scale, and thus almost all of the milkfish larvae are captured from the wild. For example, in 1970, in Taiwan only, 207 million fry were captured from the wild (Huisman, 1979).
According to the regions, capture techniques and material can differ. In Java, the best collection zones are on the north side of the island, and the fry are abundant 7 months out of the year. The best time for capture is at the spring tide, and the three days before and following the full moon. Sandy beaches, a slight slope, and clear, calm water are ideal.
The fish are often concentrated in the zone near estuaries where the influence of freshwater is high. They will often take refuge leeward of sand bars. If no sand bar exists, the fishermen will build a rock wall perpendicular to the beach into the ocean, or build artificial shelters using palm branches.
As the fry are collected, they are transferred into basins or similar containers, and the sea water is immediately diluted with freshwater. This will kill off many of the other organisms inadvertantly collected, and also acclimate the fry to brackishwater. Other species of fish are also captured, and these must be sorted out. This needs a certain level of expertise and experience on the part of the fisherman.
In the Philippines, the fry are captured by using seine nets or traps. In Mauritius, the fry are captured usually in July using a fine mesh seine net, and are transferred into acclimatization tanks. The mortality is usually high in the tanks (Bardach et al, 1972; Iversen, 1976).
Mullet species are cultivated around the world, and have a large distribution. Mullets belong to the genera of Mugil or Liza, many species are known. The artificial reproduction is possible, however there are still many instances where fry available in important quantities is collected from the wild. The mullets reproduce in the sea, and will thrive well in the sea, in estuaries, in brackishwater, and sometimes migrate to freshwater. When they are sexually mature, they form schools, and the reproductive products are released into the sea where the fertilization occurs. The eggs are pelagic, and hatch in the following two days. After hatching, the larvae begin to swim near the coast where they usually arrive in mass after 2 months. At this point, they have reached a size of 25 mm. On the coast, they form small schools and begin to move towards the estuaries. The capture is usually done as they enter the estuary, at low or rising tide, using seine nets, deep nets, etc. The fish are euryhaline and have a good resistance to abrupt changes in salinity (Bardach et al, 1972; Iversen, 1976).
Among the shrimps used in aquaculture, there are two main groups, the Penaeidae and the Caridea. Their biology and life cycles are different. For an example of the Penaidae, Penaeus notialis is widespread and often farmed. Spawning is carried out in the sea and the female is recognized by well developed ovaries and coloration. She molts shortly before spawning. The male places a spermatophore on the female theca which is kept until egg deposition.
The various stages of post egg development occur in the sea. After roughly 3 weeks, the young, termed post-larvae, begin to migrate to estuaries. The larvae are attracted by light, and are often collected by seine nets and deep nets at night. In other instances, the farm is filled with water known to contain large numbers of postlarvae. Larvae can also be collected by using bunches of palm or coconut branches lashed together and placed on the bottom of the lagoon. After a few days, the traps are surrounded by a fine mesh net and the branches removed.
For the group of Caridea, the principal species raised is Macrobrachium rosenbergii. This species spends its life in brackish-water or freshwater, and migrates to the estuaries or lagoons for spawning. Either gravid females or post-larvae are captured. Again traps, deep nets, etc. are used for capture.
In certain cases, it is not possible to reproduce fish species in captivity however; but the capture of fry from the wild presents some problems. Sometimes the sorting out of species at the fry or fingerling stage is difficult, which will result in a mixed stocking population of different species – some fast growing, some slow growing, some predators and some competitors also – into the pond. Identification keys for fry and fingerlings are not often available. Diseases and parasites affecting wild fish can be introduced into the pond.
On the other hand, fry collected in nature have usually already passed the most critical stages of their life cycle where the mortality is highest, and have good survival rates when stocked. If reproduction and larval rearing is carried out on the farm, each problem at each step of the whole process must be solved. This may be expensive if infrastructure, equipment and skill is needed. For some species, as tilapia, however, not much is needed.
However, if using natural stocks, their abundance and demand will vary the price, and some years, the fry availability may be very low. Optimum collection periods will vary from year to year, and the ponds may not be properly prepared when the fry becomes available.
In the two cases, it is important to have a sound understanding of the biology of the fish species either to duplicate reproductive conditions on the farm, or to know when and where to collect the fish. The necessary time spent in studying the fish, in collecting the necessary data on reproduction, etc, and then procuring the necessary equipment for capture may in the end be as expensive as artificial reproduction.
There are many different forms of controlled reproduction, but all achieve the result of fertilized eggs. In some cases, the adult fish are captured from the wild, in other cases, the fish come from the farm itself.
Perhaps the most important fish where the mature broodstock are captured from the wild are the atlantic or pacific salmons, where the gravid females and ripe males are captured on their spawning migration into freshwater rivers. These fish will die after a single spawning. The females are captured, either the eggs are stripped, or in some cases the ovaries removed and the eggs obtained through killing the fish, and the eggs are fertilized with milt of males captured at the same time. In Europe, gravid pikes (Esox lucius) and catfish (Siluris glanis) are captured in their natural spawning habitats. Gravid sturgeon females are collected for preparation of caviar. If broodstock is not to be sacrificed, care must be taken not to injure the usually large fish.
Actually the only fish species with potential future for aquaculture in Africa where the broodstock is captured from the wild for breeding in pond is the mullet. Hybridization of mullets (different species) have been achieved with that technique. Collection of broodstock or adult fish is always an alternative, if no selected strain is available or if inbreeding is to be avoided.
Generally, the broodstock used in artificial reproduction will come from the farm. In this way, the farmer can choose and select his fish in view of genetic improvement. The sexes are usually kept separate, and the broodstock feed with a diet optimum for egg production. The size and age of this broodstock will of course vary with the species and geographic location, temperature and the management techniques of a particular farm.
Broodstock must be prepared for spawning, and this is why they are usually kept in special broodstock ponds. Sexual maturation is a process, usually directed by external factors (temperature, photoperiod, water depth, salinity, pH etc) with a corresponding internal change brought about by hormonal activity.
Proper health of the fish can be assured by adequate feeding of the broodstock for a sufficient time before spawning occurs. Separation of fish according to their sex for some time prior to spawning is generally beneficial for Clarias spp. and for carp.
Maturation of the gonads needs a certain “amount” of temperature, which can be quantified as hour-grades, or day-grades, i.e. the number of hours or days at a given temperature needed to accomplish gonadal maturation. Length of photoperiod is also important but not for equatorial species, where the change of length of days according to the solar year is minimal. Light conditions of the breeding environment play also an important role, some species breed in day light, other in the dark. Catfishes prefer muddy water than brightness. Water conditions must also be optimal as regards to D.O., pH, etc. Stocking densities should be low, to assure e some quietness to the fish, avoiding stress and disturbances, that may affect the fish and the maturation of the gonads (Woynarovitch et al, 1981).
Feeding of brood stock is important, although not much information is available on the specific needs for feeding broodstock. Most of the research has been done on the effects of a lack of particular nutrients in the diet. Carotencides are present in maturing ovaries. They are mobilized from the muscles and transferred and concentrated in the ovaries. Deficiencies of Vitamin A or carotenoides in the female will decrease the chances of survival of the eggs and the larvae (Shehadeh, 1975). During maturation, there is an accumulation of non saturated fatty acids in the ovaries and increase of level of protein and dry matters. If sufficient feed is not given to the broodstock, there will be some cellular modifications in the hypohysis and a reduction of secretions in gonadotropins.
This shows the need for proper feed during the period prior to the breeding. Some aquaculturists recommend to avoid too fatty diets, as fat accumulates in the tissues and affects the breeding (Shehadeh, 1975). Feed given to breeders after spawning brings about deposition of fat and protein in the tissues which will be used for subsequent vitellogenesis. In this context, the function of anabolic estrogens, of prolactine and thyroxine in the process of vitellogenesis is not well understood and would need further studies. Estrogens present in the feed of females has a negative effect on the development of the ovaries and can cause sterility if the level of estrogens is high enough. There is need to be careful with some of the oilcakes, which contain naturally high levels of estrogens (Sundararaj, 1981). One should check the levels of natural anabolic estrogenes present in some food stuffs as this may interrupt the spawning process. In general, the effects of feed on broodstock are little understood, and more work needs to be done in this area. As a general rule, feed should be a composite using as many different ingredients as is feasible to avoid any unknown deficiencies or excess that may be present if fish are fed only one ingredient. Protein, lipids, calcium, phosphate should be high enough to allow for egg yolk production.
The reproduction of a species has to assure the survival of the species (see Fig. 1.1). The development of the oocytes is a long process that occurs over a relatively long period of time, and spawning occurs when the moment is the best for the survival of the fry, abundant food, shelter etc (see Fig. 1.2). Reproduction is controlled by internal factors (intrinsic) and environmental factors (extrinsic). When the fish is about to breed, proper environmental conditions are not enough to get the process of spawning started. There is need for the female to encounter a male in the place suited for laying the eggs. Often the behaviour of male and female has to be coordinated in such a way that male and female of the same species come close to assure fertilization of eggs by the male’s milt. The adjustment of the respective movements to this effect is fixed in the specific pattern of the breeding behaviour. Sometimes this behaviour is quite elaborate, as for tilapias, and is extended beyond the time of laying the eggs to the time the fry is able to sustain on their own and provide to their own needs. After spawning occurs, the eggs may be left or abandoned, or there may be some degree of parental care provided for by the male and/or female (see Fig. 1.3).
Several methods can be used in Aquaculture:
Fig. 1. 1. Adaptation for the survival of the species – Reproduction (After Woynarovich and Horvath, 1980)
Fig. 1. 2. Spawning places of freshwater fishes (After Woynarovich and Horvath, 1980)
Fig. 1. 3. Principal types of parental care in fishes (After Woynarovich and Horvath, 1980)
Hormonal control of reproduction is controlled and relayed by the excretion of successive hormones by various centres of the fish. The environmental stimuli are received by the brain, which transmits to the hypothalamus which transmits to the pituitary, which transmits to the gonads, which produce the sexual hormones (see Fig. 1.4). Control can be obtained at three levels, the hypothalamic, pituitary, or gonadal level (see Fig. 1.5) (Harvey and Hoar, 1979). The technique used most frequently is the use of pituitary extract (hypophysation).
The pituitary gland is extracted, and can be conserved in either, alcool, or glycerine. Various techniques have been developed and can be found in the literature, particularly for Indian carp culture (see Fig. 1.6 and 1.7). The whole pituitary gland can be used fresh or dried; it is ground, placed in a suitable solvent, centrifuged, and the supernatant is injected into the female fish and sometimes the male (Fig. 1.8). Actually, purified gonadotropins from pituitaries are also used and commercialised.
Ohter hormones in use are mamalian hormones, which are available in most pharmacies. Human chorionic gonadotropin (HCG) is most frequently used, although some success has been possible using luteinique hormone (LH) or FSH, follicule stimulating hormone.
The gonadotropins, isolated with varying degrees of purity from fish, are glycoproteins, the composition is similar to mamalian LH. However, the researchers are not yet in agreement whether there exists only one or two gonadotropins, as is in the case of mamals. Probably there are two; one hormone controlling vitellogenesis, and another for the final maturation and ovulation. Vitellogenesis begins with a mobilization of fats, which are synthesised into glycophosphoproteins in the liver, and are deposited into oocytes. This action is supported by the sexual hormones produced by the ovary. The gonadotropins act on the follicule tissue of the ovary to produce an ovarian steroid, which in turn stimulates the maturation and ovulation. The estrogenes themselves stimulate the synthesis of vitellogenine, a glycolipophosphoprotein in the liver. The mechanisms of action and retroaction of hormones guiding reproduction are then very complex, and can vary between species. This is why some hormones will work in some species and not in others.
Both pituitary extract and mammalian hormone are widely used. The dose used by fish breeders vary considerably and there is no standard dose. For a pituitary extract, a general rule of thumb is to use one pituitary gland from a fish of equal size/injection. Others talk of 2 to 6 mg/kg of body weight. Experiments have been conducted using anything from 0.5 to 80 mg of pituitary extract/kg of broodstock. The hormones may be administered in one single injection followed by spawning within 24 hours, or there may be repeated injections, usually at 2 to 3 day intervals. The dose and frequency of spawning depends on the quality of the pituitary extract, and the particular stage of maturity of the female. In most cases, experimental work would be needed to determine the proper dose in each case.
Fig. 1. 4. The course of natural spawning (After Woynarovich and Horvath, 1980)
Fig. 1. 5. Endocrinological links in the chain between the reception of environmental stimuli and ovulation. Circled numbers refer to those stages where artificial intervention has been at least partially successful in bringing about ovulation in captive fish (After Harvey and Hoar, 1979)
Fig. 1. 6. The extraction of pituitary gland from fish (After Woynarovich and Horvath, 1980)
Fig. 1. 7. Preparation of acetone-dried pituitary gland (After Woynarovich and Horvath, 1980)
Fig. 1. 8. Preparation of pituitary gland to be used for induced breeding (After Woynarovich and Horvath, 1980)
For human chorionic gonadotropin, doses vary from 45 UI/kg to 12500 UI/kg again depending on the species. Again, a rule of thumb would be to start with a lower dose, if this is not effective, double it following 2 days, and so on until ovulation or spawning occurs (Shehadeh, 1975; Harvey and Hoar, 1979).
Hormonal activity of the pituitary is controlled by the hypathalamus, in particular by the nucleus preopticus and the nucleus lateralis tuberis (NLT). These two organs are connected anatomically to the pituitary. The neuro-hormonal factor that influences the pituitary is the LHRH, the hormonal factor controlling the gonadotropines. This decapeptide can be synthesised in the laboratory and has been used for inducing reproduction since 1978. There seems to be no species at this time where it does not work and the main effect is the release of gonadotropins in the blood.
Other similar hormones, LHRHa, Ayerst 25205 and Hoechst 766 have been synthesized and appear to be even more effective than the decapeptide. The doses used vary from 5–10 ug/kg however for the LHRHa, doses of 1 to 0.002 ug/kg are effective for chinese carps with an intramuscular or intraperitonal injection.
The ovary produces several non-estrogenic C-19 and C-21 steroids, as well as estrogenic C-18 steroid. Among the steroids produced by the ovary, one can name estrogene steroids C-19 and C-21. The estrogens are responsible for the secondary sexual characteristics and behaviour. Several have been identified, for example the estradiol, estrone, estriol, etc. The gonadotropins induce the maturation of the ovules by the synthesis of C-21 steroids, which are generally synthesized by the ovary, but sometimes by the interrenal. Among the C-19 and C-21 steroids, the 11-deoxycorticosterone, the 11- deoxycortisol, the epipregnanolene, the pregnanediol and others have been identified. Progesterone has been identified in the ovaries, but has only been found in the blood of the winter flounder.
The estrogens are implicated in the first stages of the oocyte development in particular in the vitellogenesis, and are not particularly effective in the later stages of maturation. Ovulation has been induced by an injection of progestrone and 17-hydroxy-20β- dihydroprogesterone while the estrogenes, androgenes, and pregnanolone were ineffective. It appears that the 17α- hydroxy-20β- dihydroprogesterone is only effective at the final stages of maturity of the oocyte, which greatly restricts its use.
Another way of using the ovarian steroids is by the retroaction that they exercise on the secretion of gonadotropins. An increase in estrogenes provokes a slowing down of the release of the gonadotropins. An injection of a anti-estrogene can block this action by interfering with the negative feedback (Donaldson, 1975). Two anti-estrogenes can be used, the clomiphene or cis-clomiphene and the tamexifene (Donaldson, et al, 1981). The dosage of these hormones is very delicate. At low dose doses, they provoke ovulation, at higher concentrations, they inhibit it. The clomiphene at a dose of 1 mg/kg provokes an increase in the secretion of gonadotropines in the common carp, however at 10 mg/kg, it has the opposite effect. An antiestrogene can only be used when the rate of gonadotropins in the blood is sufficient.
Corticosteroidsare also used, in particular the DOCA or deoxycorticosterone acetate. It is used principally for catfish, experimentally in India (Heteropneustes fossilis), and for routine production of Clarias gariepinus in the seventies. The mechanism of action is not well understood, but it appears that it intervenes as a interrenal steroid, intermediate in the synthesis of ovarian steroids. It is injected at a dose of 50 mg/kg of fish. Lower doses have also been successful.
The carp will reproduce in ponds when conditions are favourable: water temperature at 18 – 22 degrees, and up to 25 degrees, submerged aquatic plants covering the bottom, clean water, rich in dissolved oxygen and little turbidity, and stable meterologic conditions. Increasing the water depth acts as a stimulant. Even under these conditions, spawning is not controlled and erratic. For this reason, farmers turn to semi-artificial or artificial reproduction. Spawning ponds are small, 100 to 1000 m2, shallow, and one places “kakabans” or bunches of grasses to collect the eggs at 20 to 30 cm from the surface (Fig. 1.9). One needs 1 to 1.5 m2 of kakabans per female. The pond is filled with clean water at the correct temperature the same day as the fish are introduced. the broodstock receive an injection of pituitary extract at a dose of 2 to 3.5 mg/kg using an intramuscular injection. As soon as the eggs are seen on the kakabans, they can be picked up and transferred to a hatching pond or first stage larval pond.
Artificial reproduction of common carp is also carried out using pituitary extract. Generally two doses of extract are given at 14 to 24 hour intervals, the initial dose is 0.3 to 0.5 mg/kg, and the second dose of 3.5 to 6 mg/kg for the females, the males receive one single dose (0.5 to 2.5 mg/kg) at the same time as the second dose for the females. The genital papilla of the female is sutured to avoid a premature release of eggs. The female is ready for egg extraction 9 to 13 hours after the last injection (9 hours if the temperature is 26 degrees and 13 h if the temperature is 19°C). If the male is kept with the female, he will indicate the right time for the mating; he will start following and courting the female. At this point the broostocks are captured and sexual products harvested. The eggs and milt are collected dry. Eggs are obtained as soon as the suture is removed using slight abdominal pressure (Fig. 1.10).
Fig. 1. 9. Kakaban (egg recipient) and placing of kakaban for pond spawning of common carp for collecting fertilized eggs with or without hormone treatment (After Woynarovich and Horvath, 1980)
Fig. 1. 10. Techniques of stripping of common carp (After Woynarovich and Horvath, 1980)
The eggs have a sticky substance that allows them to adhere on the natural spawning grasses. If water is introduced onto the egg and milt combination, this stickly substance becomes activated and all the eggs will stick together, for this reason, farmers use a fertilization medium made of 30 g of urea and 40 g of sodium chloride mixed in 10 l of water. This solution is added at 1/10 to 1/5 of the egg mass volume, and the entire solution is stirred for 3 to 5 minutes to insure fertilization. The eggs will begin to swell. The volume of the egg will increase around 10 times, so a large enough container is needed. One continues to add the fertilizing medium at the same time removing the fluied containing the disolved adhesive. After about 1½ hours, the eggs have reached their maximum size and can be treated with a second solution to remove any remaining adhesive materials. This solution is made from adding 5 to 8 g of tannis to 10 l of water. Two to four liters of tannin solution are added to 2 or 3 liters of eggs, mixed for 3 to 5 seconds, and clean water is added. The process is repeated, and then the eggs are allowed to settle to the bottom of the bucket. The eggs are then washed with large quantities of water to remove any remaining tannin, and transferred to the incubators (Fig. 1.11).
Heterotis niloticus was the object of a large amount of research in the 1960’s, and the species was reproduced naturally in ponds. Reproduction is possible without hormones, but by simulating natural spawning conditions in the ponds. It isn’t possible to distinguish the sexes from the exterior, which is a major inconvenience. The fish are sexually mature when they have a length of 400 mm, and weight around 600 g, and generally fish that weight 800 g to 1 kg are mature. This is around 20 to 24 months old. Heterotis has only one gonad, located on the left side. The spawning behavior is complex. It begins with the spawning couple looking for a nest site in shallow water filled with weeds. The fish pulls out the weeds with his teeth and then swims in a circle to push out weeds and debris. He then digs a nest, the top of which is just about at the surface of the water. The fish can enter and leave through a passage way that is left. The nest is usually 1 to 1.5 m in diameter, and surrounded by a band or vegetation, the depth is 30 to 50 cm. Both parents participate in the nest bulding. Spawning occurs inside the nest. After spawning, the brooders will guard the eggs and fry, the fry eventually group into school and will remain in this formation in a length of time. Spawning can occur several times in a season. Reproduction is stimulated by three principal changes in the environment; increase in photoperiod, beginning of the rainly season, general lowering of the pH.
Mortality of the fry is very high and continues until the fish reach 50 g. Heterotis can reproduce without difficulty in larg ponds (more than 1 ha) with a lot of peripheral aquatic vegetation. They can also spawn in smaller ponds of 200 to 1000 m2. A slight slope is advantegeous (2 to 2.5%), and the pond should be desilted before the rainy season to avoid dissolved oxygen depletion. The pond is cleaned of all weeds except for a few square meters in the corners of the ponds. These weedy area are isolated from the rest of the pond by constructing a picket barrier. These areas are the spawning sites, and as the rest of the pond is free of weeds, there is less change of predation on the fry. Enough brood stock should be introduced to assure that the two sexes are present. The fry can be removed as soon as parental care has stopped.
Fig. 1.11. Artificial propagation of common carp – (a) (After Woynarovich and Horvath, 1980)
Fig. 1.11. Artificial propagation of common carp – (b) (After Woynarovich and Horvath, 1980)
Fig. 1.11. Artificial propagation of common carp – (c) (After Woynarovich and Horvath, 1980)
Work on Clarias gariepinus has been going on since the early 1970’s; initially in Central Africa, and more recently elsewhere. Both natural and induced spawning are possible.
Natural spawning occurs in ponds of 8 – 15 m2, where the pond has been prepared by covering the bottom with river gravel. A peripheral canal in the shape of a U is dug around the edge of the pond and the brood stock, 2 males and 1 female are put in the morning. Broodstock are chosen by external characteristics, the female can be chosen if eggs are obtainable by slight pressure on the abdomen, the males chosen by a slightly coloured genital papilla. Water level is slowly increased until nightfall. Spawning should occur that night. If it doesn’t, the pond level is lowered, and the slow filling is repeated the next day. As soon as spawning has occured, the broostock is removed, and the fry are harvested after 25 to 30 days. The major problem is the survival of the fry.
Induced spawning is done using DOCA at 50 mg/kg or pituitary extract from common carp or Clarias at 4 mg/kg. The female is injected the morning and left in a concrete tank until night. Males are chosen by their agressiveness; after an hour of combat, the winner is chosen. At nightfall, 8 to 10 hours after the injection, the couple is placed in a tank of about 1.5 m2 of surface area, where the bottom is covered with gravel. Running water is needed. The couple spawns at night, and the brooders are removed the next morning. The eggs stick to the gravel and hatch the following day. As soon as the yolk sac is absorbed, the fish are transferred to a fry pond.
Artificial reproduction is similar to induced spawning. The female is injected, after 8 to 12 hours the eggs are mature. They are extruded using abdominal pressure, and mixed with sperm. Sperm is obtained by pulverising the testes of the male fish as their particular anatomy makes it impossible to obtain milt by normal abdominal pressure. The fertilized eggs are then passed into an incubator. The major inconvenience is the sacrifice of the male, however more eggs are obtained than in induced spawning. An expanded chapter on C. gariepinus breeding follows these general considerations.
For some years now, research officers have been trying to breed Chrysichthys as it is an important fish. Recently, C. nigrodigitatus has been successfully bred in captivity in the Ivory Coast (Hem, 1986) The sexual maturity of the broostock is monitored when approaching the breeding season. The criteria for the female are the swelling of the abdomen, along with the softness of it and the diameter of oocytes, removed from the ovaries by biopsy (catheterization). There is a significant relation between the diameter of oocytes and the time to spawn and therefore it is possible to predict the time needed before the spawning can occur from the diameter of a batch of eggs removed from the ovaries (Fig. 1.12). The breeding male is characterized by an enlargement of the head and thick soft lips. As for the female, there is also a significant relation between head width and body width (Fig. 1.13) measured as interopercular width, which remains quite constant. With these two relations, it is possible to predict more or less accurately the time of spawning.
Fig. 1.12. Relation between oocyte diameter and number of days prior to spawning of Chrysichthys nigrodigitatus. (After Hem, 1986)
Fig. 1.13. Morphometric parameters used for quantification of maturity of spawners (After Hem, 1986)
Fig. 1.14. Spawning receptacle for Chrysichthys nigrodigitatus. (After Hem, 1986)
Fig. 1. 15. Position of spawning receptacles in concrete spawning tank for Chrysichthys Nigrodigitatus (After Hem, 1986)
A few (3–4) weeks prior to spawning, the brood fish is placed two by two in PVC pipes of 30 cm diameter (Fig. 1.14 et 1.15). Both sides of the pipes are sealed leaving only openings for water circulation (fine mesh screens). The fish are not fed. When the time of final maturity is reached, the fish will spawn and the eggs are deposited in a batch inside of the pipe. The fish can then be removed and the eggs are incubated in troughs suspended in tanks and kept shaken. This shaking is necessary for good aeration of the egg mass. The incubation of eggs – contrary to other african fishes – takes a long time, up to 10 days according to temperature.
Tilapia culture is usually characterised by excessive reproduction in the culture ponds. However, there are many uses for the deliberate spawning of Tilapia species; hybridization to obtain all male fingerlings, culture in cages or enclosures, genetic selection, extension work in rural areas, hormonal sex reversal. For the purposes of discussion, all species of Oreochromis, Sarotherodon, and Tilapia are considered together, as although their breeding and parental care of fry changes, the techniques used in spawning are very similar. In general, Oreochromis species are female mouth brooders, Sarotherodon species are male or male and female mouth brooders and Tilapia species are substrate spawners, and the eggs are not incubated in the mouth of the parent.
The simplest technique for spawning tilapia is pond spawning. The brooders are introduced at a rate of 1 to 3 fish/m2 depending on the ferility of the pond and any supplemental feed. Depending of the breeding habits of the species, the sex ratio varies. For female mouth brooders, the ratio is 3 to 5 females to 1 male. and in some cases up to 7:1. For male mouth brooders, the ratio is 1:1. For substrate spawners where both male and female are involved in egg and fry guarding, the ratio is 1:1. The brooders are left for a period of 6 to 7 weeks, and a small mesh seine net is pulled through the pond, capturing as many fry as possible. Brooders are returned to the pond and the net is hauled again at 2 week intervals. After about 6 months, the harvest goes down, and the pond is emptied and the cycle begins again with new brood fish.
A variation of this technique is to remove the brooders after one month, leaving the fry in the pond. The brooders are transferred to another pond, and the fry left in the spawning pond for 1 month, at which point the pond is drained and all fish are harvested. This second technique will result in fish of a more calibrated size, there is no possibility of canibalism, and will generally result in more offspring. This second technique is necessary when using hybrids to produce all male off-spring.
Although Tilapia species in nature usually build some sort of nest or have specific spawning preferences, they will readily spawn in concrete fiberglass, or metal tanks. While certainly more elaborate, expensive, and labor intensive, the number of fry that can be produced are substantial, and these techniques are preferable when hormonal sex reversal is practised as the fry can be harvested at just the right age.
Although overpopulation in Tilapia ponds can be avoided by using predators, a lot of attention has been given to the rearing of mono-sex male fish, as the growth of the males is generally far superior that the females. Three techniques are currently used; manual sexing, hybridization, and hormonal sex reversal.
Manual sexing is usually done when the fish have reached 25 to 40g in average weight. Even at this point, the males are usually larger than the females, and if the fish were calibrated when stocked in the pond, mechanical size separation will result in a population of mostly males. Generally, though, sexing is done by hand and looking at the urogenital opening. Staining the opening with some kind of dye (ink, methal blue, etc.) is helpful. Females are discarded.
Hybridization is done by breeding two different species and due to the chromosome make up, all male populations result.
The following crosses have been successful:
|Female||Male||% males of offspring|
|O. mossambicus||O. hornorum||100|
|O. niloticus||O. hornorum||100|
|O. niloticus||O. macrochir||100|
|O. niloticus||O. variabilis||98 – 100|
|O. niloticus||O. aureus||100|
|S. spilurus||O. hornorum||98 – 100|
|S. vulcani||S. hornorum||98 – 100|
|S. vulcani||S. aureus||98 – 100|
|O. mossambicus||O. niloticus||100|
and there are undoubtedly several more. The situation is made more complex as many fish of the Tilapia group will interbreed and hybridze in nature to some extent, thus the genetic strains are often not pure. A second problem arises as breeding between the two species is not often regular, and there can be problems in obtaining sufficient fry. In any event, one must be very careful to keep the various species and offspring separate so further inbreeding and back crossing does not occur (Lovshin, 1984).
Hormonal sex reversal is possible in certain species, and has been successfully tested in O. niloticus, O. mossambicus, O. aureus, and S. melanotheron. The fry, as soon as the yolk sac is absorbed, has yet to undergo gonadal development. By administering methyltestosterone to these fish, the hormonal action will over-ride the genetic determination. It is important that the fish receive the proper dose and for enough time, and that they have no access to other food. Doses usually vary from 20 to 60 mg/kg of feed, and the fish are fed to satiation several times a day, until they reach a weight of about 1 gram. The hormone is usually dissolved in ethyl-alcool, mixed with the feed, and then dried in an oven. Feeding usually occurs from 30 to 45 days after the yolk sac is absorbed. While this does have several advantages, there are some hidden problems.
The sex reverse females are functional males, however genetically females, and when breeding with normal females, the offspring will be highly skewed to females, and in some cases 100% female. Care must be taken not to contaminate broodstock.
As soon as the egg is fertilized, embryonic development occurs inside the egg untill hatching. At this point, the larva tears through the shell and begins an independent life. During the egg development stages (Fig. 1.16), the eggs are susceptible to predation, and attacks from bacteria and fungus growth. The time needed for hatching varies with the species, but in all cases is temperature dependent. In tropical waters where the water temperature is usually superior to 25 degrees, hatching usually occur in 1 to 5 days. In temperate climates, hatching can take several months in very cold water (see Table I.I).
Eggs can be categorized as adhesive or free. Adhesive eggs will stick to each other, and onto a given substrate. Free eggs can be floating, sinking, or have same specific gravity as water. When artificially incubating eggs, adherent eggs are usually cleaned and the sticky substance removed (urea, salt, tannin).
Incubation eggs need a continuous supply of oxygen, and usually in large quantities. Pure, clean, sterile water is ideal, and most hatcheries will use borehole water. Eggs should be protected from shock, as the cell divisions occurring inside the eggs can be disrupted. Direct sunlight is often a problem, and should be avoided. The eggs release carbon dioxide, which needs to be removed, and ammonia. Several kinds of egg incubators have been developped.
(After Woynarovich & Horvath 1980)
The type of incubator used depends on the kind of egg, but also to a large extent on available materials, water supply, etc.
These incubators are the simplest. A box is constructed with openings allowing water to flow through the sides and over the surface of the eggs. This can be modified by several methods e.g. for a system where the eggs are placed on a fine mesh material and water flow through the eggs from below to up where it is evacuated. Dimensions can vary considerably depending on the size of the egg mass etc.
The classic jar type of incubator is the Zoug Jar; a glass cylinder with a conic base. Water enters through the base and leaves through the top. Water circulates through the eggs, and any debris or dead eggs usually float out the top. Water flow is regulated through a tap at the base. These jars can be made of clay, plastic, glass, fibreglass, or plastic bottles. Zoug jars can be anything from 1.5 to 200 litres volume.
Fig. 1.16. Embryonic and larval development (After Woynarovich and Horvath, 1980)
|Fig. 1.17. Long trough and box for incubation of heavy eggs and for rearing larvae. (After Woynarovich and Horvath, 1980)||Fig. 1.18. Funnel-type incubator devices. (After Woynarovich and Horvath, 1980)|
Time required for the development of eggs of different species
|Name of fish||Optimal temperature of incubation, °C||Number of days or hours||Day-grade|
|Common carp (Cyprinus carpio)||20–22||3.5-4 days||60–70|
|Pike-perch (Stizostedion lucioperca)||10–15||7–11 days||100–110|
|Pike (Esox lucius)||8–15||8–12 days||120|
|European catfish (Silurus glanis)||22–25||2.5-3 days||50–60|
|Tench (Tinca tinca)||22–25||3 days||60–70|
|Grass carp (Ctenopharyngodon idella)||22–25||1–1.5 days||24–30|
|Silver carp (Hypophthalmichthys molitrix)||22–25||1–1.5 days||24–30|
|Bighead carp (Aristichthys nobilis)||23–26||1–1.5 days||26–30|
|Rohu (Labeo rohita)||24–30||14–20 hours||20–22|
|Catla (Catla catla)||24–30||14–20 hours||20–22|
|Asian catfish (Pangasius sutchi)||28–29||23–25 hours||?|
|Clarias macrocephalus||26–30||18–20 hours||?|
|Giant gourami (Osphronemus goramy.)||28||44–48 hours||?|
|Channel catfish (Ictalurus punctatus)||24–30||14–20 hours||20–22|
|Grey mullet (Mugil cephalus)||20–22||50–60 hours||?|
|Sapoara falsa (Curimata sp.)||25–26||15–16 hours||16–18|
|Cachama (Colossoma oculus)||25–26||18–19 hours||?|
|Copore (Prochilodus mariae)||25–26||17–18 hours||18–20|
If there is no running water, eggs can be incubated in an aerated recipient or tank. This works well for the floating mullet eggs, which must have a constant salinity. Compressed air allows for sufficient and movement of the eggs.
The hapa is made by placing two open cloth sacks, one inside the other. The dimensions are usually 2 × 1 × 1 m and the interior sack is half that size. The outside hapa is of smaller mesh. The eggs are placed in the interior sack, and the whole apparatus is placed into a pond. As the eggs hatch, the larvae swim through the mesh in the first sack, but are retained in the second.
Some eggs can be incubated in the atmosphere, where the concentration of oxygen is 20 × higher than that of water. The eggs must be kept humid by a spray. This is the usual technique in very large trout hatcheries in the United States, and is also used for some heavy, adhesive eggs of other species. The eggs adhere to submerged plants. The plants are then hung up in a room are sprayed constantly with water As the eggs are nearing hatching, the plants are placed in water. This can have the advantage of a much higher hatching rate than would be possible in a natural spawning or other type of incubators, as the eggs in the interior of the mass would not be sufficiently oxygenated in the water.
Trout eggs are placed on trays, and water is dripped over and through a battery of trays. Egg trays are inspected daily for fungus, and any dead eggs are removed.
The eggs will develop normally if kept in optimum conditions. However frequently there are mortalities that are difficult to explain. This may be from a number of causes. Dissolved oxygen depletion can be a cause, a sharp variation in the temperature, incomplete fertilization, shocks and so on can be causes of mortality. As soon as the eggs die, they become white and opaque, where as viable eggs continue to show coloration and some transparency.
The presence of small crustaceans, copepods, insects or insect larvae can be a nuisance to the eggs, as they injure the shell by scratching at them. Bacteria and fungus, in particular fungus of the genus Saprolegnia, attack dead and alive eggs, and can cause the death of live eggs.
A clean water supply is therefore essential, and some filtration of the incoming water may be necessary. The best treatment against fungus is with malachite green. A dose of 5 ppm is used for 30 to 60 minutes, or a dose of 0.1 or 0.2 ppm is introduced into the culture system and allowed to flow through. This later technique is used as a prophylaxis in very large hatcheries.
Fig. 1.19. Breeding hapa and hatching hapa used in India (After Woynarovich and Horvath, 1980)
Fig. 1.20. Spray chamber for incubation of hard eggs sticking to nests (After Woynarovich and Horvath, 1980)
Formalin can also be used to treat fungus and bacteria. Concentrations of 200 to 400 ppm are used for 15 to 30 minutes. This does tend to affect the eggs and care must be taken.
Hatching occurs when the shell is torn, which is a mechnical and biochemical action, allowing the larva to escape. This process can be alowed or accelerated by the manipulation of temperature. If the water is too warm, the egg will hatch before the larva is well developed. If the water is cooler, the larva will be more developed than normal.
One can accelerate hatching by treating the eggs with an alkaline protease enzyme similar to the one used in laundries. The recommanded doses is 0.4 to 0.5 g to 200 to 500 cc. After 3 to 5 minutes, the shells are dissolved and the larvae are free.
To synchronize hatching, one can also transfer eggs which are completely developed into water that is a few degrees warmer but with low dissolved oxygen (25% saturation). The combination of warmer water and low oxygen will cause simultaneous hatching (Bakos et al, 1975).
After hatching, the larvae are more or less active. It is best to separate them as soon as possible from the debris of egg and dead eggs. If the larvae are actively swimming, there is no problem, but for some of the more inactive larvae, it is more delicate. The same mass of eggs will not hatch naturally at once, and some larva may be lost if they can not become free of the rest of the egg mass. For this reason, simultaneous hatching using enzymes can be a help.
Larval activity is usually specific. Some swim to the surface, and then come back to the bottom by gravity, others. will latch onto a substrate, sometimes through the secretion of a gland on the head. Others rest on the surface or bottom, occasionally displacing themselves. Finally, some form schools, and swim actively, but this usually occurs one or two days from hatching.
After hatching, the larvae are still not well adapted to the environment. The digestive tube is not developed, there is no mouth, no intestine, no anus, no gills, swim bladder, scales or coloring. Feeding is on the yolk sack, a vestige from the egg. Respiration is done by diffusion, and a high oxygen concentration in the water is needed. They should be sheltered from direct sunlight. As soon as the yolk sac is resorbed, the fish begins to look for food and the whole features and behaviour of the animal changes.
During this period of yolk sac resorption, the water must be very rich in dissolved oxygen, and free from any potential predators on the fish. Larvae can be maintained in jars similar to hatching jars, where a fine mesh filter retains the fish in the jar, in boxes where the sides are of fine mesh material and placed in running water, or in hapas. They can also be kept in aerated aquarium or tanks.
A hatchery can serve for the artificial propagation of one or more species. When designing the hatchery, the most important consideration is the water supply in terms of quality and quantity. Borehole water or water from a nearby spring are ideal as the temperature is constant, and the water is relatively free of organisms and pollutants. If this is not possible, a good system of filtration should be incorporated in the design of the hatchery.
The physical installations should be designed to allow modifications in the future as techniques change or improve. Inflow pipes should be of a larger diameter than immediately needed, the building larger, etc. If possible tanks, incubators, troughs, and other recipients should be movable to allow for future modification. Water pipes, compressed air pipes, and other supply structures should be easily accessible, for example suspended from the roof, or placed against the walls. Probably the worst situation would be to build massive concrete structures, water and air supply pipes cemented into place. Any future modifications would be very costly and time consuming.
Sexes should be kept separate. If using ponds, the ponds should be large enough (3 – 10 ares) to not stress the fish, should be easily and quickly drainable, with a good abundant water supply for fast refilling. As broodstock are usually very large, care must be taken to guard the fish against theft.
Mature brood stock will be captured from the pond, and kept in an isolated tank, usually 1 × 2 × 1 m, for hormone treatments. Running water and/or compressed air should be supplied. The tank should be covered with a opaque or semi-opaque top, so as not to unduely stress the fish and prevent it from jumping out of the tank. Incubation materials, jars, boxes, etc. should have ready access to the hatchery workers, and should be of sufficient number to allow for all contingencies. Recipients for the newly hatched larvae should be sufficient, and readily accessable.
At all times, the hatchery designer and manager should keep in mind cleanliness and hygiene. Should the hatchery become infested with a bacteria or fungus; the manager should be able to disinfect the entire hatchery quickly and easily.
The newly hatched fry are usually kept in special containers until the yolk sac is recorded. At this point, they are transferred to either ponds or running water facilities, depending on the species and hatchery design. Ponds, or first stage fry ponds, are carefully prepared in advance to assure a good supply of natural feed for the fish, usually zooplankton. Running water facilities will usually be circular tanks, to allow close supervision of the fish. Feeding is done artificially.
As soon as the young fish acquire a swim bladder and the yolk sack is resorbed, they begin hunting for food. At this stage, they are very susceptible to predation. The reserves of energy in the fry are very small, and the fish must find food quickly or will die. The farmer must assure that there is abundant food of the proper quality and particle size.
The first natural food of the young larval fish is usually zooplankton, and among the zooplankton, the rotifers and Paramecium are usually eaten first, followed by the cladocerans and copepods. In an artificial medium, it is difficult to feed the fish with a natural feed, with the exception of the nauplii of Artemia, or brine shrimp, which can be produced by the farmer in any quantity wanted. In an artificial environment, one must replace the natural food with artificial feed. The first thing to look at is the chemical composition of the feed. The composition of natural food will vary considerably depending on their age and feeding conditions. The average composition of Artemia salina and the rotifer Brachionus plicatilis is the following: water 85 – 95%, protein 50 – 55% (as percentage of dry matter), fats 10 – 20%, minerals 10%. As for the proteins, there is very little information on the amino acid composition, and for the great majority of species, there is very little knowledge on the essential amino acid needs, fatty acid needs, or vitamines and mineral needs. To compose an artificial feed, one can follow the rough outlines of the composition just mentioned. The size of the particles should be 0.1 to 0.3 mm in diameter. This small size makes it all but impossible to industrially pellet feed. Certain species, notably the trout and salmon, are large enough to accept larger particles of artificial feed, and these are widely used. for most species, however, this is not possible.
In practical terms, artificial feeding of first-feeding larvae has several major differences than natural food. In the first instance, artificial feeds will contain only 10 – 15% water, compared to 90 – 95% water in natural feeds. While fish larva is capable of consuming his entire weight in natural food in a single day, care must be taken when giving artificial feeds not to over feed, as this may be harmful to the newly formed digestive system and spoil the water. A second major difference is that live, natural food is usually moving, and artificial feed is inert. It is sometimes difficult to get the larva to accept artificial feed as they are not naturally attracted to it (Van der Wind, 1979).
Experience has shown that artificial feeds should contain some portion of fish meal, fish oil, or fish products. In addition to the known advantages of the amino acid composition, there is something that will attract the fish and they will accept the feed more readily if this is present. This has been shown for Clarias, Tilapia, Chrysichths, and many other species. Other feed components can include powdered baby food, powdered milk, crushed crab or fish eggs, chicken egg, and so on. The point is to attempt to have a small particle size, and a composition of roughly 50% protein. If particle size is still too large, a simple technique, but somewhat time consuming is to place the feed mixture in a handkerchief, put the tissue in the water with the larvae, and force the mixture through the cloth with your fingers. After one or two days, the fish will usually begin to accept this feed. When artificial feeding, feeding should be several times a day, and if feasible, automatic feeders are used. the principles is to have sufficient feed available at all times. Feeding should continue until the fish have grown sufficiently to accept more conventional artificial fish feeds (agricultural by-products).
Recently, there has been some interest in the development of microencapsulation of fish feeds. Essentially, a complete feed is prepared, and encased in a shell which is biodegradable either through a lower pH as would be found in the fishes stomach, or through direct enzymes activity. A simple microencapsulated feed can be prepared by mixing an entire egg (chicken) as well as possible using a blender or mixer. Boiling water is added, and the mixture stirred. The resulting product is a microencapsulated diet with the complete protein composition of egg.
A more complete description of parasites and diseases of fish and their treatment will be given in a later course. In terms of fry and larval production, there are several important differences. Even slight parasitic infections can kill larval fish, and the usual treatments for these parasites are often toxic to small fish. Great care should be taken when treating fish, and the recommended doses may be too high for the larvae.
The first nursery ponds are kept dry if they are not stocked. They are disinfected with quicklime if disease problems are prevalent, or limed with 500 kg/ha of agricultural lime if not. Water is very carefully filtered, usually the day of spawning in the hatchery, and let into the pond. The filter should be as fine as possible (0.5 to 0.8 mm). The pH optimum is between 7 and 8, and all weeds are removed. Fertilization occurs as the water is let in. Usually 5 to 7 tonnes/ha of organic manures, 150 kg/ha of triple super-phosphate, 100 kg/ha of urea. The ponds are also treated for copepods, small crustaceans and larval and adult insects. If available, a ‘soft” insecticide is used, where the active ingredient is trichlorophone (Flibol L, Ditriphone, Masoten) at a concentration of 1 ppm. (Tamas & Horvat, 1979). At 17 to 20 degree the insecticide disappears in 6 to 8 days. For air breathing insects, diesel and motor oil is placed on the pond surface to stop these insects from breathing. The treatments 22 – 45 l/ha, is repeated twice a week in the absence of wind (Sneed, 1975).
The density of rotifes should be controlled before stocking the fish. the optimum density is 1500 per litre. Following the development of plankton in the pond, the relative abundance of the various groups of zooplankton will evolve. Initially rotifers are plentiful, followed by the cladocerans, and after 2 to 3 weeks, the copepods are the most numerous. The use of trichlorphon kills the cladocerans and copepods, but does not affect the rotifers, so their density can be kept at the optimum.
The density for stocking the larvae varies according to the size of the pond, the rate of fertilization, and what ever further inputs given by the farmer. Densities vary from 300 – 600 m2 for small ponds, and 1000–3000 for larger ponds.
Larvae and first-feeding fish can be raised in circular or rectangular tanks, aquaria, etc. The densities are usually higher, and a good supply of fresh, clean water is needed.
If feeding the nauplii of Artemia, the number needed is very high. For 100,000 carp larvae, they will consume 3 to 4 litres of nauplii in the first 7 days of life. Fish should be fed every ½ to 1 hour.
Another method of creating natural feed in an artificial environment is to cover the bottom of the tank with straw or hay, and fertilize the water using manures, and/or chemical fertilizers. The tank is filled with 10 to 15 cm of water, and shortly a rich fauna of paramecium and rotifers will develop. The straw is removed shortly before introducing the larvae. The water is stagnant, and each day the level of the water should be increased by 3 to 5 cm of good, oxygenated water. After 3 to 4 days, one can to begin feeding mesoplancton of 0.15 to 0.25 m in diameter, Moina, and other Cladocera. After 6 or 7 days, one can begin artificial feeding using well sifted feed particles.
The major inconvenient is the cost of construction, adequate feeding of the fry and the pollution that results with the associated diseases and lack of oxygen.
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