Notes Milenge



Fish Farming (Aquaculture)

Fish farming is the principal form of aquaculture, Fish farming involves raising fish commercially in tanks or enclosures, usually for food. A facility that releases juvenile fish into the wild for recreational fishing or to supplement a species' natural numbers is generally referred to as a fish hatchery. Worldwide, the most important fish species used in fish farming are carp, salmon, tilapia and catfish.

There is an increasing demand for fish and fish protein, which has resulted in widespread overfishing in wild fisheries. Fish farming offers fish marketers another source. However, farming carnivorous fish, such as salmon, does not always reduce pressure on wild fisheries, since carnivorous farmed fish are usually fed fishmeal and fish oil extracted from wild forage fish. In this way, the salmon can consume in weight more wild fish than they weigh themselves.

The various activities involved in the process of fish farming are:

1 Capture of brood fish

Breeders can be collected from nature or from fish ponds. The capture of wild brood fish is recommended during: (i) breeding season when mature catfish aggregate in or towards shallow spawning grounds or (ii) dry season when they are relatively concentrated in their reduced natural habitats especially small pools and streams.

Handling and transportation of wild breeders should be done with care. At arrival at the hatchery, the wild breeders should be desinfected and deparasitised with a formaldehyde bath (15 ppm for 6 hours). A bactericide (Furaltadone or Furazolidone, 10 ppm for 1 hour) together with a fungicide (Malachite green 0.2 ppm) should be applied daily during 4 days in order to prevent outbreak of infections, especially on those parts of the skin which have been injured during handling/transportation. Biting during transportation may also be one of the reasons for injuries.

An alternative to capturing brood fish from spawning grounds or natural habitats, is the rearing of fingerlings up to breeders in fish ponds. At harvest, breeders are selected and transferred to the holding unit in the hatchery, or to a special brood fish pond.

2 Size of brood stock

Various factors influences the size of the stock of female breeders. Some of the most important factors are listed below:

(i) Hatchery management

Hatchery management is simplified if only two age-groups are kept in the hatchery e.g. brood fish and one batch of juveniles up to 1 g at a time. One gram fingerlings is the minimum size required for proper stocking of on-growing ponds. It was found that brood-stock maintained for at least one year under controlled conditions in a hatchery will lose its seasonal reproductive cycle. This would mean that mature breeders will be available year round. Consequently two stocks of brood fish should be maintained in the hatchery e.g. (i) actual brood stock for propagation and (ii) conditioning brood stock.

(ii) Weight of the breeders

Individual brood fish of weight 0.5–1.0 kg is preferable. They have a substantial quantity of mature eggs and are easy to manipulate.

(iii) Incubation capacity

A moderate hatchery (annual production of 500,000 fingerlings) required a total incubation capacity of 800g of fertilized eggs batch. This amount of eggs can be produced by about 16 females of 500g (see section 3.5.1) and incubated in 4 incubation troughs (see section 3.10.1).

(iv) Length of nursery period

The period of indoor rearing of fry up to early fingerlings of about 1 g (depending on water temperature and feed quality) varies between 6 to 8 weeks. This would mean that artificial propagation should be carried out every 6–8 weeks. If nursing is done in ponds, artificial breeding should be carried out once or twice every month in order to meet the annual production of 500,000 fingerlings.

(v) Repeated artificial propagation

It has been found that the same female brood fish can be induced to reproduce artificially every 4–6 weeks without affecting neither quality or quantity of eggs obtained after stripping(egg collection method).

The size of the male brood stock depends on the number of males required for each artifical propagation and the number of artificial reproductions per year. One rarely two males, needs to be dissected for the procurement of milt( collection of sperm).

3 Rearing of brood fish

Optimal hatchery management including the following factors are required to maintain brood fish under these optimal breding conditions:

Containers: The breeders have to be maintained in rectangular troughs about 1.0 to 1.5m3 (see section 3.10.1). The water inflow is at one end, while the superfluous water flows out at the other end through a turn-down pipe (flow-through system).

Stocking density: Each trough may be stocked with 100 to 150 kg of fish per cubic metre.

Oxygen/Water supply: The oxygen requirement and consequently the water supply depends on the water temperature and the feeding level. The best way to control the oxygen supply for the brood fish is to measure the dissolved oxygen content of the outflowing water. This oxygen level should not fall below 3 ppm.

Temperature: The optimum temperature recommended for rearing and conditioning brood fish for artificial propagation is 25°C. This optimum temperature with a minimum of fluctuation is a prerequisite for quantitatively and qualitatively adequate gonadal development year round.

Although the temperature of the hatchery water fluctuates with the temperature of the water source (stream, reservoir or borehole), some simple improvisations can increase or decrease the water temperature as close as possible to the required optimum.

Light: Little is known about the influence of light on the gonadal development of C. gariepinus. Light periodicy does not seem to be a decisive stimulus for gonadal development. Illuminated environment may irritate catfish since their preferred habitat are turbid waters. It is, therefore, recommended to recover ¾ of the trough starting from the inlet with a non-transparent cover (wood or plastic). Artificial light is placed on the open part of the tank (near the outlet), about 20–30cm above the water level, to accentuate the difference between the dark part (gathering of healthy fish) and illuminated part (diseased fish) and to facilitate monitoring of fish and cleanness of the rearing device.

Tranquility: It is believed that frequent disturbances interfere with normal gonadal development (Woynarovich and Horvath, 1980). Stress decrease considerably the appetite of catfish and will reduce its resistence against unfavourable environmental factors causing diseases. Conditioning and rearing of brood fish should, therefore, be done in a quite, dark-some place free of disturbances. The entrance of visitors and intruders into the propagation and rearing unit of the hatchery, (see section 3.10.1) must be minimized as much as possible.

Water quality: The non-ingested food and faecal particles should be washed out once a day by exchanging about 20% of the water volume of the container. Once every 6–8 weeks, or earlier if required, algal growth on the trough walls needs to be removed with a brush.

Health management and hygiene: Adequate hygiene is one of the most crucial factors of proper health management in a catfish hatchery. Without such a management, healthy sexual gametes cannot be guaranteed. To achieve this goal, every unit of twin trouhs must have proper tools as netting, brushes, bucket, rubber tubes etc., which are maintained in a small container filled with a disinfectant solution (Benzalkonium chloride, 1°/00; Formaldehyde, 0.1%, or Iodine solution 1–2% e.g. Wescodyne or Betadyne). The first compound, a non-corrosive and little aggressive detergent is preferred. The hatchery floor should be disinfected once every week. The hatchery operator should also use this solution to disinfect his hands before and after each cleaning operation. Under proper farming conditions, brood fish does not require regular prophylactic treatments.

Nutrition: Adequate food supply is also of foremost importance to brood fish. A well balanced compounded diet containing all the essential nutrient requirements, particularly the amino-acids, vitamins and minerals is a prerequisite for proper gonadal development. Artificial diets should be made with locally available agricultural by-products. In most African countries feed ingredients containing high amounts of animal protein such as fish and blood meal are scarce and costly. Therefore, it is easier to meet the high protein requirement by using ingredients containing large amounts of vegetable protein as oil cakes and oil meals. These ingredients are more common, cheaper and generally available in large quantities. A feeding level of 1% of the body weight should be applied for brood fish of 500 g or more. Careful hand feeding, avoiding stress and over-feeding is recommended. The feeds are given in three or even better four portions during day time near the inlet. Supplementary feeding with under-sized tilapias is recommended whenever they are available. Feeding must be stopped when catfish stop showing interest for the feed. This type of feeding allows to control the appetite and at the same time, the condition of the brood stock is monitored. It also avoids feeding of the tank instead of feeding the fish. Automatic feeders, which generally results in a higher food intake, may be used but the appetite and the health of the brood fish must be monitored daily for the above mentioned reasons.

Rearing of fry and fingerlings

Monoculture is usually practised for rearing fry to fingerlings. The fry are left in the hatchery tanks for three to five days before being transported to nursery ponds of 2-3 mu. The ponds are cleared to eliminate predators and treated to control infections. They are also fertilized a few days prior to stocking, with organic manures. The manures stimulate the growth of planktonic organisms upon which the young fry and fingerlings will feed. The stocking density varies from one farm to another, but, in all, the practice is to have a high density at the beginning to be reduced after a few days.

Application of manure is carefully controlled so as not to over-fertilize as this leads to depletion of dissolved oxygen which can kill off all the fry.

Fingerlings may be raised in the same fry ponds but the ponds have to be cleaned out and limed to get rid of pests and prevent disease outbreaks. Great care is taken to select healthy strong fry for stocking in fingerling ponds.

Feeding of fingerlings is done in the same way as for fry. However, more reliance is placed on artificial feeds like rice bran, soya-bean cake, peanut cakes and wine residues. Silkworm pupae, crushed snails, and fish meal, are also used with chopped soft grass and vegetable tops being added for grass carp fingerlings.

Precautions

(i) ensure that water colour and quality are right and if fish are surfacing, taking prompt remedial action;

(ii) guarding against fish escaping owing to broken dikes or overflow of ponds after heavy rain;

(iii) adequate feeding of the fish;

(iv) cleaning of the ponds, especially the feeding places, by applying bleaching powder to sterilize the feeding areas in order to reduce the incidence of fish diseases; and

(v) preventing predation by scaring off fish-eating birds, and controlling other predatory organisms.

Induced Breeding:

 Induced breeding is a technique whereby ripe fish breeders are stimulated by pituitary hormone or any other synthetic hormone introduction to breed in captive condition. The stimulation promotes timely release of sperms and eggs.

 

History of Induced breeding: The technique of induced breeding was first evolved in Argentina after producing pituitary extract by Houssay 1930 where viviparous fish was injected with the hormone to make premature birth. In the year of 1934, Brazilians were succeeded in induced breeding by pituitary extract. This technique was also followed in America (Merlin & Hubs) and in Russia (Gerebilisky). In India first attempt of induced breeding was made by Khan in 1937 on Cirrhinus mrigala. Later in 1955 Dr. Hiralal Choudhuri applied this technique in minor carps (Esomus danricus, Pseudeotropius atherinoides). Ramaswamy and Sunderaraj first induced to breed Clarias batrachus & Heteropneustes fossilis. The first successful induced breeding on major carps was done by Dr. Hiralal Choudhuri 1957– Cirrhinus mrigala, C. reba, & Labeo rohita. Parameswaran & Alikuni successfully bred the exotic Chinese carps – Hypophthalmichthys molitrix & Ctenopharyngodon idella in 1963.

 

Why Fish does not breed in Captivity??

 

Many cultural farm fishes like IMC do not breed in captivity. The reason may be environmental and consequently hormonal. Certain environmental parameters like photoperiods, rain, temperature, current of water influence the hormonal activity from pituitary and gonads. Disturbances arise in environment may cause the insufficient release of hormones in captive conditions and thus, the fish does not breed in captivity.  

Other factors like poor foods or insufficient natural foods, exposure to biocides and other pollutants badly affect the maturation of ovary.

 

Why induced breeding is necessary??

 

The technique of induced breeding gives very promising result in fishery point of view due to –

 

•       It gives pure spawn of certain species of fishes under cultivation. Spawn collected from natural water is not pure as because some undesirable wild species may come with them in culture pond. Sorting of pure seed is quite impossible in those stages. In later stages it is possible, but time consuming.

•       It assures timely available of pure seed, where as in nature the availability of seed is quite uncertain.

•       It can fulfill any quantity of demand in any time.

•       It also cuts short the holding potential spawners over long periods in uncertain hope of their breeding in time. Many carps take their full maturity in confined water but do not breed.

•       The technique is very simple and does not need too much technical assistance or knowledge. It can be easily learnt by a layman without much training.

•       The cost of expenditure is very low than the natural collections of spawns.

 

Technique of Induced breeding:

 

Preparation of Pituitary Extract – For preparation of gland extract the glands are removed carefully from freshly killed fish called donor fish. For best result the donor fish should be fully ripe and mature. Common carp is the best donor fish, because it breeds through out the year and the individuals are available in all parts of the world. The pituitary glands of such species are relatively large. The gland should be collected prior to spawning. However the gland doesn’t show species specificity and any carp species can be used as donor. However the glands of relative or closely related species show best result.

 

Removal of Glands –

 

The removal of glands can be done by following two processes:

 

1. Removal through foramen magnum – the foramen magnum was first exposed by removing vertebral parts adhering to skull. Fat is removed first by means of forceps and then cotton piece. A pair of forceps then inserted into foramen magnum dorsally to the brain and anterior part of the brain now detached and remaining is carefully lifted out through the foramen magnum. The gland is then located and removed.

2. Removal of gland by dissecting head – This technique is not used commercially as because the heads are damaged by this process. The first method of removal is less time consuming and economical as the heads are used for human consumptions later. At first the head is dissected using sharp butcher’s knife, a portion of scalp is chopped off in a clean cut with one stroke. Fat surrounding the brain is removed with the help of cotton. Olfactory and optic nerves are now severed, and then brain is lifted up and removed. Locate the gland. The gland may come up along with the brain or may remain behind on the floor of brain cavity often covered with a membrane. In any case the gland is carefully removed after separating it from membrane or the brain proper. The gland must not be damaged or torn.

  Preservation of Glands:

 

The gland after removal needs to be preserved for certain periods or for future use. The glands are taken in absolute alcohol and can be stored in room temperature. In certain countries like Russia the glands are preserved in acetone at 10°C. The glad may be preserved in refrigerator immediately for certain periods. But alcohol preservation of glands is very common and easy methods. It is widely used in India. Glycerin preservation is another technique but less popular in our country.

 

Preparation of Pituitary Extract:

 

The preserved glands of known quantity are taken out and macerated in a homogenizer after evaporation of alcohol with little amount of distilled water. Then the extract is freed of suspended particles by means of centrifugation. It is the diluted with required amount of distilled water or 0.3% saline water or a suitable physiological solution. The extract is now ready for use. 

Istruments for Gland Extract Preparation   Removal of Pituitary Gland

 

Preservation of Extract:

 

The extract can also be preserved for future use. In this process in place of saline water glycerine is used and extract can be preserved in room temperature or in refrigerator. Other methods of preservation are done by propane and trichloro-acetic acid in place of glycerine.

 

Selection of Brooders:

 

Proper selection of are the key of success in case of induced breeding. The breeders should be healthy, fully ripe and of medium sized. They should preferably come into the age group ranging from 2 – 4yrs and have the weight of 1 – 5kgs. Large sized breeders are avoided for difficulty in handling. For ripe male and female carps, it can be easily identified. The male shows roughness on pectoral fins when belly pressed milt freely oozes out. The ripe female shows relatively smooth pectoral fins and operculum. The eggs are released when the belly is pressed smoothly in female. The belly of ripe female is generally soft and round or budged. The vent is swollen, protruding and pinkish in colour. It is wiser to practice to keep ready adequate stock of potential brooders. For this a few months before breeding season potential breeders are kept away under care, and fed on supplementary feed (rice bran and oil cake mixture).

 

Injection to the breeders:

 

The pituitary extract is administered into the body of breeders by means of hypodermic syringe either intra muscular or intra peritoneal. To ensure a higher percentage of fertilisation during induced spawning it is necessary that there is synchronisation between ovulation and milt shading. This difficult to achieve with a set of breeders having one male and one female. Therefore the common practice is to use a set consisting of one female and two males.

 

Determination of correct dosage of pituitary extract to be given to the breeders is very important though a difficult matter. Dosage depends upon the size and state of maturity of the recipient (breeders) as well as upon the state of maturity of the donor for the glands. It has been found that the potency of the gland is influenced by the size, the age, the sex, the state of sexual maturity of the donor fish as also the size of the gland itself. Great difficulty is encountered because it is not easy matter to ascertain the state of maturity of fish from external examination. Usually the female is given a preliminary dose of 2-3mg/kg of body wt. The preliminary dose is not given to the male. After an interval of time about 6hrs a second dose of 5 – 8mg are given per kg of body wt of female. The male was given then the first dose of injection with female @ 2-3mg/kg of body wt. The dose may be depending upon the maturity of fish, age, sex and also the environmental conditions.

 

For intra muscular injection the fish is laid on its side while held in hand net and the needle is inserted either in the caudal peduncle or in the shoulder. For intra peritoneal the injections are given in the bases of paired pectoral fins. But it is avoided because less expert hand can puncture heard of the fish.

 

Spawning:

 

After injection to the brooders a set of brooders are released into breeding hapa. In hapa breeding the hapa is the fine netting, rectangular in shape and is held by four bamboo poles one at each corner. Closed meshed mosquito netting is preferred for that purpose, as its meshes will allow a good circulation of water and will also not let the laid eggs and milt escape through the meshes. The hapa measures the range of 3m × 1.5m × 1m for breeders weighing to 3 to 5kgs. The height of the hapa should remain about 20cm above to the level of water. The roof can be open or closed. The roof can be opened or closed.

 

The spawning takes place with in 3-6hrs following the second dose. It turns out the midnight if the second injection was given in the evening. Successful induced breeding results in the spawn of fertilised eggs. The fertilised eggs are transparent, pearl like where as unfertilised eggs are opaque or whitish.

 

Factors influencing the breeding:

 

Climate - 24°C to 31°C with cloudy days and rainy periods. Light drizzling following heavy rains is ideal. In absence of rain artificial showers are used.

Water – Flowing water is preferred.

Turbidity – 100ppm 1000ppm.

Light – It is known to bring that light may help in early maturation and spawning of fish.

Types of fish farms

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Open net pens or cages enclose fish such as salmon in offshore coastal areas or in freshwater lakes. Net pens are considered a high-impact aquaculture method because waste from the fish passes freely into the surrounding environment, polluting wild habitat. Farmed fish can also escape and compete with wild fish for natural resources or interbreed with wild fish of the same species, compromising the wild population. Diseases and parasites can also spread to wild fish living near or swimming past net pens.

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Ponds enclose fish in a coastal or inland body of fresh or salt water. Shrimp, catfish and tilapia are commonly raised in this manner. Wastewater can be contained and treated. However, the discharge of untreated wastewater from the ponds can pollute the surrounding environment and contaminate groundwater. Moreover, the construction of shrimp ponds in mangrove forests has destroyed more than 3.7 million acres of coastal habitat important to fish, birds and humans.

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Raceways allow farmers to divert water from a waterway, like a stream or well, so that it flows through channels containing fish. Farmers usually treat the water before diverting it back into a natural waterway. Some governments require strict regulation and monitoring of on-site and nearby water quality. In the U.S., farmers use raceways to raise rainbow trout. If untreated, wastewater from the raceways can contaminate waterways and spread disease. Farmed fish can potentially escape and compete with wild fish for natural resources. Escaped fish can also interbreed with wild fish of the same species, putting the health of the wild population at risk.

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Recirculating systems raise fish in tanks in which water is treated and recycled through the system. Almost any finfish species such as striped bass, salmon and sturgeon can be raised in recirculating systems. Recirculating systems address many environmental concerns associated with fish farming—fish cannot escape, and wastewater is treated—but they are costly to operate and rely on electricity or other power sources.

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Shellfish culture means that farmers grow shellfish on beaches or suspend them in water by ropes, plastic trays or mesh bags. The shellfish farmed using these methods—oysters, mussels, and clams—are filter feeders and require only clean water to thrive. Filter feeders can actually filter excess nutrients out of the water, but farming shellfish in high densities in areas with little current or tidal flow can lead to the accumulation of waste. Also, historically, some shellfish culture has been responsible for the introduction of exotic species that can sometimes out-compete native species for natural resources.

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