Fisheries :: Sea Fishes
India ranks second in culture and third in capture fisheries production and is one of the leading nations in marine products export. The present marine fisheries scenario is characterized by declining yields from the inshore waters and increasing conflicts among different stakeholders, whereas the increasing demand for fish in domestic and export markets indicate good prospects for large scale sea farming and coastal mariculture. The mariculture potential of India is vast as there is great scope for developing farming of shrimps, pearl oysters, mussels, crabs, lobsters, sea bass, groupers, mullets, milkfish, rabbitfish, sea cucumber, ornamental fishes, seaweeds etc. Although about 1.2 million ha is suitable for land based saline aquaculture in India, currently only 13 % is utilized. In India till date mariculture activities are confined only to coastal brackishwater aquaculture, chiefly shrimp farming.
(Source: CMFRI, 2004)
Depending on the geographical and ecological diversities of the country, there are vast differences in the availability and suitability of areas that can be developed for mariculture and also in the candidate species available for cultivation. Species like shrimps and the finfish like grey mullets, milkfish, pearl spot, seabass, groupers, redsnapper, breams and pompanos are suitable for farming all along the Indian coast especially along the south west and south east coasts.
Open sea cage culture
The open sea cage culture has been expanding in recent years on a global basis and it is viewed by many stakeholders in the industry as the aquaculture system of the millennium. Cage culture has made possible the large-scale production of commercial finfish in many parts of the world and can be considered as the most efficient and economical way of rising fish. It has now been realized that further conversion of wetlands and mangroves into traditional aquaculture farms has to be limited. Cage culture has several advantages over other culture systems. The cage culture system can optimize the carrying capacity per unit area since the flow of current brings in fresh water and removes metabolic wastes, excess feed and faecal matter. Simple cage designs for inshore waters are relatively easy to construct with minimal skilled labour. Cage culture is a low input farming practice with high economic return. The Indian coast offers many ideal locations for cage farming.
In the area of marine fish culture, the country is still in the experimental phase only. Attempts are being made to develop suitable hatchery and farming technology for mullets (Mugil cephalus, Liza macrolepis, V. seheli), groupers (Epinephelus tauvina), seabass (Lates calcarifer), milkfish (Chanos chanos) and pearlspot (Etroplus suratensis). The Central Institute of Brackishwater Aquaculture (CIBA) has developed an indigenous hatchery technology for seabass using captive broodstock. Some of the seafishes that are grown in India and those that have immense potential from Kerala’s perspective is dealt herein.
The Asian seabass (Lates calcarifer) known as "Kaalangi" or "Narimeen" in Kerala is an important candidate finfish species for farming. Seabass is a euryhaline fish, growing rapidly up to 3-5 kg within a growing period of 2-3 years in both freshwater and brackishwater environments. For maturation and spawning it migrates to the sea while the postlarvae and juveniles migrate to lagoons and backwaters for growing. It is a voracious carnivorous fish. However, the juveniles are omnivorous, feeding mainly on crustaceans and other small fishes. Seabass attains maturity at the age of 3-4 years at a length and weight range of 60 to 70 cm and 2.5 to 4.0 kg respectively. Males are generally small and in the size range of 2.0-3.0 kg and the males convert into females as they reach a size above 5.0 kg. The fecundity is between 2.1 to 17.0 million depending upon the size of the fish.
The required important facilities for a seabass hatchery complex are
(a) broodstock holding tanks
The saline water can either be drawn from a borewell in the intertidal area or from open sea. Water drawn should be stored in reservoirs and filtered through biological filters, rapid sand filters and ultra-violet, filter to maintain the required water quality.
Broodstock development and maintenance
The source of the broodstock can be either from the wild or from rearing ponds/cages. Brood fishes range in size from 2.0 to 10.0 kg, smaller size for males and larger ones for females, which can be induced to mature and spawn within 6 months. Transportation can be done using open or closed containers lined with soft materials like rubber foam mattress. Before transferring the fishes into the broodstock holding tanks (12m x 6m x 2m or 7m x 4m x 2m; rectangular in shape: with PVC inlet and outlet), they should be acclimatized in the holding tanks for. 2-3 days.
Stocking of 1 kg fish biomass/m3 is recommended for a 100 tonne of water, i.e. 10 females each of average weight 6.0 kg and 16 males each of 2.5 kg.
Water quality management
Water exchange to an extent of 70-80% of the total volume should be done daily. A flow through arrangement for water exchange is desirable. Important parameters of water quality to be maintained are as follows: water temperature: 28-32°C; salinity: 29-32 ppt; alkalinity (CO3): 80-120 ppm; pH: 6.8-8.0; dissolved oxygen: above 5 ppm; phosphate: less than 10 ppm; unionised ammonia: less than 5 ppm; ionised ammonia: less than 1.5 ppm;
Trash fishes like Tilapia/sardines can be procured, cleaned and packed as 2-3 kg blocks and frozen in deep freezers. At the time of feeding this can be thawed and given as feed to the broodstock @ 5% of the total biomass. Conditioning the broodstock fishes to feed on frozen trash fish should be done gradually, first with live and frozen fish and later with frozen fish only. Excessive feeding may be avoided and left-over feed should be removed immediately, since it will pollute the rearing medium.
Parasites like Caligus sp., Lernanthropus sp. are common in seabass. The most problematic parasite in seabass broodstock maintenance is the monogenic trematode Diplectanum latesi. Infected fishes do not feed, become lethargic and swim in isolation. Bath treatments with 100 ppm formalin for crustacean parasites and 1 ppm organophosphorus pesticide Dichlorovos, for 1 hr, is effective in controlling the parasites.
Seabass matures spontaneously in captivity during June to October. The maturation process can be accelerated by hormonal pellet implantation. The maturity stages of the broodstock should be monitored every fortnight. For assessing the maturity stage of female, ovarian biopsy is done by catheterisation. A polythene cannula of 1.5 mm diameter is inserted into the oviduct through the genital opening and the eggs are collected. When the diameter of eggs is more than 0.450 mm in size, the fish can be induced to spawn In matured males, white creamy milt will ooze out when the abdomen is gently pressed.
Matured seabass can be induced to spawn by hormonal manipulation. The hormones like LHRH-a (Luteinizing Hormone Releasing Hormone analogue), HCG (Human Chorionic Gonadotropin), ovaprim, ovatide and carp pituitary extract can be used for induced spawning, of which, the LHRH-a hormone is known to give consistent results. Female and male fish in the ratio of 1:2 are selected for hormone treatment. Hormone is administered normally in the early hours of the day to facilitate the spawning in the evening hours of subsequent day. Full moon or new moon days are preferred for spawning. LHRH-a hormone is administered intramuscularly in a single dose @ 60-70 mg/kg body weight for females and 30-35 mg/kg body weight for males and the fishes are released into spawning tanks of 15-20 tonne capacity.
The abdominal swelling and the courtship behaviour of the fish externally indicate the ovulation response. Good water quality and aeration should be provided in the spawning tank. After 35-36 hrs of hormone injection, the fish spontaneously spawns. Seabass is a protracted spawner (i.e. ova are spawned in batches by the same fish). The same female can be induced to spawn 3 times in a season with an interval of 15 days. Fertilization is external and the fertilized eggs, which are transparent, measure 0.78-0.80 mm in diameter and float on the surface. The unfertilized opaque eggs sink to the bottom. The rate of fertilization may be 60-90%. From a single spawning, 0.75 to 1.5 million eggs can be obtained.
Incubation and hatching
The floating fertilized eggs can be collected from the spawning tanks by a scoop net made of bolting cloth and stocked @ 80-100 nos./ litre in cylindro-conical shaped incubation tanks of 500 litre capacity. Hatching takes place 17-18 hrs after fertilization. Newly hatched larvae measure 1.4-1.6 mm in length and the healthy ones are transferred to larval rearing tanks.
The larvae and post larvae are reared at first in indoor tanks until they metamorphose into fry at about the 20th day after hatching. Larvae are stocked @ 30-40 nos./litre in fibre glass/RCC rectangular tanks of 4-5 tonne capacity. Larvae thrive on the yolk for the first 3 days. The 3-day old larvae should be fed with rotifers (Brachionus plicatilis) @ 2 nos./ml initially and gradually increased to 5 nos./ml till 8th day. From 9th day onwards, larvae should be fed with 5 nos./ml. rotifer and Artemia nauplii. Artemia nauplii density should be maintained @ 1 no./ml initially and increased to 3 nos./ml upto 15th day. From 16th day to 25th day, larvae should be exclusively fed with Artemia nauplii at a density of 4-5 nos./ml. All the feeds should be given in 4 doses at an interval of 6 hrs. After 20 days the larvae metamorphose into fry. The survival rate from hatching to fry stage is about 35-42%.
Nursery rearing of seabass fry in ponds and cages to stockable juvenile size is essential before release into the grow-out ponds. Nursery ponds may range in size from 500-2000 m2. A water depth of 50-80 cm is desirable. In a prepared nursery pond, fry of 1.0 to 2.5 cm size can be stocked @ 20-50 nos./m2. Water exchange to the extent of 30% is required daily. Fry must be fed with supplementary feed of chopped and ground fish (4-6 mm size) @ 100% of the body weight, twice a day, in the first week. The feeding rate is gradually reduced to 60% and 40% during second and third week respectively. Though seabass prefers live fish food it could be weaned to trash fish within 5-7 days. The nursery-rearing period is about 30-45 days.
On 25th day, the fry measures 1.0 cm, which should be transferred to nursery tanks in the hatchery or nursery hapas at the farm site. Fry are stocked @ 1000 nos./m3 in 4-5 tonne capacity tanks. The cooked and minced fish meat, made into small pieces of 1.5-2.5 mm, should be given as feed ad libitum during the nursery rearing. Grading (removal of shooter fish) should be done on alternative days to reduce cannibalism. In the case of rearing seabass in the hapas (2m x 2m x 1m size), fry should be stocked @ 500 nos./m2. Feeding and grading should be done as in the case of tank rearing. The expected survival rate would be 80-86% with an average size of 1.25g in 30-35 days of rearing in the tank or hapa. Nursery cage size may range from 3 m (3x1x1 m) to 10 m (5 x 2 x 1 m) with a mesh size of 1 mm. Stocking density with fry of size range 1.0 - 2.5 cm may vary from 80-100 nos./m. Feeding schedule to be followed as in pond rearing. Cages should be checked and cleaned regularly. The fry on reaching a size of 5-10 g at the end of a rearing period of 30-45 days can be stocked in the grow-out system. Usually a survival rate ranging from 50-70% could be obtained.
Extensive culture of seabass as a traditional activity is followed in the Indo-pacific region. In low lying coastal ponds, juveniles of assorted sizes collected from estuarine areas are introduced and fed with the forage fishes like tilapia, shrimps and prawns available in these ponds. These ponds receive water from adjoining brackishwater or freshwater canals or from monsoon flood. Harvesting is done after 6-8 months of culture. Since seabass exhibit differential growth, the size of the harvested fishes varies from 0.5 to 5.0 kg. Production up to 2 ton/ha/7-8 months has been obtained.
The traditional culture of seabass can be improved by stocking uniform sized seed at specific density and feeding them with low cost trash fishes/ formulated feed. Water quality is maintained through periodic exchange. Fishes are allowed to grow to marketable size and harvested. Seabass culture can be done in more organized manner as a small scale/large scale activity in both brackishwater and freshwater ponds and also in cages.
The size of the cage may be 50 m (5 x 5 x 2 m) with mesh size depending upon the size of the fish to be stocked (0.5 cm for 1-2 cm size fish, 1.0 cm for 5-10 cm size fish, 2.0 cm for 20-25 cm size fish and 4 cm for fish larger than 25 cm in size). In cage culture, both floating and stationary net cages are used.
Floating cages can be set on coastal waters where tidal fluctuation is wide. The net cages are hung on GI pipe, wooden or bamboo frames. The cage is kept afloat by styrofoam drum, plastic carbuoy or bamboo. The most convenient dimension for a cage is that of a rectangle and a volume of 50 m3 (5.5m × 6m × 2m). The cage unit is stabilized with concrete weights at each bottom corner. The cage unit has to be anchored to the bottom.
This type is fastened to wooden poles installed at its four corners. Stationary cages are usually set in shallow bays where the tidal fluctuation is narrow. The mesh size of nylon net would depend on the size of fish. Fingerlings should be transferred to nylon net (mesh size of 2.0 cm) for about 2 months of culture period and then they are moved to a cage net of 4.0 cm mesh size until harvest.
Stocking with seed of uniform size is desirable to avoid cannibalism. The stocking density may be about 40-50 fish / m initially. However after 2-3 months of rearing, depending upon the survival rate etc., it may be reduced to 10-20 fish/m. Periodic transfer of fish from one cage to another is essential in order to grade the fish and maintain uniformity in size. Feeding schedule is similar to that in pond culture. The cages should be cleaned regularly and periodically inspected for damage. A production level up to 15-17 kg/m can be obtained in cage culture over a period of 7-8 months.
The two-week nursery reared fingerlings are suitable for pond culture. The production pond can have concrete walls and a soft bottom, ranging in area from 0.1 ha to a few ha, water depth of up to 2 m and salinity of 5-10 ppt is suitable. Seabass culture in ponds can be carried out either by polyculture method or by feeding with low cost fishes like tilapia/oil sardines or with extruded floating pellets. The pond is at first dried, tilled, leveled and manured with raw cow dung @ 1000 kg/ha. If required, lime is added @ 50-200 kg/ha to maintain soil pH above 7. Urea @ 100kg/ha and super phosphate @ 50 kg/ha can also be added to enhance the algal bloom. Sea water/fresh water is then filled to a depth of 60-70cm in the pond. When the pond water becomes light green in colour indicating sufficient development of algae in the pond, forage fishes are introduced.
In pond culture, stocking with seed of uniform size (5-10 g), @ 3000-5000 nos./ha is desirable. Feeding of fish is carried out following two methods. In the first method, the fish are fed exclusively with chopped trash fish @ 10% of biomass twice daily (08.00 & 17.00 hrs) and reduced to 5% subsequently. In the other, the food is made available in the pond in the form of forage fish like Tilapia (Oreochromis mossambicus). Pelletized feed can also be given. In a well-prepared pond, manured/fertilized with raw cow dung @ 1000-1500 kg/ha and urea @ 100-150 kg/ha, Tilapia adults (male and female in the ratio 1:3) are introduced and reared for 1-2 months prior to stocking with seabass. To maintain natural food production for the forage fish, periodic manuring at fortnightly interval is done @ half the initial dose. 20% of pond water is exchanged on alternate days. Harvesting is done by draining the ponds or by using seine nets. Grow-out pond culture of seabass can yield a production of 2-3 tons/ha within a rearing period of 7-8 months.
For sea bass farmed in cages, harvesting is relatively straightforward, with the fish being concentrated into part of the cage (usually by lifting the net material) and removed using a dip net. Harvesting sea bass 'free-ranging' in ponds is more difficult, and requires seine-netting the pond or drain harvesting. After harvesting, the barramundi are placed in ice slurry to kill them humanely and preserve flesh quality. Fresh barramundi is generally transported packed in plastic bags inside styrofoam containers with ice. There is a limited market for live barramundi in Kerala. Fish are usually transported live in tanks by truck.
Seabass is prone to diseases caused by parasites, bacteria, fungi and virus. But diseases and abnormalities due to environmental stresses and nutritional deficiencies have also been recognized.
Lymphocystis disease is commonly found in seabass raised in cages especially among juveniles 4–7 cm in total length. It has been observed at all temperatures in rather high salinity. A gross sign of the disease is massive enlargement of the cell within the dermis layer of the fish skin, which resembles the cauliflower disease. Transmission is from fish to fish.
Fish reared in intensive culture conditions are exposed to extreme environmental fluctuations, and they may be more sensitive to stress than wild populations.
a. Aeromonas bacteria
Whenever fishes are exposed to environmental stress or injury, aeromonas causes serious outbreaks of homorrhagic disease with high mortality. Temperature, pH, high CO2 and DO depletion, decomposition products, and free ammonia in the water, all of these can be considered as possible factors for Aeromonas infection. When seabass are overcrowded and water salinity is low for long periods, the diseased fish caused by A. punctata could be observed. Gross signs and behaviours are usually shown by hemorrhage on the fin and tail. In a heavy case, erosion of tail and fin can be seen.
b. Vibrio bacteria
Diseases caused by Vibrio sp. typically appear as ulcerative hemorrhagic septecaemia. The typical symptoms of vibrio disease include congeston of the fins, eccymoses and petechiae on the body surface and frequently, haemorrhages and ulceration of the skin and muscle tissue. The tissues surrounding the infected anus (the vent) are usually reddened and inflamed. Internally, there is congestion and haemorrhagia of the liver, spleen and kidney, frequently accompanied by the presence of necrotic lesions. The gut and particularly the rectum may be distended and filled with a clear viscuos fluid.The body is completely covered by a thick layer of mucous. Occasionally, small-unbroken lesions are present. There may be a reddening of the caudal funs and vent. Internal organs appear normal. Young fish die more rapidly than adults.The pathogenic vibrio which have been isolated from seabass include Vibrio parahaemolyticus, V. anguillarum and V. alginolyticus.
c. Columnaris disease
Columnaris disease caused by Flexibacter columnaris is one of the diseases commonly found in juvenile seabass which are raised in water of low salinity during rainy and winter seasons. Gross signs are observed by saddle-shaped lesions in the mid-body position about the dorsal fin of the fish. The bilaterally symmetrical lesion appears as a fuzzy, pale yellow white plague, with dark margins, often eroding in the epidermis. Clinically, the condition may be chronic, acute or peracute. The gram negative, aerobic bacillus (about 12 um) can be isolated from the lesion of the diseased fish.
Protozoans are probably the most important group of animal parasites affecting fish. Many reports from all over the world indicate great losses in fish culture caused by protozoans. Obligate parasites such as the ciliate ichthyophthirius and certain species of the cnidosporidians are responsible for many of these losses. Many species, which are considered as commensal protozoans, may become pathogenic under certain conditions. Environmental factors affect the susceptibility of fish to certain protozoans. Oxygen concentration and temperature are the factors affecting both hosts and parasites. Since many protozoans transfer from fish to fish through the water, fish population density is an important factor. Tremendous infestation of protozoans can occur in a relatively short time where fish populations are dense. Other factors, such as host size, age, host specificity, immunity, and the aforementioned influences of host condition also play an important part in the host reaction to invasion by protozoans. Most host reactions to invasion by protozoans are directed towards expelling or isolating the parasite.
Protozoans cause harm to fish mainly by mechanical damage, secretion of toxic substance, occlusion of the blood vessels, depriving the host of nutrition and rendering the host more susceptible to secondary infections. Some of the most common clinical signs are changes in swimming habits, such as loss of equilibrium, flushing or scraping, loss of appetite, abnormal colouration, tissue erosion, excess mucous production, haemorrhage, and swollen body or distended eyes.
a. Cryptocaryon sp.
Cryptocaryon is a marine counterpart of the freshwater Ichthyophthirius species and similarly causes the white spot diseases in marine fish. Its morphology and life cycle is quite similar to that of the “Ich”. The surface of invaded fish reveals white pustules or numerous minute, greyish vesicles which are nests of cilliates burrowing under the epidermis. They feed on the host's cells underneath the epithelium and cause heavy irritation resulting at first in excessive production of mucous and finally completely destroying the fine respiratory platelets of the gill filaments. On the skin, this parasitic protozoan causes considerable lesions resulting in destruction of large areas of the epidermis. Secondary infection may complicate the situation and the host dies. The incidence of Cryptocaryon sp. in seabass showed a distinct peak during low water temperature period, with a marked prevalence during February. This ciliated protozoan probably causes more damage to fish populations over the entire world than any other single parasite.
b. Trichodina sp.
Members of the genus Trichodina with about 60 species described from marine fish and related peritrichous ciliates are the most common parasitic protozoans that are especially harmful to young fish. The species attach themselves to the gills of marine fish. More than half of juvenile seabass heavily infected with this parasite died. Trichodina also causes problems to crowded seabass in cages.
Clinical signs of trichodinosis include excess mucous production, flushing, debility and hyperplasia and necrosis of the epidermis. The fin may become badly frayed in heavily infected fish and this may be accompanied by sluggishness and loss of appetite. Excessive number on the gills of infected fish interferes with respiration.
c. Henneguya sp.
Henneguya is a flaggelate found to attach mainly to the gills of seabass in cages. In heavy infections, it may be found in the skin. Gross signs are hyperplasia, bronchitis plus necrosis. Life cycle involves a free-swimming dinospore, which moves by means of 2 flagellae. It attaches to the host and transforms to a sac-like trophont, which has elaborate attachment mechanism. It feeds and grows and detaches from host, sinks to substrate where it encysts and produces dinospores.
d. Epistylia sp.
Epistylis is another protozoan found in seabass especially in freshwater environment. Epistylis belongs to the sub-class Peritrichia, and is common ectocommensals; however, it occasionally turns pathogenic. It attaches to the fish with its stalks. This protozoan may be present at a variety of temperatures and its number may be large enough to cause a grey mat on the epithelial surface.
Worm diseases with the possible exception of those produced by monogenetic trematodes have not yet appeared to be a serious problem in seabass culture. This is probably due in large part to their complex life cycle and the difficulty in completing such cycles in the culture system. Helminthis parasites, which have been found in seabass, include monogenetic trematode, digenetic trematide, nematide and acanthocephala.
a. Monogenetic trematodes
Monogenetic trematodes can be observed throughout the year. Abundance of these parasites and their seasonal distribution have not been studied. It has been reported that temperature apparently plays an important role in determining outbreaks of certain parasitic Monogenera. Peak infections of monogenetic trematodes usually occur among young susceptible fish. Such behaviour is advatanegous for the spread of the organism in a fish population.
b. Digenetic trematodes
Lecithochirium sp. was found in the intestine of seabass especially in wild fish. Incidence of infection was 86.0 percent and average parasite burden was 5.5 Another digenetic trematode which was commonly found in the intestine of wild seabass is Pseudometadena celebesensis. Its incidence of infection and parasite burden were 100 percent and 9.3, respectively.
Although many species of nematodes are found either as adults or larvae in fish, few have been implicated as serious pathogens of their hosts. In seabass, nematode of the genus Cucullanus was found more common in the gut of larger fish than in that of young fish.
Acanthocephalid worms, despite their fearsome-looking proboscis with its rows of hooks, have not been observed as serious pathogens of fish. The great majority of acanthocephalus in seabass are found as adults in the gut.
The parasitic copepods are among the most devastating of fish parasites. The mature female usually attaches to the fish and feeds on the host. After copulation the female matures and produces egg sacs while the male dies.
a. Caligus sp.
Caligus sp. has caused big problems in cultured seabass. They attach to the gills, buccal and opercular cavities, occasionally on the skin and fins of the seabass. Heavy infections can cause mass mortalities especially in young fish.
b. Lernanthropus sp.
Lernanthropus are found attached to the gill of seabass especially in cage cultured fish. Large numbers of this parasite can cause anaemia to the fish host.
Treatment is usually in the form of chemotherapy, possible combined with some of the preventive measures listed above. Chemical control should be a “last resort” in disease control.
To treat the pond accurately, the volume of water in the pond must be known. To determine the volume of a pond, multiply the number of surface unit area of water by the average depth of the pond.
The following chemicals are often used in the treatment of various fish diseases:
For small ponds, dilute the chemical in a bucket of water and distribute evenly over the pond using a dipper. In large ponds, the chemical should be mixed in a large drum and distributed evenly over the pond.
When treating fish, it is advisable to know the quality of the water because such things as pH and temperature greatly affect treatment results. Treat a few fish first and see how they react before treating the entire group. Use only the drugs and chemicals that have been cleared by an authorized agency for use on food fish. Here are some general treatments for specific groups of pathogens:
Remember the following precautions during treatment:
The Pearl spot, Etroplus suratensis commonly known as “Karimeen” in Kerala is an indigenous fish extensively found along the east and south-west coasts of Peninsular India. It is an important candidate species for aquaculture in ponds in both brackishwater and freshwater environments. It is cultured in traditional ponds in Kerala where it is considered a delicacy fetching a high price up to Rs. 150/ kg. Though growth is slow, at a high stocking density table-size fish can be harvested in 9-12 months culture period.
Seed of pearlspot is available throughout the year along the east and south-west coasts of India. The peak season of abundance is during the months of May-July and November-February. It can be easily collected from both the brackishwater and freshwater tanks and ponds. A simple method of seed collection is adopted taking advantage of the tendency of the fish to congregate in large numbers for feeding on epiphytic growth. In this method twigs or branches are kept submerged in the water a week ahead of day of collection. The juveniles congregating for feeding purpose are trapped using an encircling net or trap. Fecundity of pearl spot is low and has been estimated to be around 3000-6000; hence a successful hatchery production of seeds is difficult. However, Central Institute of Brackishwater Aquaculture (CIBA), Chennai using the technique of environmental manipulation, has successfully demonstrated the hatchery seed production of pearl spot.
Before letting in water, the ponds are drained and lime is applied at the rate of 300 kg/ha. In undrainable ponds, piscicide (Mohua oil cake @ 200-250 ppm) may be used to eliminate the weed fishes etc. After a time gap of 10-15 days for the neutralization of the residual effect of the piscicide, water is let in through screens to avoid the entry of undesirable fishes. The pond is filled up to the appropriate level (1.2 m) and cow dung applied at the rate of 1500-2000 kg/ha for promoting plankton production.
Acclimatization and stocking of breeders
Adult Etroplus in the weight range of 50-125 g procured from the wild or culture ponds are stocked @ 5000/ha after one week of fertilization of the pond. Breeders collected from the wild are to be disinfected by dipping in 1% commercial formalin and acclimatised before introducing into the pond. Since the fish is monomorphic, sex differentiation is difficult and it has to be assumed that the sex ratio of the stocked breeders is approximately 1:1. Additional breeders have to be added to the existing stock from the second year onwards to compensate the natural mortality of breeders. Breeders once stocked will be normally viable for three years.
Provision of spawning surfaces
In the natural environment the fish attaches its eggs to submerged substrata like stones, aquatic plants etc. As a prepared pond may not have such natural spawning surfaces, materials like palmyrah leaves tied in bunches to fixed poles, coconut leaf petioles, coconut husks, bricks, pieces of asbestos sheets etc., have to be provided in the ponds.
Water quality monitoring and management
Maintenance of optimum water quality is important for the successful breeding of Etroplus. Water quality parameters like salinity (15-30 ppt), dissolved oxygen (>3.5 ppm), pH (7-8), temperature (24-32°C), transparency (>50 cm) and ammonia (<1 ppm) have to be maintained. Optimum water level in the pond is 1.2 m. The loss of water due to seepage and evaporation is to be compensated by pumping in water. Exchange of pond water through sluice or any other means is not desirable, as it will lead to escape of hatchlings and fry. Salinity should not be allowed to go beyond 30 ppt. The fishes should not be disturbed frequently.
Feeding of the breeders has to be initiated within 3-4 days after stocking. Artificial feed prepared with groundnut oil cake 40%, rice bran 45% and fish meal 15%, fortified with vitamin and mineral mix @ 2.5 kg per 100 kg feed, is to be supplied daily @ 3-5% of the fish biomass, either in pelleted or in dough form. Feed can be supplied in feeding trays kept at the bottom of the pond. The feeding trays should be examined daily and cleaned outside the pond. The quantity of the feed can be reduced whenever left-over feed is present in the trays, to avoid wastage and water pollution. The presence of hatchlings indicates that the pond is to be manured with cow dung @ 500 kg/ha for the production of plankton, which forms the food for the hatchlings. Small quantities of the artificial feed (250-300 g/pond of 1000 m2) also can be broadcast in powder form during early morning.
Fry production and harvesting
The experiment conducted in a pond of 100 m2 area gave an actual production of 3500 fry from five sets of spawnings in a year (projected production rate is 3.5 lakhs/ha/yr) when the breeders were stocked at the rate of 6800/ha. Fry production from a pond of 300 m2 area from 3 sets of spawnings over a period of 5 months was 9600 and this works out to more than 5 lakhs/ha/year from five sets of spawnings, when the breeders were stocked at the rate of 5400/ha. This probably indicates that the rate of fry production may be enhanced from a bigger pond even at a low stocking density of breeders.
The eggs are oblong in shape, about 1 to 2 mm in diameter, attached at one end by means of a short stalk to the nesting object. The newly laid eggs are yellowish in colour and as the embryo develops, the colour becomes brownish and the yolk sac becomes pigmented. The incubation period lasts from 82 to 100 hours. During hatching, the egg membrane bursts first over the head of the larvae, which is at the free end, and this continues along the upper side by the waving of the tail. The early larval stage lasts for 7 days during when the larvae develops into a free-swimming individual. During late larval stagethe larvae, though free swimming, are quite different from the adult. The tail remains long and the caudal fin is continuous with dorsal and anal. After a fortnight, the primary chromatophores on the back disappear and permanent colour bands begin to appear. The larvae assume adult form within a month after hatching and measure about 18mm.
The young ones feed almost exclusively on zooplankton, the advanced fry on aquatic insect larvae, filamentous algae and other vegetable matter, while the adult fish subsists mainly on filamentous algae, aquatic macrovegetation and planktonic organisms. Worms, shrimps and insect larvae also form part of its food. Adult pearl spot can be fed with pelleted fish feeds.
The pearlspot is suitable for culture in confined, fresh and brackishwaters. The fish is cultured on a small scale mainly in the state of Kerala. It is cultured in the traditional manner, in the 'Pokkali' fields (paddy fields). An annual yield of 3 to 5 tonnes is obtained from these fields, of which, prawns constitute 80%, while the mullets and pearlspot form 20%. In mixed-culture operations along with prawns and other fishes ranged from 768.2 kg/ha/3 months at a stocking density of 25,200/ha (24000 prawns + 1200 fish) to 845.4 kg/ha/110 days at a stocking density of 20,300/ha (20,000 prawns + 300 fish) have been reported. The culture of pearlspot is more economical under polyculture system especially with milkfish and mullets than under monoculture.
The fish can attain a marketable size of 120-150 g over a period of 8-10 months. Though growth rate is relatively slow, high stocking density with low input management can yield optimum production. Under monoculture at stocking densities ranging from 20,000 to 30,000 / ha, an average production of 1,000 kg/ha/year can be obtained in brackishwater ponds. The fish can also be reared in the backyard ponds and tanks in the rural areas. Being a herbivorous fish it is suitable for polyculture. Pearlspot farming could be adopted to any scale integrating with other occupations like poultry farming. The poultry droppings form good manure for natural food production in the culture ponds.
Adult fish in the weight range of 50-125 g are stocked in ponds @ 5,000 nos./ha. The fish are fed with supplementary feed @ 3.5% of the body weight (prepared with groundnut oil cake 40%, rice bran 45% and fish meal 15% fortified with vitamin and mineral mix @ 2.5 kg per 100 kg of feed). The feed is supplied in pellet or dough form. The hydrographical parameters desirable for the breeding and seed production of pearl spot are: water temperature 24-32°C, salinity 15-30 ppt, dissolved oxygen > 3.5 ppm, pH 7.0 to 8.0 and transparency > 50 cm. To facilitate egg attachment, the pond is provided with substrate materials such as palmyrah leaves, coconut leaf petioles, coconut husks, wooden twigs, bricks etc. Breeding occurs within 30-40 days of introduction of the brooders. A production of upto 6 lakhs fry/ha/year can be achieved. Harvesting is usually undertaken by draining the water from the ponds and operating a seine net, cast net or a drag net for capturing the fish.
Pearl spot is prone to many diseases mainly caused by wide fluctuations in environmental parameters. The most common disease causing agents are bacteria including Pseudomonas, Alcaligenes, Flavobacteria, Moraxella, Vibrio and gram-positive Micrococci, Athrobacter and Bacillus spp.
Bacterial diseases of E. suratensis
The milkfish (Chanos chanos) is one of the most ideal finfishes for farming in coastal areas. They are fast growing, tolerates a wide range of temperature, oxygen and salinity. They feed mostly on filamentous algae from the bottom of the pond and are free from major diseases and parasites. Milkfish are cultured in large scales in countries like Indonesia, Philippines and Taiwan in ponds called “Tambak”. In India too the popularity of its farming is growing especially in Tamil Nadu and Kerala.
The milkfish has a moderately compressed, spindle-shaped, elongated body covered with small scales and without scutes along the belly. In its early stages of life history, in natural environment, the fish enters estuaries, backwaters, lagoons and rivers, which serve as nursery grounds and spend early part of its life there for about a year or more.
Since milkfish does not mature and breed in confined waters and culture ponds, development of a hatchery technology has been difficult. However, induced breeding has been successfully carried out in this species, but the final survival rate has been less and hatchery operations hence are not economical. In this respect the most suitable method is collection of seeds from natural sources. In India, the seeds of milkfish measuring 2-7 cm in length occur along the coast of Orissa, Andhra Pradesh, Tamil Nadu, Kerala and Karnataka. The main fry season extends from March to June. Milkfish seeds prefer clear calm inshore waters of gently sloping beaches including tidal creeks, estuaries, brackishwater bodies and mud flat areas where the temperature is about 23-25 oC and salinity varies between 10 and 32 ppt.
The seeds are collected from low-lying areas using scoop nets, dip nets and hand nets. In estuaries and lagoons, drag nets or seine nets may be used, while in mud flat areas, a scare line made of coir rope of about 3-3.5 m length with palmyrah leaves attached is dragged below the water surface and seeds collected in fine meshed cloth. Soon after collected the seeds are conditioned by keeping in a limited volume of clear water for a definite period without food. Seeds are transported in containers with diluted seawater of 10-15 ppt salinity and at a rate of about 100 fry/L.
Nurseries are ponds for rearing the fry until they attain 5-7 cm in length. The area of nursery ponds ranges from 500 to 5,000 m2. At the nursery site the fry are acclimatized to the salinity of the pond water. Preparation of pond for stocking with fry has to be started about one or two months in advance. The ponds are drained and dried for about 10 to 15 days and later tilled and raked. Lime is added @ 1000 kg/ha and fresh water is let in. Pond water is fertilized with organic and inorganic fertilizers. Once the algal bloom develops more saline water is added to a height of 10 cm. Within 3-7 days, a complex of blue green algae, diatom, bacteria, nematode worms develop at the bottom of the pond called “Lab-Lab”. This algal consortium is most vital for developing frys of milkfish. Stocking to the nursery pond is usually done only after “Lab-Lab” has developed in the pond. The fry are stocked at densities of 20-50/ m2. The threat of predatory fishes, crabs and snakes can be screened from entering the pond using nets. Erecting poles along the embankments and crisscrossing with strings can discourage predatory birds. One serious cause of mortality of seeds in fry ponds is the sudden reduction of temperature and salinity due to heavy rains. Filling the ponds with brackishwater before rains may prevent such an eventuality.
The fry feed actively on Lab-Lab and phytoplankton and grow rapidly. By the end of one month they measure 5-8 cm long and weigh 1.5-5 gm, when they are ready for transfer to the production pond or pen structures for rearing. They are captured by partially draining the fry ponds at low tides, when the fry usually congregate near the water gates, for which seine nets are used.
Farming in earthen ponds
The ponds where milkfish fingerlings are reared are called “production ponds” or “rearing ponds”. The production pond ranges from 0.5 ha to 3 ha in area and are rectangular in shape, with water depth ranging from 0.3 to 0.7 m. The best bottom for these ponds is a soft, jelly like, colloidal and biologically active mud containing about 4% humus and large amounts of clay. The silt, sand and clay should be in the proportion of 64%, 32% and 4%. In Taiwan, a system of canals is provided on the outer boundary of the ponds to give protection to fish against extreme conditions of temperature.
About two months prior to stocking, the pond management begins for augmenting the growth of blue-green algae in pond bottom. These are similar to those practiced in nursery pond viz., draining of the water, drying, tilling, leveling and raking. Manuring of the ponds is always carried out. Usually, green manure is used, such as leaves and twigs of mangrove plants, rice straw, copra, rice bran, oilcakes, pig manure, chicken manure etc. The rate of fertilization usually varies from 500 to 2,000 kg/ha. Beside organic manure, inorganic fertilizers containing nitrates, phosphates and potassium (NPK) such as superphosphates, triple superphosphates, urea etc may also be applied at required rates. Water is then let into the pond and the depth is increased to about 30 cm gradually. Within two weeks the algal-periphyton complex (Lab-lab) develops at the pond bottom. Stocking in earthen pond follows only after the growth of Lab-lab. Usually fingerlings of 7 to 15 cm length are stocked at a rate of 2,000 to 10,000 per ha. When the growth of Lab-lab in the pond decreases, fresh dosages of manure are added.
Farming in pens
Inexpensive pen structures for farming the milkfish are constructed in shallow natural creeks, swamps, lagoons, lakes and bays, ranging in depth from 1 to 3 m. The bottom in pen culture sites should be of firm clay or mud so that poles and posts can be driven sufficiently deep to make them support the pen structure. Traditionally pens are made up of wooden planks, split bamboo etc. But in recent times, nets materials made of synthetic materials such as nylon, polypropylene, polythene etc are used commonly. A part of the vertical net barrier is buried inside the mud or ground with the aid of a footrope and small weights, secured to a chain link between concrete sinkers. At the upper level, floats are provided. Fingerlings stocked in usually feed upon the natural food in the lagoon or lake and no artificial food is provided.
Eradication of predators and pests
Certain predatory fishes, crabs, water snakes are the common threat to milkfish culture ponds and pen structures. These can be prevented from entering the farming area through use of fine meshed screens. Apart from these larvae of certain insects also pose threat to fingerlings of milkfish. Application of 0.5 ppm Baluscide, lime, urea etc. in the initial preparation of the ponds helps to reduce their growth.
Milkfish has a higher growth rate in its first year in brackishwater, during when it grows to a marketable size of 30-45 cm long and 300-800 gm in weight. The periodicity of harvesting depends upon the number of batches stocked. During harvesting the pond is drained using pumps, while in the case of pens, the lowest tidal period is the best time for harvest. If trenches were provided in culture ponds, it would be easier to gather all the fish inside the trenches by draining the water and then capturing them. Usually, seine nets are operated for capturing farmed fish. The survival rates ranges from 80 to 95% amounting to a production ranging from 500 to 1000 kg/ha in ponds and 250 to 500 kg/ha in pens.
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