Seed supply
Due to their larger size and better survival, captured wild seeds were used commonly in south Asia for extensive ponds, which require a minimal amount of seed for stocking. However, the use of wild seeds has been reduced, due to overfishing and the outbreak of white spot disease in shrimp nursery grounds. Therefore most
Penaeus monodon grow-out farms now rely solely on hatchery-produced seeds.
Broodstock
Healthy females (25-30 cm body length and 200-320 g weight) and males (20-25 cm; 100-170 g) captured from the wild are preferably used as broodstock in the induced ovarian maturation process. Broodstock from greater depths (60-80 m), or more than 20 miles offshore, are preferable due to the lower prevalence of shrimp diseases, which are higher in coastal shrimp farming areas. Once the shrimp have recovered from transport stress for a few days, they are stocked in a circular maturation tank that is normally covered and kept in a dark room. The same stocking density (2-3/m²) is used for both females and males. Shrimp are subsequently induced to moult by manipulating the salinity of the water. After mating has occurred, which is easily determined by the presence of a spermatophore in the thelycum and hardening of the shell, the eyestalk of females is unilaterally ablated for endocrine stimulation. Broodstock are fed with squid, mussel or cockle meat, supplemented by polychaete or
Artemia biomass to enhance reproductive performance.
The early stage of ovarian development can be first observed within a week after ablation. Later, gravid females with ripe stage eggs, which can be observed by the opaque diamond-shaped ovary under torchlight, are collected and transferred into spawning tanks. After spawning, these females can be re-used in the maturation process a few times, while males can be further used for several months, depending on prawn health and tank conditions.
Whether the spawners are caught directly from the sea or from an induced maturation tank, they generally spawn on the first or second night in the hatchery. However, spawning can be delayed for long distance or overnight transportation if each spawner is placed tightly in a PVC pipe to straighten its body. Gravid females should be placed individually in a small spawning tank to avoid the spread of disease that may occur in mixed spawning.
After spawning, eggs are generally kept in the same tank for fertilization until hatching. Nauplii are then collected and cleaned (rinsed with flowing seawater to remove the fat and debris released by the spawner) for transfer to larval rearing tanks or for transportation to other remote hatcheries. In Thailand, thousands of specialized small-scale or backyard hatcheries in inland areas buy nauplii and grow them to PL 12-15 because they cannot conduct the costly broodstock maturation operation. Broodstock maturation facilities located at the coast require a large volume of clean, clear seawater, while closed system larval rearing generally needs much less seawater. Therefore the inland hatcheries, whose land costs are much lower, are able to operate economically by purchasing seawater or brine that has been transported by trucks from the sea or from salt pans.
Hatchery
Smaller indoor concrete tanks (4-5 tonnes) than those originally used have now proved more efficient and manageable for larval rearing, particularly during the application of closed systems for disease prevention. If an outdoor system is unavoidable, due to economic constraints, tanks should be covered by black cloth or roof tiles in order to avoid the diurnal fluctuation of water temperature, and also to reduce light intensity. Nauplii are usually stocked at 100 000/tonne and cultured up to late mysis or early PL with approximately 70-80 percent survival rate. They are then transferred to a new tank and cultured up to PL 12-15; a further 70-80 percent survival rate is achieved in this stage. Diatoms (either
Chaetoceros, or
Skeletonema or
Tetraselmis) that have been reared in monoculture, are fed at an approximate density of 30 000-50 000 cells/ml, starting from the protozoea stage and continuing until early PL (4-5). Diatoms can be replaced by micro-encapsulated diets or dry formulated feeds if their production is interrupted by rain.
Artemia nauplii, at an average of 50 g of cysts per 100 000 larvae are given from mysis up to early PL stage.
Artemia flakes are also used to supplement
Artemia nauplii for cost reduction. From PL 4 to PL 15, artificial diets are commonly used to reduce the deterioration in water quality that occurs when fresh feed is applied. From hatching, it takes about 26 days to reach PL 15.
Nursery
Due to its benthic habit, the harvesting of nursed juveniles in separate earthen ponds is difficult; thus nursing of hatchery produced postlarvae is impractical. Nursing in concrete tanks also yields poor survival, due to the cannibalistic behaviour of PL at high stocking densities. Since intensive ponds are well treated to eliminate all fish predators, it is safe to stock PL 15 prawns directly into the grow-out ponds. If the pond has not been well-prepared in time, or some predators have been observed, or the PL seem weak, PL 15 may be acclimatized by impoundment in nets, pens or small enclosure within grow-out ponds for less than a week before release.
In semi-intensive ponds, where postlarvae are not fully fed by artificial diets and some fish predators still remain, postlarvae are commonly nursed for a few weeks in an earthen compartment (5-10 percent of pond area) within the grow-out ponds. This enables the feed to be concentrated in this small nursing area, resulting in juveniles that are larger and thus better able to escape from the remaining fish predators after release into the grow-out pond.
Ongrowing techniques
There are three on-growing culture practices: extensive, semi-intensive and intensive, which represent low, medium, and high stocking densities respectively. Due to its benthic feeding habit
Penaeus monodon is commercially cultured only in earthen ponds, under widely varying salinities from 2 to 30‰.
Extensive
Commonly found in Bangladesh, India, Indonesia, Myanmar, the Philippines and Viet Nam, extensive grow-out of shrimp is conducted in tidal areas where water pumping is unnecessary. Ponds with an irregular shape according to land boundaries are generally larger than five hectares and easily constructed by manual labour for cost reduction. Wild seeds, which either enter the pond through the gate by the tide or are purchased from collectors, are usually stocked at a density not exceeding 2/m². Shrimp feed on natural foods that enter the pond regularly on the tide and are subsequently enhanced by organic or chemical fertilizers. If available, fresh fish or molluscs may be used as supplementary feed. Due to the low stocking densities, larger sized shrimp (>50 g) are commonly harvested within six months or more. The yield is lowest in these extensive systems, at 50-500 kg/ha/yr. Due to the increase in land costs and the shortage of wild seeds, almost no new extensive farms are being constructed today. After gaining experience in shrimp farming, many farmers have upgraded their ponds to semi-intensive systems to provide better incomes.
Semi-intensive
Semi-intensive ponds (1-5 ha) are commonly stocked with hatchery-produced seeds at the rate of 5 to 20 PL/m². Water exchange is regularly carried out by tide and supplemented by pumping. The shrimp feed on natural foods enhanced by pond fertilization, supplemented by artificial diets. Production yields range from 500 to 4 000 kg/ha/yr.
Intensive
Intensive farms are commonly located in non-tidal areas where ponds can be completely drained and dried before each stocking. This culture system is found in all
Penaeus monodon producing countries and is commonly practiced in Thailand, the Philippines, Malaysia and Australia. Ponds are generally small (0.1 to 1.0 ha) with a square or rectangular shape. Stocking density ranges from 20 to 60 PL/m². Heavy aeration, either powered by diesel engines or electric motors, is necessary for internal water circulation and oxygen supply for both animals and phytoplankton. Feeding with artificial diets is carried out 4-5 times per day followed by feed tray checking. Final FCR is normally between 1.2:1 and 2.0:1. Since the outbreak of white spot disease, reduced water exchange and closed systems have become commonplace, due to their lower risk of introducing viral diseases through intake water. However, feed and phytoplankton blooms need to be carefully monitored and managed to avoid deterioration of the pond bottom and water quality due to wastes.
P. monodon has a habit of slowly nibbling feed on the pond bottom; this causes substantial nutrient losses because pellet stability is generally not longer than two hours. Efficient feed management is the major criteria for a successful crop, since feed represents over 50 percent of the production costs in intensive systems. Water quality parameters such as pH, salinity, dissolved oxygen, alkalinity, Secchi disc, H
2S and unionised ammonia are regularly measured. If closed system culture is applied, stocking should be minimized, otherwise the pond must be harvested earlier (within 3.5 months instead of 4-5 months) and smaller shrimp will be yielded (20 g, instead of 30-35 g as achieved in semi-intensive and intensive systems with water exchange). Production yields of 4 000 to 15 000 kg/ha/yr are commonplace.
Feed supply
As shrimp feed technology has been readily available, each major producing country has developed its own commercial feed factories instead of relying on expensive imported feed. Prolonged storage of imported feed caused by sea transport or because of the necessity to import economically large volumes in each order, tends to reduce feed quality due to the rancidity.
Harvesting techniques
Bamboo traps are traditionally used for the partial harvest of selected large shrimp in extensive culture. Semi-intensive ponds are commonly harvested by draining the pond by tide through a bag net installed at the outlet sluice gate. Intensive ponds are normally harvested similarly to semi-intensive ponds. If the tide does not allow harvesting, the drainage canal can be blocked to allow the water to be pumped out to reduce the water level. It is still necessary to pick the remaining shrimp by hand after the pond has been drained.
In Thailand, artificial sluice gates are temporarily installed inside the pond for the harvesting of many closed system ponds where a sluice gate for water exchange is not necessary. Shrimp are then trapped in this artificial gate during the pumping out of the water. For the live shrimp market, ponds are partially harvested by cast net in the early morning. Due to its burrowing habit, a drag net is not practical unless it is installed with electric shock gear to stimulate the shrimp to jump.
Handling and processing
If shrimp are sold directly to processing plants, specialized teams for harvesting and handling are commonly used to ensure the first grade quality of their raw materials. After rough sorting, shrimp are washed, weighed and immediately killed in iced water at 0 °C. The most difficult job is to clean the shrimp picked by hand from pond bottoms at the end of the harvest, because they contain a lot of mud, organic matter and debris. Shrimp are then kept in ice in insulated containers and transported by small pick-up trucks for short distances or by large insulated trucks over long distances, either to processing plants or shrimp markets. For transportation of live shrimp from farms directly to aquarium tanks in restaurants, the shrimp are kept in aerated plastic containers at a density of 0.2-0.3 kg/litre of water. The containers are typically placed on small pick up trucks with a roof. For the export of live shrimp from Thailand to Hong Kong and China, water temperature is gradually reduced to 16-17 °C until the shrimp become inactive. The dormant shrimp are then packed alternately in layers of chilled sawdust or polystyrofoam beads in insulated boxes for export by air. This dry packing can minimize freight costs and shrimp can survive for 12-15 hours. Domestic markets mainly require chilled product supplied directly from farms or from shrimp markets.
In processing plants, shrimp are properly cleaned and sorted according to export standard sizes. Depending on market requirement, shrimp are processed in several categories before quick freezing at -10 °C and stored below -20 °C for further export by ship or air cargo. Due to an increasing demand and higher profit margin, many processing plants increasingly operate value-added product lines.
Production costs
Production costs always vary depending on the site, season, scale of production, water management system (such as water exchange versus closed system), irregular production yield affected by culture problems, outbreak of diseases, etc. Operational costs for seed production regionally average at about USD 2.5/1 000 PL.
Production costs for adult shrimp are summarized as follows (USD/kg):
Seed |
0.53 |
0.58 |
0.59 |
Feed |
– |
1.41 |
2.02 |
Labour |
0.85 |
0.20 |
0.19 |
Electricity & fuel |
0.21 |
0.36 |
0.33 |
Chemical, materials & supplies |
0.16 |
0.18 |
0.26 |
Overheads |
– |
0.13 |
0.37 |
Depreciation |
0.20 |
0.66 |
0.52 |
Total |
1.95 |
3.52 |
4.28 |
The major disease problems are included in the table below. There are no chemicals or drugs available to treat the viral infections listed but good management of pond, water, feed and health status of stock inputs can reduce their virulence. Outbreaks of the most serious virus (WSD) always occur after dramatic changes in water parameters such as temperature, salinity caused by heavy rain, DO
2, hardness, and the stress to shrimp caused by deterioration in water quality and pond bottom environment. Pond preparation by proper bottom cleaning or regular scraping of the fouled layer is also a key factor for prevention of the shrimp stress caused by built up waste and toxic gases, and also for the elimination of virus carriers, particularly crustaceans. For confirmation, the Polymerase Chain Reaction (PCR) test for white spot disease or other viruses is widely used for screening of broodstock before spawning, nauplii before larval rearing, late PL before pond stocking, and shrimp in ponds for regular monitoring.
In some cases antibiotics and other pharmaceuticals have been used in treatment but their inclusion in this table does not imply an FAO recommendation.
White spot (WSD)Otherwise known as WSBV, WSSV |
Part of the white spot syndrome baculovirus complex |
Virus |
Acutely infected shrimp show rapid reduction in food consumption; lethargy; high mortality rates with cumulative mortalities reaching 100 percent within 3 to 10 days of the onset of clinical signs; acutely infected shrimp often have loose cuticle with white spots (which represent abnormal deposits of calcium salts by the cuticular epidermis) of 0.5 - 2.0 mm in diameter that are most apparent on the inside surface of the carapace; in many cases moribund shrimp display a pink to reddish-brown colouration due to expansion of cuticular chromatophores & few if any white spots |
Screening of broodstock, nauplii, PL & grow-out stages; avoiding rapid changes in water conditions; avoiding shrimp stress; avoid use of fresh feeds, particularly crustacean; minimizing water exchange to prevent virus carriers entering the pond; treating infected ponds or hatcheries with 30 ppm chlorine to kill infected shrimp & carriers; disinfect associated equipment |
Yellowhead (YHD) Also known as Yellow-head shrimp disease, Yellow-head virus (YHV), Yellow-head baculovirus (YBV), Yellow-head disease baculovirus (YHDBV) |
Not yet described |
Virus |
Acute epizootics with high cumulative mortalities that may reach 100 percent within 3-5 days after appearance of clinical signs; infection is horizontally transmitted; PL 15 have been found to be resistant but PL 20-25 & on-growing juveniles through to sub-adults are highly susceptible; initially, feeding increases, followed by reduced feeding in later stages of the disease; pale body; yellowish swollen cephalothorax & hepatopancreas; whitish-yellowish-brownish gills; presumptive diagnosis can be made on basis of pond history, clinical signs, gross changes & histopathology |
Screening of broodstock before hatchery operation & PL before stocking in pond; avoiding rapid changes in water pH, alkalinity, & dissolved O2; avoiding fresh aquatic feeds; proper cleaning of pond bottom before stocking; infected ponds & hatcheries must be disinfected similar to WSV (see above) |
Baculoviral Midgut Gland Necrosis (BMN)Also known as midgut gland cloudy disease, white turbid liver disease, and white turbidity disease |
Baculovirus |
Virus |
Generally infects larvae & early postlarval stages in which it can cause high mortalities; apparent white turbidity of the hepatopancreas caused by necrosis of tubule epithelium & possibly also the mucosal epithelium; larvae affected but later stages (late postlarvae) tend to show resistance; source of infection documented as wild-caught female spawners; larvae float inactively on the surface & exhibit a white midgut line through the abdomen |
Wash fertile eggs through a soft gauze by running clean seawater to remove excrement or faeces of spawner; if infected, culture facility must be disinfected to avoid re-introduction of virus |
Nuclear Polyhedrosis BaculovirosesAlso known as Monodon baculovirus disease (MBV) |
Baculovirus |
Virus |
Lethargy, anorexia, dark coloured shrimp; reduced feeding & growth rates; often increased surface & gill fouling with various epibiotic & epicommensal organisms; severely affected larvae & postlarvae may exhibit a white midgut line through the abdomen; acute MBV causes loss of hepatopancreatic tubule & midgut epithelia &, consequently, dysfunction of these organs, often followed by secondary bacterial infections; linked with high mortalities (>90%) in late postlarvae & juvenile shrimp in many culture facilities; usually juvenile & adult P. monodon are more resistant to MBV than larval shrimp; MBV may predispose infected shrimp to infections by other pathogens; |
Reduce stocking density, use of chemicals & environmentally induced stress; prevent contamination of fertilized eggs from spawner faeces by washing in formalin or iodophore treated seawater; if infected, culture facility must be disinfected & stock should be removed & sterilized |
Suppliers of pathology expertise
No specific institutes or laboratories named, but shrimp pathology expertise is now readily available.