It is not always necessary, or even desirable, to use hatchery stock for grow-out operations. In many areas, there are abundant supplies of naturally occurring postlarvae which can be utilized at a fraction of the cost of hatchery-produced postlarvae. These postlarvae are available, and there is no need to wait for hatchery technology to be developed. This point is especially important to small artisanal farmers who can utilize their own labour to collect wild post-larvae.
In Thailand, Singapore, Indonesia, Malaysia, and the Philippines, naturally occurring fry are taken into the ponds with incoming water and held for varying periods of time until they grow to a larger size. The main problems with this practice is that many undesirable organisms enter the ponds with the postlarval shrimp and stocking density of the shrimp is usually unknown. An improved method is to take unfiltered seawater containing fry into a nursery pond as frequently as tidal conditions permit. After 30 days, the pond water is treated with teaseed cake at a rate of 10 to 25 ppm. Teaseed cake, a residue from the processing of wild tea, Camelia sp., contains saponin, a chemical that at the recommended dosage, kills fish but not shrimp (see also Section 9.2.1). After the fish are killed, the shrimp are released to a larger pond where they are grown to marketable size.
Postlarvae are attracted by lights, a characteristic which can be utilized to increase the number of shrimp entering a pond at night. A lantern is suspended in front of the closed sluice gate. After allowing sufficient time for the shrimp to accumulate, the water surface is slapped loudly to scare away the fish fry. The shrimp fry are not disturbed by the noise, but the fish fry are, and they swim away. The sluice gate is then opened, and the rapid flow of water carries the postlarval shrimp into the pond.
There is only limited use of collected wild postlarvae at present. Milkfish fry collectors in Indonesia and the Philippines separate and sell P. monodon fry to farmers, usually for polyculture with milkfish (Chanos chanos). They normally do not separate and sell other penaeid species such as P. merguiensis, M. ensis or M. monoceros even though they are available in substantial numbers. Many of these shrimp, such as P. merguiensis, are cultured in some areas, and if pond conditions were improved, they could be farmed successfully throughout the region.
Methods of collection which have been developed for P. monodon usually occurs in only limited numbers. This shrimp's habit of clinging to objects has been put to good use in the first three methods.
Small bunches of branches or twigs are fixed in the bottom in shallow areas. Collections of shrimp are made during low tide by placing a scoop net under the bunch twigs as it is lifted up.
Lure lines made from bunches of saltwater grass (Paspalum vaginatum) or twigs suspended from ropes are used along beaches, in rivers and in estuaries. The line, about 20 m in length, is strung out with stick supports to keep it above the water surface. The bundles or lures are suspended from the line at short intervals and rest in the water. Several lines are worked at one time. Shrimp are collected by placing a scoop net under the lure and then lifting up the lure and net to the surface.
In grassy areas, scoop nets are run through the grass.
Push nets and scissor nets, with or without cod ends, are used along the beaches, and in rivers and estuaries. They can be either hand or boat-operated.
Tray method. Trays are baited with mud which is high in organic content suspended off the bottom. To collect postlarvae the tray is lifted and placed in a tank on a boat.
An improved method is to collect the postlarvae in fine mesh nets (Fig. 3) placed in tidal passes, water supply canals, or sluice gates. It is not necessary to attach wings to this type of collecting net if the water current is fast. In fact, sometimes wings can do more harm than good, by funnelling large organisms like jellyfish into the collecting net and causing it to become clogged. In mouths of small rivers or bays where tidal flaws are small, wings are needed. The nets are fished on incoming tides. It is important that the net be attended to constantly and the catch removed to a holding container at short intervals.
The best time to collect fry is on the rising high tide during the period of the new moon when tidal fluctuation is greatest. Subrahmanyan and Rao (1970) report that the best catch for P. monodon, P. semisulcatus and P. indicus is during the third, fourth and fifth hours of the rising tide. They state that large numbers of P. indicus are sometimes caught during the second hour. P. indicus was by far the more abundant species captured by a “shooting net”. During October, for instance, the average number of P. indicus caught per hour of sampling was 13 275. For P. monodon the highest average monthly catch rate per hour during two years of sampling was 190, while for P. semisulcatus, it was only 68.
In most methods of collecting, the fry are mixed with many unwanted pests. As it is time consuming to separate the fry manually, other methods may be tried.
Chemical treatment. After the catch is completed, the water in the holding container is treated with compounds like saponin which kill fish but not shrimp (see Section 9.2). This procedure eliminates the tedious job of separating the shrimp from fish fry by sorting individually, and prevents the accidental stocking of fish. When the fish are dead, the shrimp can be stocked directly with no further separation if there are no crab larvae present. If crabs are present, they must be sorted manually.
Screen and light. The fry are sorted by placing them in a box with walls of mosquito netting. The top should be covered so that light can not penetrate and the interior remains dark. The fry will swim out towards the light and the large predators and trash will be left in the box. The shrimp fry are then sorted from the other unwanted fry.
Sometimes there can be confusion about the identity of the shrimp collected. The same characteristics which are used to distinguish adult penaeids can be used to identify postlarvae. In penaeid shrimp, the first three pairs of walking legs have chelae (pinchers) and the first abdominal segment overlaps the second. However, it is usually necessary to use a microscope to observe these characteristics in postlarvae. The various parts of the shrimp are illustrated in Figure 4. Probably, the animal most commonly mistaken for postlarval shrimp is Acetes spp. (Sergestidae). These frequently occur in great abundance, but can be easily distinguished by their long, bright orange antennae which have a prominent sharp bend in them. Postlarval shrimp have short, colourless antennae. In addition, postlarval shrimp do not have statocysts (in live animals these appear like small bright spots to the naked eye) on the tail (Mysidacea), and their eyes do not extend laterally at a 90° angle (Sergestidae). The uropods of Mysidacea are not spread fan-like as in shrimp, but are parallel, directed posteriorly. Initially, some time will be required to identify the various forms, but after a short while, a collector will be able to distinguish penaeids readily, as they are quite distinctive. Some guides to gross visual identification are presented in Figure 5.
Separating species of penaeids from each other is a little more difficult. First, only Penaeus and Metapenaeus postlarvae should be found in inshore waters in this region. Penaeus postlarvae are long and thin, while Metapenaeus are relatively short and stout. Metapenaeus are generally coloured a mottled grey or brown. Postlarvae within the genus Penaeus are generally almost colourless, however, P. monodon and P. semisulcatus are coloured rust-brown. When the pigment chromatophores expand, a prominent bluish or reddish-brown streak appears on the ventral side of the body in P. monodon postlarvae. In P. semisulcatus, the chromatophores are prominent only on the sixth (last) segment. P. monodon is longer than P. semisulcatus and has a distinctive habit of swimming with its head lower than its tail, the body at about 45° angle. Other species of Penaeus may also be pigmented, however, as postlarvae of all the species indigenous to the region have not been described.
Prawirodihardjo, et al (1975) found that postlarvae of P. monodon and P. semisulcatus can be separated by the following characteristics (Fig. 6).
|P. monodon||P. semisulcatus|
|-||Brown pigment distributed evenly but more intensively on posterior part of telson||-||Brown pigment distributed only on the anterior base and about one-third posterior tip of telson.|
|-||Endopod of uropod may be totally or partly pigmented||-||Only posterior part of endopod of uropods pigmented.|
|-||Exopod of uropod is mostly unpigmented; in some cases with a small blotch on the inner lateroposterior margin||-||Exopod of uropod mostly with small blotch on the inner lateroposterior margin as continuation of pigmentation of inner plate.|
PROVISIONAL KEY FOR THE IDENTIFICATION OF POSTLARVAL
PENAEUS MONODON, P. INDICUS AND P. SEMISULCATUS
(Subrahmanyan and Rao, 1970)
|(a)||Five to seven reddish-brown (or yellowish) chromatophores on the ventral side of the sixth abdominal somite. One reddish-brown chromatophore at the anterior end of the sixth abdominal segment laterally||P. indicus|
|More than eight chromatophores on the ventral side of the sixth abdominal somite. One reddish-brown chromatophore at the anterior end of the sixth abdominal segment present or absent||b|
|(b)||Eight to eleven reddish-brown (sometimes bluish) chromatophores on the ventral side of the sixth abdominal somite. One reddish-brown chromatophore at the anterior end of the sixth abdominal segment laterally. One or two reddish-brown chromatophores are also present on the dorsal side of each abdominal segment||Penaeus semisulcatus|
|Fourteen to nineteen reddish-brown (sometimes bluish) chromatophores on the ventral side of the sixth abdominal segment. No lateral chromatophore on the sixth abdominal segment anteriorly. The ventral chromatophores on the sixth abdominal segment appears as a bluish or reddish-brown streak in expanded condition||Penaeus monodon|
Some areas have such a large number of indigenous species of Penaeus that identification might be difficult. In such places, one approach to identification is to rear the young postlarvae in aquaria until they reach identifiable size.
The most common method of transporting shrimp fry is in plastic bags filled with oxygen. The following points should be considered when using plastic bags for shipment.
Thin polyethylene bags are permeable to oxygen. So when post-larvae are to be shipped for long periods of time, better results can be obtained by placing the bags in an impermeable container and sealing the container tightly. If this polyethylene bags are used, shrimp should not be held in the bag for more than six hours without a change of water and new oxygen.
Postlarvae should be held without food for several hours before packing. Then they should be screened thoroughly and placed in clean water to eliminate as much trash as possible. Decomposition of the trash depletes oxygen and the trash itself serves as nutrients for harmful bacteria. Small amounts of activated charcoal can be added to the bags to absorb harmful waste materials produced by the shrimp.
The bags can be punctured easily, so as a safety measure, double bags should be used.
Containers holding fry should not be exposed to the sun, but kept in the shade. It is better to make shipments at night.
The most commonly used polyethylene bag is 50 × 75 cm. Five to six liters of water are put in the bag. Then the shrimp are added, as follows:
|Total length 10 mm||=||15 000|
|Total length 17–18 mm||=||5 000|
|Total length 20–24 mm||=||3 000|
Soft twigs can be placed inside the bag for the postlarvae to attach to so they do not group together on the bottom. The bag is then closed down so there is no air in it, only water. Next sufficient oxygen is added to fill the bag which is then sealed with elastic bands or string. The bag is then placed in a container which is impermeable to oxygen. Styrofoam is preferable if it is available. Two fist size pieces of ice in a plastic bag are placed in the container alongside the bag holding the postlarvae. The ice is to cool the water and reduce metabolism of the shrimp. The container is then sealed shut with tape. The shrimp can then be transported for one day with a survival rate of 90 to 100 percent.
It is better to hold and grow shrimp to a size of 2.5 to 3.0 cm than it is to stock small postlarvae directly into rearing ponds. This can be done in well-prepared nursery ponds or tanks. Hapa nets have been found to give poor survival. It is better to stock directly into a pond than use hapa nets.
One of the problems with stocking directly into a rearing pond is that it is almost impossible to tell if the fry survive. A farmer usually stocks a pond then has to wait until the shrimp grow before he knows if they lived or died. A good practice is to keep a few postlarvae in a container with pond water and watch them for several days. If these fry die a farmer will know to check his pond carefully and arrange for restocking, if necessary. This practice is particularly recommended for fry obtained from hatcheries, because the chance for disease is greater in hatchery-produced fry.
When nursery ponds are used, the shrimp should be acclimatized gradually to pond conditions to prevent death or damage from the shock of rapid change in temperature or salinity.
Some farmers keep the fry in plastic shipping bags and float the bag in the pond water for a short time to acclimatize them. In cases where temperatures are nearly the same and the shrimp are healthy, the floating technique will reduce losses. However, if they have been subjected to low oxygen levels and high levels of carbon dioxide and ammonia, such as would occur on a long trip, it would be more harmful to keep them in the bags exposed to unfavourable conditions (Spotte, 1970).
It is best to use a tank with aeration for acclimatization. Water in the tank should be adjusted to near the temperature and salinity of the water in which the shrimp were transported. After the postlarvae are added to the tank, the water in the tank is gradually adjusted to pond salinity and temperature (see Section 12.3 for salinity measurement). The period of adjustment depends on how much the temperature and salinity must be changed, but usually a half-day is adequate. The fry should not be released into a nursery pond during the heat of the day. Evening is best. The fry can be stocked at fairly high densities in nursery ponds, up to 25 per square meter, or 250 000 per hectare. The young shrimp should be kept in a nursery pond for from two weeks to one month, or until they reach an average size of 2.5 cm.
Several types of culture tanks have been found useful.
Boxes made from marine plywood. Dimensions of the tank are 4 × 8 feet (1.2 × 2.4 m) and 4 feet deep. Aeration is provided by air stones. Feed is mussel meat, ground fish and pelleted fish food. A daily exchange of 25 to 50 percent of water in the tank is recommended. An initial stocking rate of 10 000 to 50 000 of postlarvae 5–6 mm long per box results in 70 to 90 percent survival to a size of 25 mm. This method should not be used without aeration.
A round plastic tank with a bottom area of 25 m2 and depth of 60 cm has been used effectively without aeration. The bottom is covered with 10 cm of sandy garden soil. Rate of stocking is 2 000 to 4 000 P10 (P. merguiensis) per m2. Feed is Acetes meal (40%), rice bran (20%), coconut oil cake (20%) and cassava flour (20%). The fry are fed 100 percent of their estimated body weight daily. One-half of the water volume is changed daily. After one month the shrimp reach an average size of 30 mm and survival is from 60 to 95 percent. Survival was found to be lower at higher densities and in tanks without soil on the bottom.
A more complex tank system developed in the United States (Mock et al, 1973) is being adapted for use in this region. Postlarvae are grown in shallow oval shaped tanks in which airlift pumps keep the water circulating and maintain food particles in suspension. Stocking density is 10 000 postlarvae per m2 and survival is usually over 90 percent. A small wooden unit suitable for individual use is shown in Figure 8. It is constructed from marine plywood. Dimensions are 4 × 8 feet (1.2 × 2.4 m) and 2 feet (0.6 m) deep.
It is difficult to harvest shrimp or transfer from one pond to another without injuring or killing a large number of them. Transfer should be done by making them move with water flow. This is not always easy to accomplish, especially with P. monodon. The following methods are suggested for inducing shrimp to move with the flow of water during transfer from one pond to another:
Transfer at the time of month when tidal amplitude is greatest.
Transfer at night, and use a light to attract the shrimp to the sluice gate.
Let a little water into the pond on the high tide preceding the transfer. This makes the shrimp become active. The pond is then drained on the next low tide.
Change pond conditions to make the shrimp become active and ready to move out of the pond. One way of doing this would be to lower the water level so the temperature of the pond is increased.
If the growing pond is not adjacent to the nursery pond, the shrimp must be caught and transported. They can be caught in the out-flowing water with a minimum of injury by using a net of the same design as that shown in Figure 3. The net is fastened to a wood frame which is placed in the sluice gate. As the water is drained from the nursery pond the young shrimp are caught in the floating catch box. They can be scooped out periodically and transferred to suitable containers for carrying to the growing pond.