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 that 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 is especially important to small artisanal farmers who can utilize their own labor to collect wild postlarvae.
There is some use of wild postlarvae now. Indonesia and the Philippines have indigenous milkfish fry collectors who separate and sell Penaeus monodon fry to farmers, usually for polyculture with milkfish (Chanos chanos). They normally do not separate other penaeid species for sale, even though they are available in substantial numbers. Many of these shrimp, such as P. merguiensis, are cultured in other areas and if pond conditions were improved they could be farmed successfully throughout the region. In Thailand, Singapore, Indonesia and Malaysia and the Philippines naturally occurring fry are taken into the ponds with incoming water and held for varying periods of time. The problem with this practice is that many undesirable organisms are also taken in, and stocking density is usually unknown.
Two improved procedures have been used to acquire wild postlarvae with good results. A method used by a few farms in Thailand is to pump unfiltered seawater into a nursery pond as frequently as tidal conditions permit. After thirty days the pond water is treated with tea seed cake at a rate of 10 to 25 ppm. Tea seed 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. After the fish are killed the shrimp are released to a larger pond where they are grown to marketable size.
The second method is to collect the postlarvae in fine mesh nets (Figure 1) placed in tidal passes, water supply canals, or sluice gates. It is not necessary to attach wings to this type of collecting net. In fact, wings can do more harm than good, by funnelling large organisms like jelly fish into the collecting net and causing it to become clogged. The nets are fished on incoming tides and it is important that the net be attended constantly with the catch removed to a holding container at short intervals. After the catch is completed, the water in the holding container is treated with tea seed cake to kill the fish, and the postlarval shrimp can be stocked in ponds. This procedure eliminates the tedious job of separating the shrimp by sorting individually, and ensures against the accidental stocking of fish in the pond.
Sometimes there can be confusion about the identity of the shrimp collected. A group of farmers in the Philippines evidently stocked a pond with Mysidacea thinking they were penaeid shrimp. The same characteristics used to separate adult penaeids from other shrimp-like animals can be used to separate postlarval forms. In penaeids 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 characters. Probably the animal most commonly mistaken for postlarval shrimp is Acetes sp. (Sergestidae). These frequently occur in great abundance, but can be easily distinguished by their long, bright orange antennae. Postlarval shrimp have short, colorless antennae (Figure 2). In addition, postlarval shrimp do not have statocysts (in live animal this appears like a small bright spot to the naked eye) on the tail (Mvsidacea), and their eyes do not extend laterally at a 90 degree angle (Sergestidae). Initially, some time will be required to identify the various forms collected, 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 2.
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 colored, a mottled grey or brown. Postlarvae within the genus Penaeus are generally almost colorless, however, P. monodon and P. semisulcatus are colored rust brown. 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 a 45 degree angle. Other species of Penaeus may also be pigmented, however postlarvae of all the species indigenous to the region have not been described.
Some areas have such a large number of indigenous species of Penaeus that identification might be difficult. In these cases, one approach to identification is to rear the young postlarvae in aquaria until they reach identifiable size.
There is great interest in shrimp hatcheries in the region, however, building and operating a shrimp hatchery requires a great deal of complicated technology and should not be attempted by anyone without a good technical background. It is not the purpose of this paper to present explicit plans for hatchery construction and operation, as hatchery technology in other regions is already well documented. Rather, the intent is to present suggestions for improving production from existing hatcheries and to suggest general guidelines so those planning new hatcheries can minimize their problems. Detailed recommendations for hatchery design and operation, tailored specifically for this region are needed, but due to the extensive scope of the subject this is planned for a future paper.
Almost every country in the region has, or is planning for, experimental hatcheries. Nevertheless, development of commercial scale hatcheries has been disappointingly slow. This has been caused in part by the difficulty in transferring technology. The hatcheries in the region are patterned after those used successfully in Japan, but problems have been encountered in their operation in the tropics. Algal cultures do not bloom, disease occur, etc. Even though biologists have undergone training in hatchery operation in Japan, more experience is needed in problem-solving required to adapt the system to local conditions. Most planners have assumed that there would be a few problems of this kind, and as a result the hatcheries built are large, but have few facilities for research. Hatcheries constructed in the future should be small and equipped to do the research needed to solve local problems.
The problem of obtaining sufficient numbers of gravid spawners has been a major constraint in hatchery development. Recently, there have been promising breakthroughs in maturing shrimp in captivity. Many species including P. monodon, have been matured, spawned, and the larvae reared successfully. Basically the procedure involves cutting off one of the shrimps' eyes. This induces ovarian maturation. The main requirements are a supply of good quality, high salinity seawater (above 34 ppt) and the right food. For feeding, a high percentage (50%) of squid with the remainder crab, shrimp or shellfish is suggested. Many details still need to be worked out, but we can probably look forward to being able to keep broodstock and obtain spawning at will within the next few years.
Every hatchery should be conducting research to develop their holding facilities for broodstock. Various researchers have been having success in holding facilities that range from 0.5m3 tanks to large net enclosures (250m2 by 4 m deep). It is best if the shrimp can be observed daily without being disturbed, and ripe females removed. One of the most successful groups is using 6 m diameter round tanks, shaded from direct sunlight. The tank bottoms are bare and there is a center drain. Once a day tanks are flushed clean, by running water in forcibly to start a rapid circulation around the tank. This carries all debris to the center where it is then sucked down the drain. Water depth is about 0.6 m. About 20 shrimp are held in each tank, half males and half females. One eye of each female is pinched off by fingernails.
One of the biggest problems in all the shrimp hatcheries visited is parasitic infections (mainly fungus) which cause large scale mortalities. This problem can be minimized if possible sources of infection are eliminated.
Water - Some sort of water treatment is necessary. Mechanical filtration does not eliminate fungi, unless treatment facilities are elaborate, but it is a prerequisite for further treatment. Batch treatment with chlorine is probably the easiest and safest method of eradicating fungus in the water. A tank is treated with sodium hypochlorite solution (10%) at a concentration of 150 ppm. The chlorine is neutralized 12 hours later with sodium thiosulfate, 45 g/m3 (Kurata and Shigueno, 1976). Care should be taken to aerate the water thoroughly before use as the chemical reaction depletes oxygen. This method is useful in that it sterilizes the tank as well as the water. A separate tank is needed to treat and hold water needed for periodic exchanges in the hatchery tanks.
Broodstock - It is probable that at least some diseases occasionally come in with the broodstock, and the following procedures are recomended to minimize this. Spawn females in smaller tanks and add eggs or nauplii to large rearing tanks after washing. Besides helping to control disease, this eliminates the large amount of organic material added to the water incidental to spawning and which contributes to fouling of the water. Also, the number of larvae per tank can easily be adjusted to desirable levels. The eggs could be treated with malachite green (5 ppm for 2 min), a method which has been found effective as a preventive against fungus infections in lobsters (Fisher, et al, 1976).
Sea water used in culturing algae should be pre-treated with chlorine as described above.
There have also been reports of stalked ciliated protozoan (probably Zoothamnium sp.) infestations on the body surfaces of larvae and postlarvae. It is the author's experience that this organism occurs when the larvae or postlarvae are growing slowly (not molting frequently) and water quality is poor. In some hatcheries water is not changed frequently, in fact one hatchery in the region does not change water for the first ten days after spawning. A portion of the water in each tank should be changed regularly, daily if possible.
One of the main reasons for the relatively low survival in hacheries is the poor water circulation in the tanks. Dead algae, feces, etc. settle to the bottom and decompose anaerobically. This puts stress on the system by producing ammonia and sulfide compounds. If circulation is sufficient to keep the bottom clean, the detritus stays in suspension and degradation is aerobic and the by-products are not so harmful. Also, the particles remain available to serve as food for the larvae. Some hatcheries have installed large mechanical stirrers to provide good circulation of water, but these devices are very expensive and subject to mechanical failure.
Airlift pumps are extremely useful in inducing strong water circulation in tanks. They are inexpensive and have no mechanical parts to break down. Properly installed they move the water in the tank vigorously so the bottom is swept clean, keeping food particles and/or detritus up in the water column. An airlift water circulating device that can be installed to improve the efficiency of existing hatchery tanks is shown in Figure 3 (a,b,c) which illustrates placement on two sides of a 50 ton tank. For larger tanks, say 200 ton, airlifts may be required on all four walls to assure strong movement of water in the tank. Wood strips coated with fiberglass are fastened to the sides of the tank with epoxy or other suitable glue, and the plastic sheets (or any other available material such as asbestos board) fastened to the wood with aluminium screws. If the tank walls are not straight the wood supports can be cut and glued on in sections to fit the contour.
With slight modification this system can be adapted for use in larger hatchery tanks or smaller tanks for algal culture, brine shrimp hatching, etc. Additional information on airlift water circulation devices can be found in Spotte (1970) and Salser and Mock (1973).
If hatchery water is treated with chlorine to prevent disease, the algae and zooplankton will not be available for food as in the traditional practice and, consequently, the hatchery tanks must be seeded with diatoms. This is not too difficult, but provision should be made for tanks in which to culture the diatoms.
Many hatcheries have experienced difficulty obtaining sufficient supplies of Artemia to feed the later larvae. This has led to a search for for alternate foods. Brachionus sp. seem to be acceptable as food from the third protozoeal stage. The most successful method of Brachionus culture seen by this writer was conducted in two very large, uncovered, concrete tanks at the Southeast Asian Fisheries Development Center (SEAFDEC) Aquaculture Department Tigbauan, Philippines. A marine Chlorella is cultured and used to feed the Brachionus. The management procedure is as follows (Gabaza, personal communication, 1976):
Fertilize Chlorella with urea (100 g/ton) and ammonium sulfate (15 g/ton). Add as needed depending on the density of the culture,
When the Brachionus reach a density of 500/ml, one half of the tank is drained. The amount drained is replaced with two thirds Chlorella culture and one third sea water;
The Chlorella tank is refilled with clean seawater and fertilized as above;
The Brachionus are harvested daily to feed the larvae.
The airlift device shown in Figure 4 can be used to harvest Brachionus from a large tank. The Brachionus would be harvested continuously and be ready for feeding whenever needed. The water flowing through the catch net keeps the Brachionus alive.
The following scheme is suggested for managing food organisms in the hatchery tank. Practicing traditional procedures, tank is seeded with a diatom such as Skeletonema sp. which is known to be a good food for the protozoeal stages. The water in the hatchery tank is fertilized appropriately. The diatom then grows in the tank and serves as food for protozoea I and II. When all the larval shrimp have reached the third protozoeal stage, Brachionus are added. The shrimp can grow on Brachionus alone and the Skeletonema is not required any longer. The Brachionus need to feed, however, so when they are added to the hatchery tanks as food, the tanks should also be seeded heavily with Chlorella to serve as food for the Brachionus. The hatchery tank should then be fertilized as given above to maintain the bloom of Chlorella. this procedure is recommended because, while a dense growth of Chlorella is much easier to maintain the Skeletonema, it is not a good food for the protozoea.
Some thought should be given to trying out the system of larval culture used in the United States [Mock and Murphy (1970) and Salser and Mock (undated)] in this region. By utilizing small tanks where good aeration and water circulation is assured, frozen algae (Brown, 1972) or frozen Brachionus can be fed. The use of frozen food eliminates the problem of coordinating availability of live food organisms with the availability of spawning stock. The small round hatchery tanks used in the United States were originally designed because good water circulation could be attained in them. With the development of side-wall airlift pump water circulation system, the expensive round fiberglass tanks are no longer needed. Simple wooden or concrete tanks like those shown in Figures 5 and 6 are suitable. Air stones are not required. A screening device such as that described by Salser and Mock should be installed in the tank to facilitate exchange of water.
Sometimes algal cultures become contaminated with zooplankton, primarily copepods. These can be eliminated with biodegradable insecticides. Two which have been employed successfully in mass cultures of Chlorella used in shellfish hatcheries are Dipterex (0,0-dimethy 1–2, 2, 2-trichloro-1-hydroxy-ethyl phosphate) and TEPP (containing 40 percent tetraethyl pyrophosphate). They are added at a concentration of 0.1 ppm. After three days in a dense Chlorella culture, these compounds lose their toxicity and the algae can be used as food (Loosanoff, et al, 1957).
Holding and rearing postlarvae to a size suitable for stocking in earthen ponds or other grow-out facilities has been a problem. Attempts to hold and grow postlarvae in hatchery tanks have usually resulted in heavy mortalities. This is because most hatcheries are designed for culturing larvae, and they are not well suited for growing postlarvae. A hatchery represents one of the largest capital investments in shrimp culture and consequently should be used as efficiently as possible. This means it should be used only for the culture of larvae and early postlarvae. Postlarvae should be transferred from the hatchery three days after the larvae change from myses to postlarvae.
Shipment of postlarvae is much more efficient if they are transported while still small. It makes sense that postlarvae should be sent to the farmers as soon as possible. The individual farmer, or centrally located growers, can then grow the postlarvae to the size he desires to stock. With only slight adaptations, an intensive culture system developed at the Gulf Coastal Fisheries Center, Galveston, Texas, United States (Mock, et al, 1973) could be used by the farmers. The postlarvae are grown in large, shallow, oval tanks where strategically placed air lift pumps keep the water circulating and retard settling of food particles or detritus. A dried processed food is used, most of the freeze dried flake foods used to feed tropical fish should be adequate. Survival to pond stocking size (2 cm) is consistently over 90 percent. A small wooden unit suitable for individual use is illustrated in Figure 7. A piece of fine mesh screen should be placed over the bottom of the air lift pipe to prevent the postlarvae from entering the pipe. Recommended stocking density is 10 000/m2.
Benefits to be derived from adaptation of this procedure would be:
Increased survival from egg to stocking size. This is important because one of the greatest hindrances to large scale hatchery operations is an adequate supply of spawning stock. Even when maturation of broodstock in captivity becomes routine, it will still be costly to rear and hold broodstock.
If a hatchery is used only for growing larvae, larval density in the tanks can be increased and individual units made smaller, with resulting savings in both capital and operating costs.
Hatchery time will be reduced to two weeks per hatch, greatly increasing hatchery capacity. Twenty-three crops could be produced in one unit every year. If the hatchery unit was designed to yield 500 000 postlarvae per hatchery run, total production per year would amount to 11.5 million postlarvae. Using the intensive system, four tons of tank capacity would be sufficient for this.
Costs of postlarvae will be reduced, and this will permit more flexibility in pond management to optimize production. For example, in June 1976, a farmer in the Philippines informed the writer that P. Monodon fry cost 93 to 97 centavos (US$0.12 to 0.13) each. In order to make a profit, this farmer has to stock at low densities and grow the shrimp to the largest size possible. As will be seen in a subsequent section, this increases risk. If postlarvae cost less, he could stock heavier and harvest at a smaller size obtaining the same yield with less risk.
Being on site, the farmers can acclimate the postlarvae to pond conditions before stocking. This will reduce mortality in some cases.
