9. PREDATORS, COMPETITORS AND PESTS

9.1 Kinds

The following have been identified as causing problems in shrimp culture.

9.2 Methods of control

9.2.1 Fish

(a) Prevention

The most effective method of control is prevention. If the proper precautions are taken in maintenance and pond preparation, fish will not ordinarily be a problem during the culture period.

1. Proper pond maintenance. Predators and competitors can enter ponds through crab holes and other leaks in the dikes. Also, postlarval shrimp can escape from the ponds through the holes. Regular maintenance should be performed to stop all leaks in the dikes. Crabs should be eliminated and their holes stopped up. Closure boards in the sluice gates should fit tightly. Prompt stoppage of leaks is especially important in tidal ponds where there often can be a reverse flow of water.

2. Drying the pond bottom. Thoroughly drying the pond bottom before stocking will eliminate the fish. The farmer must be sure that there are no puddles or moist places left in the pond. If the pond cannot be thoroughly dried, the portions with water should be treated with chemicals to kill the fish. “Gusathion” and “Bux 300” have been used successfully. “Bux 300” degrades in less than one week. Use at the dosage prescribed on the package.

3. Poisoning before stocking. If a pond cannot be completely drained, fish poisons should be used before the pond is stocked. Rotenone or derris root is recommended, 4 to 5 kg dry root for a one-hectare pond with water depth of 5 cm (see also Section 9.2.1). As “Bux 300” degrades in one week, it may be suitable for use as a pretreatment at the dosage recommended on the package.

4. Screening water as it enters the pond. After fish are eliminated from a pond, it is important that all water let into the pond is screened. The screen must be fine enough to prevent entry of fish eggs and larvae as well as adult fish. Ordinary plastic mosquito netting is not suitable, because the holes in it are too large. A fine mesh nylon or plastic screen with a hole size of 0.5 mm is recommended. Nets with such small holes are easily stopped up. For this reason, it is often necessary to have a series of screens with different mesh size and to increase the surface area of the finest screen by making it into a bag. If the end of the bag is connected to a floating screen box similar to that shown in Figure 3, trash and fry will collect there and can easily be removed with a dip net. The shrimp fry can then be separated and stocked in the pond. In ponds which are filled by pumping, the nets should be placed before the pump. The initial surge of water when a pump is turned on can sometimes break a net. The inlet canal can be widened and additional nets used. This reduces the water velocity through the netting and increases their efficiency. One example of how to do this is illustrated in Figure 10.

   Even if the methods of prevention are followed, accidents happen occasionally and fish enter the pond. It is then necessary to remove the fish without harming the shrimp. If the number of fish in a pond is not too great, the cost of chemical control is not justified. It costs too much. Fish can be caught by using hand lines, traps, gillnets or seine nets. When using traps or nets, care should be taken that the mesh size is large enough for the shrimp to escape. Some kinds of fish gather near the sluice gate when water is let in. A seine can be used to catch them while they are concentrated there.

(b) Selective poisoning

When the number of fish in a pond is large, the most effective method to get rid of them is by the use of selective poisons. The use of natural products such as teaseed cake or derris root is recommended. These are safe because they are not harmful to man in small amounts and they break down and lose their toxicity shortly after application.

Use of any of the chlorinated hydrocarbon group (DDT, Endrin, Chlordan, gamma BHC, etc.) in fishponds is not recommended because of their long-term residual effects. In fact their use is discouraged. Unlike the chlorinated hydrocarbons, organo-phosphate pesticides do not leave a toxic residue for more than about two weeks after application. Gusathion belongs to this group. If Gusathion, or other organo-phosphates, are used, they should be handled and applied with extreme caution. Fish which are killed with chlorinated hydrocarbon and organo-phosphate poisons should not be eaten by people or animals. Though some of the chemicals are not lethal, the sublethal effects are uncertain (Anonymous, 1976a).

In order to apply the correct dose of a chemical, the amount of water in a pond must be estimated. First, the total water surface area of the pond must be known. If the pond is square or rectangular, the length and width are measured to the nearest metre. The length is then multiplied by the width to get the number of square metres of pond surface. This is then multiplied by the average depth to obtain the number of cubic metres of water in the pond. To estimate the average depth of water, first a mark is put on the gate to record the water level in the pond. Then depth soundings are taken by wading in the pond over a preset grid pattern. The depth sounding stick should have a flat board attached to the base so it does not sink into the pond bottom. An average is taken of the soundings to get the average depth of water in the pond. This is then recorded on the sluice gate. With this mark as a reference, a permanent scale can be marked on the sluice gate for future reference when the water level is at a different depth, such as when lowered for treatment with chemicals.

(i)   Saponin. Saponin is the best known compound to selectively poison fish without damaging the shrimp or food organisms in the pond. It is 50 times more toxic to fish than to shrimp and so it is safe to use, while shrimp are in the pond. At the recommended dosage, it does not affect rotifers and copepods. It is bio-degradable and losses its toxicity after a short time, probably two or three days.

The most commonly used source of saponin is teaseed cake, a residue from the processing of oil from the seeds of Camellia. The cake contains from 10 to 15 percent saponin. The effectiveness of saponin decreases with decreasing salinity. A treatment of 1.1 ppm killed Tilapia mossambica in one hour at a salinity of 35 ppt. At a salinity of 10 ppt, it took 14.5 to 16.5 hours to kill the fish at 1.1 ppm (Terazaki, et al, 1976; Tang, 1961). Consequently, the recommended level of application is:

Salinity above 15 ppt = 12 g teaseed cake per m³ of water

Salinity below 15 ppt = 20 g teaseed cake per m³ of water

To apply teaseed cake, it must be ground up. Heating in an oven dries out the cake and makes it more brittle and easier to grind. The proper weight of ground cake should be soaked in water for 24 hours to extract the saponin. The water containing the saponin can be filtered and the filtered fluid applied to the pond water. It is not essential to filter the water, however, since the teaseed cake residue acts as a fertilizer.

When using teaseed cake, or any other chemical control, the level of water in the pond should be lowered as much as possible without causing damage to the shrimp by increased temperature. It is best if the water level is lowered in late afternoon or evening, and the chemical applied then. Water level in the pond could then be raised the next morning before the sun heats up the shallow pond water. When saponin is applied, the amount of dissolved oxygen in the water decreases somewhat. This is usually not serious, but saponin should not be applied to a pond in which low levels of dissolved oxygen are occurring. Dead fish should be removed from the pond.

(ii)   Rotenone. Rotenone has been used to selectively kill fish, but not shrimp. However, the difference between the lethal limit for fish and shrimp is small and great care must be taken when using it. The volume of water in the pond must be estimated accurately as an overdose will kill the shrimp. Rotenone is most effective in freshwater and works better in low salinity water than in high salinity water.

Studies in freshwater show that rotenone has a harmful effect on benthic invertebrates and zooplankton (Neves, 1975). Recovery of benthic organisms is fairly rapid, but that of zooplankton is much slower. In one experiment in which rotenone was applied to a lake at a rate of 0.6 ppm, the plankton volume decreased 97 percent within 24 hours. It took about six days for the population to return to normal. From this, it can be seen that perhaps it would be useful to add supplemental feed for several days after rotenone is added to a pond with shrimp in it.

Rotenone is available in several forms.

Rotenone powder usually contains 5 percent rotenone, but sometimes, a 4 percent rotenone product is sold. The recommended level of treatment is 0.2 ppm rotenone. This requires 4 g of 5 percent derris powder per m³ of pond water. This does not kill eels. Treatment of 8 g of 5 percent derris powder per m³ of water is required to eliminate eels.

Derris root. Fresh roots are more effective than dried roots which had been stored. Rotenone content appears to be higher in small roots than in large roots. Rotenone content of the roots also appears to vary with location (Yang, personal communication). The roots should be cut into small pieces and soaked overnight in water. After soaking, the roots are pounded to crush them. The crushed roots are replaced in the water in which they were soaked and squeezed so as much of the rotenone as possible goes into solution. The solution is then added to the pond. Four grams of dry root are required per m³ of pond water.

(iii)   PCP-Na (Sodium pentachlorophenate). This is an agricultural chemical used widely as a weed killer. It kills fish at treatment levels which do not kill shrimp. The recommended level of treatment is 0.5 ppm. Levels of 1.5 to 2.0 ppm are toxic to shrimp which have just molted, but shrimp held for eight days in a concentration of 1.3 ppm had no ill effects and molting was normal. PCP-Na decomposes when exposed to direct sunlight. Toxicity to fish is reduced by 90 percent after three hours. After six hours of sunlight, it is no longer toxic (Anonymous, 1976a).

Before application, the pond water should be reduced to as low a level as practicable. The correct amount of PCP-Na is dissolved in freshwater and solute is then spread evenly around the pond. As soon as the fish are killed, freshwater should be let into the pond to dilute the concentration of PCP-Na.

PCP-Na is toxic to man in large doses and care should be taken in its use. A stick should be used to mix the chemical with freshwater or rubber gloves should be worn. Fish killed with PCP-Na should not be eaten.

9.2.2 Crabs

Crabs are one of the worst pests in a shrimp pond. The swimming crabs (family Portunidae) especially are fierce predators of shrimps. These should be removed from the pond by trapping. Fish with firm meat such as catfish or shark are recommended as bait. Other suitable baits are trash fish, snake meat, toads, and uncooked bones. Shrimp will also be attracted to the bait and if they are caught in the trap, many will be killed by the crabs. This can be prevented by constructing the trap of material with large enough holes so that the shrimp can escape.

One of the major causes of water leakage through pond dikes is holes made by burrowing crabs. In a pond with a large number of crab holes in the dikes, maintaining the proper level of water is a problem. Water flowing through crab holes can cause a dike to wash out and result in costly repairs. Postlarval shrimp will leave a pond by swimming out through a crab hole. Similarly, predators and competitors can enter a pond through crab holes. Several of the methods used to kill burrowing crabs are given below.

1. A widely used insecticide “Sevin”, is effective in killing crabs. “Sevin” is also toxic to shrimp, so care must be taken in its use in a pond. “Sevin” is, however, relatively safe for humans and domestic animals. For use, the “Sevin” is mixed with ground up fish. Small balls of the mixture are placed in crab holes above the water line. It is also possible to put the fish balls in crab holes below the water line and then close up the hole so the shrimp can not eat the poison and it can not get into the pond water.

2. Calcium carbide is put into crab holes and enough water is poured into the hole to wet the carbide. This produces acetylene gas which kills the crab.

3. Tobacco dust, “Brestan” and “Aquatin” can kill on direct contact.

4. Rice hulls are burned and the residue is used to fill up crab holes. The hulls stop up the gills of the crabs and they die.

9.2.3 Burrowing “shrimp” (Thalassina)

In some areas, damage to dikes caused by burrowing Thalassina (Anomura) is common. Their burrows can usually be distinguished from those of crabs because they make a very high mound at the hole entrance which is above the water line. The same method used to kill crabs can be used to kill Thalassina. Especially designed trigger type traps made of bamboo can also be used.

9.2.4 Snails

Snails (Cerithidea) compete for the natural food in a pond. Most pond operators feel that production is lower in ponds with a large number of snails. If their numbers are high, they disturb the bottom algae loosening the sediments. As a result, on windy days, the pond can become muddy and/or the “lab-lab” breaks loose from the bottom and floats to the surface. Wind action carries it to the pond bank where it settles and decomposes and produces large amounts of H₂S.

It has been reported that snails can be eliminated during pond preparation by using tobacco rejects or dust, but this is open to controversy. Treatment at 200 kg/ha was used to kill snails by one person and it took six months for them to become reestablished. Application was by broadcasting over a dry pond bottom. Water was let in to a depth of 10 cm initially and gradually raised to a depth of 1 m after one week. Tobacco dust at 400 kg/ha and tobacco stem at 150 kg/ha was not effective when used by others who broadcast the tobacco into shallow water, 5 cm. Nicotine is very toxic to shrimp so ponds where tobacco dust is used must be flushed well before stocking.

The commercial preparations “Brestan”, “Aquatin” and “Bayluscide” will kill snails when used at the dosage prescribed on the package. However, these chemicals have produced a residual effect when used in milkfish culture. Shrimp production was reported to be reduced for six months after the use of “Aquatin” or “Brestan”. “Aquatin” kills Ruppia and retards the growth of “lumut”. Milkfish cultured after the use of “Bayluscide” or “Aquatin” were stunted. “Aquatin” has a residual effect for five years in pond soil. Consequently, the use of these chemicals is not recommended.

9.2.5

Small Caridean shrimp sometimes become so abundant they cause problems. In Taiwan, China, a common species, Caridina denticulata causes problems as a competitor. They either attach molting shrimp or eat the feed provided for the shrimp. The most effective method of control is to dry the pond bottom and to apply poisons before stocking to kill the larvae and eggs.

9.2.6 Birds

Predation by wading birds can be a problem in some regions. Most wading birds need a place to land. If water over the flat main portion of a pond is kept deep enough and coloured with a growth of phytoplankton, the birds can not see the bottom and will not land. Wading areas at the sides of the ponds can be reduced or eliminated by making the sides slope steeply to a deep peripheral canal. If there is a berm, it should be placed above the water line, or mangrove or other branches placed on it to prevent birds from walking along the shallow pond margin.

Special precautions can be taken in nursery ponds because of their small area. Some farmers run lines of string between posts set in the pond and attach bright coloured pieces of cloth metal to the string to scare birds.

Flashing mirrors can be used to scare some types of birds away from ponds. The device shown in Figure 11 is used at the Jitra Fisheries Station in Malaysia to scare eagles. It is simply a windmill with mirrors that revolve and flash brilliantly scaring the birds.

9.2.7 Man

Losses caused by man are perhaps the hardest to prevent. The most vulnerable point is the sluice gate, because shrimp can be caught so easily with a net when water is let out at night. If a pond owner or caretaker is living at the pond site, his house should be located near the sluice gate. If this is not possible, a strong lock should be attached to the upper board of the sluice gate. As shrimp grow larger, they become more valuable. For this reason, it is not possible to have someone living at the pond site, a watchman should be hired during the last third of the growing period.

If a farmer is using traps to selectively harvest, the traps should not be left in the ponds unattended. It is too easy for someone to come along and empty them. When making the daily inspection of his ponds, a farmer should look carefully for footprints near the water's edge. If strange footprints are observed, a watch should be maintained at night to see if someone is catching shrimp by hand or with small nets. Branches placed around the edge of the pond make it difficult for thieves to catch shrimp with cast or seine nets.

9.2.8 Organisms that degrade wood

Destruction of wood by marine organisms is one of the major causes of destruction to water control structures. The problem can be reduced greatly by constructing sluice gates from concrete. For various reasons, concrete construction is not practical in many places. In cases where wood must be used, it may be desirable to use special kinds of wood or to coat the wood with preservatives.

Wood degrading organisms can be classified as follows:

The molluscs and crustaceans cause damage by boring. One can usually tell the difference between shipworms and piddocks. With shipworms, the calcareous lining of the hole is visible externally and the holes are only 1 to 2 mm in diameter. With piddocks, no calcareous lining can be observed and the hole of adults is two to three times larger. Fungi cause soft rot.

Generally, wood is attacked more in tropical waters, and in tropical regions, destruction by borers is greatest in brackishwater. The predominant type of degrading organism varies from place to place and one should find out which one is predominant in his area and how serious the problem is.

Many types of wood are more durable than others and it is advisable to utilize them if possible. The resistance of wood is not dependent on density or hardness. Silica content is very important. Some species have a high resistance due to a toxic action or repellent substance to one species but not to another. Wood from the following species of trees is recommended for use in saltwater by Fougerousse, 1971.

A more complete list of species of trees and their resistance to the various organisms is given in Table 5.

Preservatives can be applied to wood to increase its resistance. Creosote is one of the oldest and most effective treatments. It should be applied under pressure, if possible. If pressure-treated wood cannot be obtained, the wood should be soaked in the preservative. An external coating of tar or asphalt is more effective than creosote treatment for most pholadids. Excellent results have been reported by first applying asphalt or coal tar then applying cement on the still soft coating.

Treatment of removable parts like sluice boards. These can be given more frequent applications of the preservative. They can also be soaked in chemicals to kill the pest. Borers can detect most chemicals and they withdraw their siphons and close their holes. They can not detect sodium arsenate and a dip in 25 ppm (As₂0₃) for 18 hours is effective (McQuire, 1971). If two sets of boards are made, one can simply be left to air dry and the borers will be killed.