Arief Taslihan and Sunaryanto
Brackishwater Aquaculture Development Centre, Jepara, Central Java
The pests that can be found in the hatcheries and grow-out ponds for P. monodon can be grouped into predators, competitors and destructive organisms. Methods of overcoming them by physical or chemical means are discussed in detail in this paper. The physical methods are the safest because it does not result in any side effects both for the culture animals and for human beings. However chemical method is more efficient in terms of time and effort required. Shrimp diseases can be caused by biotic and abiotic factors. The most effective way of controlling diseases is to first find out the cause and then ascertain its treatment. The best way is still through prevention through maintenance of the cleanliness of the culture environment along with the provision of appropriate quantity and quality of feed.
One venture that is viewed as the most appropriate for increasing the volume of fisheries export in accordance with KEPPRES No. 39/80 is the intensive culture of shrimps in tambaks. In order for intensive tambak operation to be successful one factor that needs to be given attention is water management which includes management of pests and diseases which is one of the points in the Sapta Usaha Pertambakan.
Pests and diseases need to be controlled since the occurrence of pests in a tambak can result in not small losses because it diminishes the productivity of the tambak. On the other hand, pest control done without any consideration is risky not only to the culture animals but also to the aquatic ecosystem. An example is the application of synthetic chemical pesticides. Chemical pesticides generally do not degrade easily and stays active for a long time in the aquatic environment. This disturbs the ecological balance and can ruin the aquatic ecosystem. For this, safety precautions ought to be taken so that the pests and diseases can be controlled without disturbing the environment, by applying the correct dosage of the proper pesticides. With good practice the ecological balance can be maintained and more importantly the harvests will be safe for human consumption.
Problems in pests and diseases emerge primarily because of short comings in managing the environment. For example, poor sanitation results in the accumulation of disease organisms within and around the culture area. One example that can be mentioned is that hatcheries as well as tambaks are often successful during the initial year or years of operation. Disease infection normally starts in the succeeding years in the form of sudden epidemics that are hard to manage. One reason for such occurrence is when a hatchery or farm in question allows its wastes to accumulate within its vicinity which later becomes a breeding place of all sorts of microorganisms. If ever such microorganisms, especially the decomposers, become abundant then the quality of water around the hatchery in question will likely deteriorate.
2.1 Types of Pests
Pests refer to any species of animals in the tambak other than that being cultured, and are considered harmful because they are likely to lower the productivity, by causing the elimination of the culture organism, by competing in the use of available energy, and/or causing damage to the facilities.
The occurrence of pests in a tambak will likely result in the reduction of feed ration through competition, in the reduction of the number of culture animals through predation, and in the destruction of tambak facilities. Based upon the type of losses that result, the pests can be categorized either as predator, competitor or destructive pests.
Predators are those animals that directly kill and eat shrimps causing reduction in the shrimp stock. Aside from reducing the shrimp stock, predators can also be directly harmful in other ways such as competing for dissolved oxygen, as well as for space, with the shrimps. In addition feed ration intended for shrimps may also be consumed by the predators hampering shrimp growth. There are many species which can be categorized as predators ranging from lower vertebrates such as fish to higher vertebrates such as lingsang (Prionodon gracillis). A list of predator species can be found in Table 1.
Fish species such as ten pounders (Elops hawaiensis), tarpons (Megalops cyprinoides), groupers (Epinephelus) and barracudas (Sphyraena sp.) are some of the species which can generally be found in the IndoPacific area which can inflict losses to tambak operators.
The occurrence of competitor species in a pond can be harmful in terms of affecting shrimp growth, by competing with the shrimps for food, both natural and artificial, competing for oxygen and competing for space. A list of competitor species can be found in Table 2.
Species which compete for food includes the polychaete worms, Dendronereis sp., Chironomid larvae and cerithid snails. Actually, the cerithid snails may be beneficial to the tambak for as long as its population is not too high because it is a scavenger which prevents the build up and decay of organic matter on the pond bottom. However if its population becomes too dense, it becomes harmful because it destroys the pond bottom structure in which the soil becomes loose. Loosening of the pond soil is followed by peeling off of the klekap and as a result toxic gases such as sulfides are released from the bottom and pollutes the water.
Lately, in some areas of Indonesia another problem has emerged in the form of mysid shrimps or jambret (Mesopodopsis sp.). This pest first emerged in 1980 in South Sulawesi and were known by the tambak operators as wereng tambak (tambak locusts) (Pirzan and Poernomo, 1985). In Jepara, when the jambret population blooms, it can cover almost the entire pond water surface especially in the morning. The effect of the occurrence of a dense jambret population is heavy competition for the oxygen supply as well as for food.
2.1.3 Destructive pests
The occurrence of destructive pests can be harmful to the tambak in terms of causing leakages in the dike and in the destruction of flush boards for controlling the water level. The most destructive pest is the mud crab (Scylla serrata) which makes holes in the dike with the following consequences:
2.2 Pest Control Procedures
Pest control may be classified into two methods: physical and chemical. The two methods are usually used one after another, but each method may be used by itself.
The safest method is physical because it does not have any harmful side effects not only to the culture organisms but also to humans. The chemical method can be used if the physical method is difficult to undertake or if efficiency in terms of time and labor is desired.
2.2.1 Physical methods
Physical methods for pest control are usually undertaken during the preparation period, however they may also be undertaken during the culture period. Physical pest control methods include the following:
2.2.2 Chemical methods
Should the physical methods encounter obstacles, for example, if total drying cannot be achieved, it would be necessary to resort to chemical methods. However in using chemical pesticides, care should be taken in terms of selecting the type as well as in the dosage because if used improperly it can be extremely harmful. The advantage of using chemical instead of physical means is its efficiency in terms of time and labor required.
2.3 Regarding Pesticides
2.3.1 History of pesticides
Pesticidal substances have been known to man even during the pre-historic period and developed together with the development of civilization itself. Ever since man became familiar with system of cultivation, efforts were already made to overcome pests which attack the plants being cultivated, even if the methods were very simple, that is with the use of traps or to directly catch the pests. Along with the rise of human knowledge, man continuously searched for substances that can be used to kill harmful pests.
The first type of chemical pesticide known to man is arsenic which apparently is very effective in killing pests harmful to potatoes. The next development was the discovery of Paris green and then of compounds of mercury and copper. Meanwhile pesticides that come from living organisms were known only in the 19th century with the discovery of derris roots which contain rotenone and chrysanthemum blossoms which contain pyrethrum.
Man continued to look for a substance which is effective in eliminating pests and is inexpensive. Finally in 1939, a man-made substance was developed, this is DDT which is short for Dichloro-diphenyltrichlorethane. Actually this chemical was accidentally discovered as very effective against insects that were attacking cotton plants. After DDT, several other chemicals were developed which were better in quality as well as in not having wide effects to the environment.
2.3.2 Types of pesticides
One way of classifying pesticides is to group them according to the type of organisms it is effective against as follows:
Some of these pesticides are plant based and are considered natural pesticides, others are man-made and are considered synthetic.
2.3.3 Natural pesticides
Natural pesticides are those which the active ingredients come from living organisms, generally plants, and are therefore known as botanical pesticides. The three most common pesticides in this category are saponin from teaseed, rotenone from derris root and nicotine from tobacco.
2.3.4 Synthetic pesticides
Synthetic pesticides are man-made chemical compounds. Based upon their chemical structures they may be either inorganic (e.g. potassium cyanide, KCN; carbide, CaC; potash, K2CO3) or organic. The organic pesticides in turn may be classified into 3 types:
Generally synthetic organic pesticides work by preventing normal transmission of nerve impulses in the target organisms. In insects this results in tremors, convulsions, paralysis and death. Since generally, the vital activities of insects are similar to other members of the animal kingdom, including man, what are toxic to insects will also likely be toxic to other animals and to human beings, with differences only in degree of toxicity.
2.3.5 Pesticides used in tambak operation
The pesticides that are usually used in tambak operation in Indonesia may be grouped into 4 categories as follows:
Thiodan is one pesticide that has been specifically banned for use in aquaculture under a Memorandum from the Junior Minister of Agriculture, Menmud UP 4 No. Hm. 530/MMUP-4/204/1984 dated 31 December 1984, Dir. Gen. Fisheries Circular No. IK 220/02/1817/85K dated 15 March 1985.
2.3.6 Application of some pesticides in tambak
Before a pesticide is ready to be applied in a tambak, there are certain preparatory steps that need to be undertaken depending upon the type of pesticide. The procedures for applying two types of pesticides that are considered safe for tambak use, namely saponin and derris roots are as follows:
The milky white liquid that has been collected can already be applied directly in the tambak. The tambak water should not be more than 10 cm depth if the derris is applied during preparation stage. If applied during the culture period, the water level should be adjusted to not more than 75 cm. The derris concentrate is diluted with pond water so as to have sufficient quantity for spreading evenly throughout the tambak. This is best done during a clear day when rain is not likely to fall since the rainwater could further dilute its concentration and reduce its effectivity.
3. DISEASES AND THEIR MANAGEMENT
The management of shrimp diseases is actually very complex because several factors have to be considered, including sanitation which maybe viewed as a primary factor. Because of this the first thing to consider in shrimp culture is how the waste water should be disposed in order not to cause pollution, such as with the use of blind drainage so as to filter the waste water before it goes into the sea. If the environmental sanitation has been well maintained then the disease problem can be managed.
3.1 Diseases and their Causes
In general, it is widely believed that any disease is caused by microorganisms. This belief is not accurate because environmental factors such as salinity, dissolved oxygen content, ammonia concentration as well as nutritional deficiency can all contribute to the shrimp getting diseased. This is due to the disturbance to the functioning of some organs.
Some experts define diseases as interruption to some functions of, or to the whole organ of, an organism. From such definition we can conclude that an understanding of diseases involves certain aspects, namely the interruption of the function of an organ of an animal and a factor which causes such interruption. Disruptive factors can be in the form of environmental factors which is made up of biotic and abiotic factors. Abiotic factors include chemical factors, for example presence of toxic substances, ammonia concentration, oxygen concentration and salinity; and physical factors which include temperature, turbidity and others.
Biotic factors consist of micro and macro-organisms which could either be pathogenic or parasitic, or may be decomposers, as well as food organisms which all form part of the food web in the aquatic environment.
The emergence of disease is actually the result of the interaction between abiotic and biotic factors. Disease usually occurs when the environmental condition is not stable, for example sudden drop in salinity following a heavy rain, very high temperature at mid-day or too low oxygen concentration all of which cause stress on the part of the shrimps. Stress condition is an internal response system to bring back the metabolic system to normal levels. Under stress condition the shrimp is at a critical level. Environmental condition which are not right for the culture organisms, such as shrimps, are probably conducive to disease causing organisms. Pathogens and parasites grow well under such condition and as a result affects shrimp growth probably because the pathogens exude certain toxic substances which affects the enzymatic actions in the shrimp or absorb food substances from the body of the shrimp thus affecting the growth of the affected shrimp.
Feed is also an important factor which should be given attention because good quality feed increases the resistance of the shrimp to unstable external factors and creates active immunity against pathogenic infections. Feed in this case includes any substance that serves as a resource of energy, body-builder, and functions as nutritionally-essential elements such as vitamins and minerals.
The method considered most effective in fighting diseases is to first of all know its cause. After knowing the cause effective measures to prevent and cure can then be taken. Based upon the cause, diseases may be classified into two general types, those caused by biotic factors and those caused by abiotic factors.
Organisms which cause diseases caused by biotic factors may be differentiated into 3 groups, as follows:
3.2 Infectious Diseases
3.2.1 Viral diseases
There are 6 types of viruses which infect shrimps, these are BP (Baculovirus Penaei), BMN (Baculoviral Midgut gland Necrosis), MBV (Monodon Baculovirus), IHHNV (Infectious Hypodermal and Hematopoietic Necrosis Virus), HPV (Hepatopancreatic Parvo-like Virus) and HPVREO (Hepatopancreatic Reo-like Virus).
Of the 6 types, the most common are MBV and IHHNV. MBV are reported to infect shrimps only at the late postlarval stages (PL-25 to PL-50) up to juvenile stages. Huge losses can be in curred when MBV infects postlarvae because it often results in mass mortality. In Indonesia MBV infection has not yet been reported.
3.2.2 Infectious bacterial disease
Infectious bacterial disease is probably one of the most serious threat to the Indonesian hatchery industry. In BBAP Jepara, the following types have already been encountered:
a. Luminous bacteria
This disease normally appears in the month of July up to February (Sunaryanto, 1987). However lately it is still being encountered up to the month of April in the BBAP hatchery. This disease infects shrimp from the larval stage up to early postlarvae. Its major characteristic is that it makes the infected larvae glow in the dark. Other manifestations include weakness of larvae which no longer actively swim about and loss of appetite. The occurrence of this disease appears to be related to water quality. Normally shrimp larvae infected also exhibits red discoloration.
Luminous bacterial disease appears primarily during the rainy season when the salinity drops, reaching as low as 20 ppt, and extreme temperature fluctuation from mid-day to evening period. This has been identified at the BBAP Pest and Disease Laboratory as being caused by Vibrio bacteria. Experiments following the identification show that the said bacteria is sensitive to various antibiotics such as Chloramphenicol at 20 ppm, Furazolidon at 10 ppm and Prefuran at 1 ppm. Test treatment in the hatchery where part of the larvae has been infected using Chloramphenicol at 4–10 ppm; Elbasin at 0.5–1.0 ppm and Erithrocine at 1–2 ppm gave satisfactory results.
Prevention procedures include filtering of water to be used as culture media for both larval rearing and algal culture; regular water change, disinfection of larval tank with calcium or sodium hypochlorite at a strength of 20–40 ppm, reduction of artificial feed, and use of natural feed, instead such as plankton, Artemia and others.
b. Bent-shrimp disease
From results of monitoring done at BBAP, two types of bent-shrimp diseases have been recognized. Type I infected larvae from PL-1 stage on the months of June, July and August 1986 but started as early as M-1/PL-1 from September through November 1986. The infected larvae showed the characteristic bent body, reddening of the body and antennae and sluggish movement.
Type II infected larvae at Z-1 stage on the months of November and December 1986. The larvae were bent, had no appetite, had abnormal coloration, and suffered from incomplete molting. Initial identification indicates that the disease was caused by bacterial infection by the genus Vibrio which was sensitive to some antibiotics such as Chloramphenicol at 10 ppm and Furazolidon at 10 ppm.
Preventive measures can be undertaken against the bent-shrimp disease by maintaining the stability of salinity and temperature, disinfection of the larval tank and the provision of highly nutritional food rich in calcium and other minerals to the larvae.
c. Brown spot disease
Brown spot disease normally attacks grown shrimps. Infected shrimps are characterized by a brownish shell which usually start as a small brown spot which gradually becomes bigger. The tissue layer below usually becomes necrotic which gives opportunity for other pathogens to cause further infection.
The cause of this disease are chitin-destroying bacteria which during later development becomes associated with other bacterial species such as Beneckea, Vibrio and Pseudomonas. Experiments at BBAP indicate that bathing the infected shrimps with Malachite green solution (1.0 ppm) or Formalin (25–50 ppm) for 30 minutes give satisfactory results.
Measures to prevent its occurrence include maintenance of water quality, separation of infected from healthy shrimps, reduction of density and improvement of food quality.
d. Red gill disease
Lately a new disease affecting shrimp spawners have been observed. Infected spawners show reddening of gills (normal gills are transparent). The spawner shows weakening and settle on the tank bottom.
Results of microbiological analysis show bacteria of the Vibrio type which are sensitive to Erythromycin (40 ppm), Prefuran (10% Nifurpirinol), Doxycycline (1–5 ppm) and Enrofloxacin (2.5–5.0 ppm).
3.2.3 Fungal diseases
Almost all fungi that occurs in penaeid shrimps can be considered pathogenic because they live in the body of the shrimp larvae and subsist on the body tissues as its live media. Fungal infection among shrimp larvae normally occurs during the dry season. This is suspected as due to the fact that humidity level is very high during such time, which favors fungal growth. Some species of fungus attact the larvae while others attack the grown shrimp, however it is those species which attack the larvae which are considered the most dangerous because they can result in mass mortality if not overcome.
a. Lagenidium sp. and Sirolpidium sp.
Among the different species of fungus the most dangerous are Lagenidium sp. and Sirolpidium sp. Infection starts when zoospores settle on the body of the shrimp larvae. The spores then grow to become hyphae. The hyphae penetrates the body of the larvae and develops into mycelium and starts to feed on the tissues. The mycelium develops and eventually invades the entire body of the shrimp larvae.
Infected larvae are usually weak, do not actively swim and settle on the tank bottom until they die. The next stage in the development of the fungus is sporogenesis which is preceded by the formation of terminal vesicles which emerge from the body of the shrimp in which the tissues have been totally consumed. For Lagenidium, the vesicles can clearly be recognized because they are spherical in shape. This is not true for Sirolpidium where the vesicles merely makes the body of the shrimp larve look hairy.
There are three ways that Lagenidium may possibly enter the larval tank, these are:
Preventive measures practised in BBAP so far consist of immersing the gravid spawner in a potassium permanganate solution (0.37%) for 30 minutes or in Malachite green (5 ppm) for 15 minutes. At SEAFDEC in the Philippines, calcium hypochlorite or formalin solution is used for the spawners and iodine solution for the eggs.
b. Fusarium sp.
Although Fusarium is not as dangerous when compared with the previous two species, it can still inflict considerable damage to shrimp culture because it results in the deterioration of the quality of shrimps. Only grown shrimps which has previously suffered wounds in the exoskeleton or has damaged tissues due to other diseases such as black gill or brown spot, are subject to Fusarium infection. In addition, the lesion caused by the fungus may give rise also to a secondary infection, for example by bacteria.
Diagnosis of shrimp suspected to have Fusarium requires scraping of the infected tissue from the lesion and examination of the tissue samples under the microscope. The presence of boat shaped or crescent shaped bodies, which are actually the conidio spores, is a definite indication of the disease. Until now there is no known preventive or treatment procedure for Fusarium except maintenance of good water quality.
One type of protozoa which is often encountered in shrimp larvae is the amoeboflagellates. Infected shrimp larvae when viewed under the microscope appears empty, and only the exoskeleton is visible because the body tissues have all been consumed by the said pathogen. During the initial stages of infection, the larvae appear weak and merely settle on the tank bottom. The tissues that are attacked usually include the muscles, eyestalk, digestive tract and other soft tissues. This protozoa species usually appears when the water quality deteriorates and the tank bottom has a thick layer of waste organic matter. To prevent its occurrence, organic matter settling on the tank bottom should be siphoned off and the water changed more frequently.
b. Cotton shrimp disease
Cotton shrimp disease usually infects grown shrimps when the organic matter in the water becomes too high (above 70%). Such organic matter comes from left-over food and shrimp faces. Infected shrimps are characterized by the white coloration of the abdominal muscles giving it the appearance of cotton, hence the name. The pathogens responsible for this disease consist of three genera, namely: Nosema, Thelohania and Pleistophora. The target tissues include the unstriated muscles and the gonad.
a. Black gill disease
There are several causes of black gill disease, one of them is the filamentous bacteria, Leucothrix sp. This disease is a more advance form and is not due to the said bacteria by itself. Whenever the population of filamentous bacteria becomes very high, they start to adhere to the shrimp body including the gill surfaces. The gills which are already covered by the filamentous bacteria start to change in color from brownish to dark green due to the deposition of detritus or plankton that has been attracted by the bacteria. The color change is the effect of both the plankton and detritus adhering to the gills. With the gill surfaces completely choked, the tissues of the gill lamillae where the exchange of oxygen and carbon dioxide takes place, starts to decay resulting in necrosis. The presence of dead tissues is the reason for the black color. As more of the gill tissues die, the shrimp starts to suffer from oxygen deficiency and eventually dies.
Preventive measures to be undertaken are aimed at controlling the bacterial population. There are several chemicals which can be used for this purpose, for example copper sulfate (1 ppm). The infected shrimps may also be immersed in potassium permanganate solution (5–10 ppm) for 1 hour. According to experiments of Le Bitoux (lightner, 1977), immersion of infected shrimps in Furanace (1 ppm) also gives satisfactory results.
b. Udang lumutan
Shrimps infected by protozoans often appear to be covered by a layer of algae to the naked eye, so that they are often locally known as udang lumutan or “algaecovered shrimps”. What appear as algae are actually protozoans adhering to the shrimp body consisting of Epistylis sp., Zoothamnium sp. and Vorticella sp.
Usually these protozoan species emerge when the water quality is poor, such as when the organic matter and ammonia concentration are high. The occurrence of Vorticella is therefore an indication of water quality deterioration because these organisms are saprophytic. The most effective method of overcoming this disease is by changing as much of the water as possible, at the same time reducing the amount of artificial feed and stimulating the growth of plankton.
Based upon experiments conducted at BBAP Jepara, treatment of larvae infected with Vorticella, with formalin solution (25 ppm), Malachite green (0.02–0.04 ppm) or copper sulfate (1 ppm) for 1 hour give satisfactory results. It is best to conduct the treatment when the larvae is not in the molting condition because formalin can destroy the epidermal tissues. Meanwhile if such infection occurs in pond shrimps, as much of the pond water as possible should be changed so as to lower the organic matter concentration and mechanical aerators, such as paddlewheels, operated so as to raise the oxygen level.
Worms that are encountered in shrimps are usually nematodes that are associated with protozoans. Generally worms infest the body of the shrimp only after this has been coated by bacteria. One control measure against worm infestation on the shrimp larvae consists of subjecting the larvae to a formalin solution (25 ppm) for 1 hour. The same treatment can be used on the grown shrimp except that the formalin solution maybe increased to 30 ppm. The formalin makes the nematodes weak and causes them to get detached from the body of the shrimps. Aside from this, because formalin is an irritant it makes the worms dehydrated because worms do not have the protection of chitin like shrimps. However formalin treatment on the shrimp postlarvae should be done with care because if it is allowed to last more than 3 hours, these could weaken the postlarvae and could cause mortality.
One species of isopod infesting the shrimp gill has been reported. It has been identified as belonging to the Family Bopyridae. This isopod species are usually encountered in shrimp spawners caught from the sea. Its presence makes the shrimp gills appear swollen. Eggs carried by female bopyrid hatches into epicardium which are then released into the water. The epicardium then starts to find a host in the form of copepods. Within the copepod the epicardium develops into microniscus and then cryptoniscus. It then will have to find a primary host in the form of adult or juvenile shrimps.
Shrimps infested by this parasitic isopod are generally weak because it sucks the blood of the shrimps through the gills. In addition it also disturbs the shrimp's respiration.
3.2.6 Environmental diseases
a. Black death disease
Shrimps infected with black death disease are characterized by the blackening of the dorsal part of the tail muscle immediately below the shell because the cell tissues have become necrotic. This disease has been traced to vitamin-C deficiency which disturbs the metabolic processes. If the symptom of this disease appears, control measures should be done immediately because it can cause mortality. For this it is necessary to provide vitamin-C supplement and natural food in the form of plankton.
The blooming of blue-green algae in the plankton often endangers shrimp life. A dense population of some species, one of which is Schizotrix calcicola which belongs to the Oscillatoria family can result in the emergence of HE (Hemocytic Enteritis) disease. HE disease occurs when the shrimps ingest the said plankton species. Inside the intestinal tract of the shrimps, endoxins are released by the algae which results in necrosis of the intestinal tissues. The shrimps die because of disturbance to the food absorption process as well as the osmotic balance.
Preventive measure involves controlling the growth and blooming of the blue-green algae by using copper sulfate (1 ppm) and continuously changing the water in order to reduce plankton density.
Actually the most effective method of preventing the occurrence of pests and diseases is by maintaining the cleanliness of the culture environment. This includes providing a place for waste disposal so that the waste water which includes organic wastes, will not pollute the water supply for aquaculture, filtration of water that is intended for various culture use, and the sterilization of facilities and equipment used for culture.
Even if antibiotics are perhaps still being suggested, proper procedures on its use should be followed, especially with regards to dosage so that no undesirable effects will emerge on the part of both the shrimps and the consumers. This is very important because some antibiotics, such as chloramphenicol can result in the destruction of red blood cells in human beings and in the process induce anemia aplastica (Goth, 1974). If used often for disease control, antibiotics can accumulate in the body of shrimps and pose a threat to people eating such products.
Lannan, J.E., R.O. Smitherman and G. Tehoganoglous (editor). 1986 Pond production system. In Disease, competitors, pests, predators, and public health considerations in principles and practices of pond aquaculture. Oregon State University Press, Corvallis, Oregon, pp. 169–185.
McEwen, F.L. and G.R. Stephenson, 1979 The use and significant of pesticides in the environment. A Wiley-Interscience, John Wiley and Sons, New York, p. 538.
Pirzan, A.M. dan A. Poernomo. 1987 Biological and ecological aspects of jambret (Mesopodopsis sp.) as tambak pests in South Sulawesi and control methods (In Indonesian). J. Pen. Budidaya Pantai, Maros.
Preston, S.T. 1967 A guide to the analysis of pesticides by gas chromatography. 2nd ed. Ply Sciences Corporation, Illionis, p. B-43.
Goth, A. 1974 Medical pharmacology. The C.V. Mosby Company, Ltd, Saint Louis, pp. 557–597.
Sinderman, C.J. (editor). 1977 Milk or cotton disease of shrimps. In Disease diagnosis and control in North American Marine Culture. Elsevier Scientific Publishing Company, New York, pp. 48–50.
Sinderman, C.J. (editor). 1977 “Black Death” disease of shrimps. In Disease diagnosis and control in North American Marine Culture. Elsevier Scientific Publishing Company, New York, pp. 65–67.
Lightner, D.V. 1984 A review of the disease of cultured penaeid shrimps and prawns with emphasis on recent discoveries and development. In Proceedings of the First International Conference on the Culture Penaeid Prawns/Shrimps, SEAFDEC Aquaculture, Iloilo City, Philippines, pp. 79– 100.
Sunaryanto, A., Arini M. dan Pudjiatno. 1987 Shrimp diseases. (In Indonesian) INFIS manual series No. 23, p. 27.
Tareen, I.U. 1982 Control of disease in the cultured population of penaeid shrimp, Penaeus semisulcatus (de Haan). J. World Maricul. Soc. 12, pp. 157–161.
Table 1. Types of predators commonly found in Indonesian tambaks.
|Fish||Payus (Elops hawaiensis)|
|Bulan-bulan (Megaslops cyprinoides)|
|Kerong-kerong (Therapon jarbua)|
|Kakap (Lates calcarifer)|
|Kerapu (Epinephelus tetradactylum)|
|Kuro (Eleutheronema sp.)|
|Amphibians||Katak hijau (Rana sp.)|
|Reptiles||Ular kadut (Carberus rhynchops)|
|Birds||Bangau hitam (Ciconidae fam.)|
|Belibis (Anatidae fam.)|
|Platuk besi (Plegadidae fam.)|
Table 2. Type of competitors commonly found in tambaks.
|Polychaetes||Cacing tambak (Dendronereis sp.)|
|Shrimps||Udang putih (Penaeus merguiensis)|
|Udang api-api (Metapenaeus monoceros)|
|Jambret (Mesopodopsis sp.)|
|Fish||Mujair (Tilapia mossambica)|
|Belanak (Mugil sephalus)|
|Kepala timah (Aplocheilus panchax)|
|Kiper (Scatophagus argus)|
Table 3. List of chemicals used for shrimp disease control.
|Cutrine plus||0,5 ppm Cu||L/PL||Filamentous bacteria|
|EDTA||5–10 ppm||L/PL||Chelating agent,|
|Formalin||25 ppm||L/J||Protozoa, Nematode|
|50 ppm-3 hours||S||Protozoa, Nematode|
|Potassium Permanganat||4–10 ppm||L/J||Protozoa,|
|Quinine Bisulphate||5 ppm||L/PL||Protozoa|
|Quinine Sulphate||5 ppm||L/PL||Protozoa|
|Quinacrine HCl||0,3–0,6 ppm||L/PL||Protozoa|
Notes: L: Larvae, PL: Postlarvae, J: Juvenile, S: Spawner
Sources: 1. BBAP Jepara
2. Tareen, I.U., J. World Maricul. Soc. 13:157–161 (1982)
Table 4. Mode of action of some antibiotics.
|Mode of action||Antibiotics|
|- Affects cell wall synthesys||- Penicillin and Cephaloof bacteria sporin|
|- Affects protein synthesys of bacteria||- Tetracicline|
Source: Hash, J.H.: Ann. Rev. Pharmacol. 12:35, 1972 in Goth, A. Medical Pharmacol.
WILFREDO G. YAP
FAO-UNDP INS/85/009 Shrimp Culture Development Project, Jepara, Central Java
The use of inter-tidal land, generally mangroves or milkfish ponds, and supra-tidal dry lands, generally farmlands, for shrimp farming are compared. While traditionally brackishwater ponds were developed primarily for milkfish, with shrimps only as a secondary crop, the situation changed when the market for black tiger shrimps developed. The extremely good market situation during the mid-1980's stimulated the development of intensive shrimp farms which require water independent of the tidal condition and thus negated the only real advantage of inter-tidal area, the capability of being watered by tidal water without the use of a pump. The period therefore saw an increasing trend towards the use of coastal drylands for intensive shrimp farming since such lands can be developed very rapidly by using machines and are generally easier to manage. The case of East Java is presented. It is estimated that within the districts of Banyuwangi and Situbondo alone of some 3,166 ha intensive shrimp farms registered, only about 33% were originally tambaks with the rest consisting of paddy fields (1,027 ha), farmlands such as corn and sugarcane fields (314 ha) and other types of dryland. The risk of salinization of adjoining farmlands and possible contamination of the groundwater supply are raised.
Brackishwater aquaculture since its very beginning had always been limited to the intertidal zone -- that was until the advent of modern shrimp farming technology. As the water rises during high tide this was allowed to flood shallow ponds through a system of canals and sluice gates and its level maintained at a desired level inside the ponds by earthen dikes even as the tidal water recedes. Thus, early literature in brackishwater aquaculture in general and shrimp farming in particular (Villaluz, 1953; Delmendo and Rabanal, 1956) had always emphasized the importance of land surface elevation in selecting sites for farming milkfish and shrimps -- the two major species in brackishwater aquaculture.
Due to the generally low yields in traditional farms it was considered too expensive to pump water except on a supplemental basis. However with the development of technology which made possible the increase of shrimp yields to two orders of magnitude (from less than 500 kg to more than 10,000 kg per ha per year), coupled with the increase in worldwide shrimp demand, pumping became not only economically viable but also technically necessary to maintain water quality. In traditional brackishwater aquaculture, water is changed whenever possible, but semi-intensive and intensive shrimp farming depends to a large extent on the capability to change water whenever necessary not just when natural conditions, i.e., the tidal level, makes it possible.
With such a change in production requirement, the need to locate a shrimp farm within the intertidal area became unnecessary. Taiwan, which never had much intertidal mangrove area in the first place, became a leading exponent in the use of above-ground ponds requiring total pumping for its water supply for shrimp culture since it had no other choice to begin with. By intensifying, a given area is able to yield as much as ten times more than a traditional pond of the same size. Probably, it is the limited area available which provided the necessary pressure for Taiwan to develop intensive shrimp culture technology, while the Philippines and Indonesia, both of which have a much longer tradition for brackishwater aquaculture, but which have hundreds of thousands of hectares of mangroves, were initially content to extensify.
As the shrimp market became lucrative enough to attract investors, Taiwanese culturists either came or were imported in droves to the ASEAN region. At first these were limited to hatchery operation, but not long after, this expanded to the grow-out phase. Not surprisingly, such grow-out ponds managed or operated by the Taiwanese were patterned after what they were accustomed to, in terms of design, equipment requirement and feeds. And very soon even those financed by local investors using local expertise started using the Taiwanese system.
2. MANGROVE VERSUS HIGH GROUNDS
2.1 Mangrove Areas
Shrimp farming per se, where shrimp fry is stocked in ponds in pre-determined quantity, is relatively new. Brackishwater aquaculture in Southeast Asia, particularly Indonesia and the Philippines, was for a long time synonymous with milkfish culture. Shrimp was only a secondary harvest with the seedstock consisting merely of wild fry which comes in with the tide. Thus, it is just logical, that when the time came for shrimp to be farmed as a primary species, with fry deliberately stocked in pre-determined numbers, the farm sites were either converted milkfish ponds or newly-opened mangrove areas. The use of mangrove areas was advantageous in the point of view of land cost, energy cost, and natural productivity.
Mangrove areas are within the intertidal zone and are more often than not, public lands. It used to be very easy to acquire a long-term lease on large tracts of mangrove areas for aquaculture development at a very cost from the government. However as mangrove forests disappeared at an alarming level, environmental concerns mount against clearing of the remaining stands. Rabanal (1977) estimates that in the Indo-Pacific area some 1.2 million hectares of mangrove has been developed for aquaculture. Doubtless during the past 12 years this must have increased considerably. At that time Indonesia was cited as having only 185,000 hectares of tambak, by 1984 this had increased to 225,000 hectares. In the Philippines, it is now almost impossible to obtain new Fishpond Lease Agreements. While in Indonesia this can be obtained only in certain islands outside Java.
In the point of view of energy cost for drawing water, the traditional tambak has zero expenditure. With a pond bottom elevation below the lower high tide level, water comes in naturally with the tide. Depending upon the tidal amplitude of the locality, pond water level of one meter or more can be attained without pumping. It should be noted though that while such practice is adequate for milkfish growing, it severely limits shrimp culture to extensive methods which rely mainly on natural productivity at most enhanced with fertilizers. This is where the natural productivity of the mangrove area becomes important. Ironically as the mangrove forests give way to aquaculture, the productivity of a given area is also adversely affected.
The use of mangrove forests also has several operational and technical disadvantages. Oftentimes clearing of the mangrove forests is a very expensive and laborious process which largely negates the low land cost. Rare is the mangrove forest which do not have a dense stand of Avicennia and the deep-rooted Sonneratia. The soft and often oozy bottom makes mechanized construction extremely tricky and applicable only on a selective and supplemental basis. The bulk of the construction work has to be undertaken manually. This greatly delays development time. A 20 hectare mangrove area can take more than one year to reach production stage.
Even after the ponds have been constructed, many new ponds may not be immediately usable due to acid-sulfate problems which is characteristic of mangrove areas. Sub-surface soil in a mangrove area is generally rich in iron sulfide. By itself it is not acidic. However, once exposed to air upon excavation, these sulfides oxidize to sulfates and in the presence of water forms sulfuric acid. With this condition no amount of liming can neutralize the acidity.
It is a well known fact that both milkfish and shrimp culture require a pH level that is slightly alkaline or at least near neutral. The recommended practice is to alternately dry and flood the pond bottom in order to leach out as much of the potential acidity as possible (Singh and Poernomo, 1984). This often takes several months to accomplish, thus prolonging the gestation period even further. Even after such reclamation the sides of the dike may still remain acidic and catastrophic lowering of pond pH can occur after a heavy rainfall.
The low elevation within the intertidal range is actually a mixed blessing. It is beneficial in letting in new water. On the other hand it has several disadvantages. One, inspite of very careful pond preparation, during the rearing period it is easy for extraneous organisms to come in through the unavoidable leaks either along the dikes or through the sluice gates, especially in old ponds. Two, harvesting has to be timed to coincide with a favorable tidal condition to facilitate draining of water. In some areas, due to the topography and size of ponds it may not at all be physically possible to harvest one pond in one operation due to the limited time when the tide is low enough to allow draining. In many ponds, due to the unavoidable seepages and leakages, it is almost impossible to dry the pond bottom completely without pumping, as is true in most of the ponds at the BBAP Jepara.
2.2 Supratidal Areas
The alternative to using mangrove swamps of course is to use higher grounds not normally reached by high tide. For lack of a better term to denote such zone, it is here referred to as supratidal areas -- meaning areas above the tide, although this may include areas which are technically intertidal but are reached by seawater only at the highest high water level which occurs occassionally during the year.
Once the concept of relying totally on pumping to supply seawater is accepted, then apart from long-term environmental issues which shall be discussed later, all of the disadvantages to the use of mangrove areas enumerated above no longer exist in a supratidal setting as shown in Table 1. Being along the coast these would already be settled in and will no longer be forested. As a matter of fact many of such areas are farms planted to rice, corn, sugar cane and other crops. These areas, being dryland, of course easily lends to mechanized development. With the use of bulldozers and backhoes, such areas can be converted into a productive shrimp farm in a fraction of the time required for developing mangrove areas. And in the business sector where profit is the major, if not the only, objective, the time factor is indeed very important in order to minimize the cost of money.
The topsoil and the sub-soil of such farmlands normally have already stabilized geochemically and will not have the acid-sulfate condition which characterizes mangrove areas. Immediately after construction these can be watered and stocked. Being relatively elevated, such ponds can be drained anytime regardless of tidal condition and can be completely dried. Furthermore since all the water required has to be pumped, the screening of predators and other extraneous organisms is easier and during the course of the rearing, such extraneous organisms have no way of entering by themselves.
3. RECENT TREND TOWARDS USE OF SUPRA-TIDAL AREAS AS SHRIMP FARM SITES
3.1 The Situation in East Java
From the above comparison it is very easy to see why the supratidal land is very attractive to investors in intensive shrimp farming. Fueled by very good world market prices, the rapid conversion of coastal crop lands was inevitable. In Indonesia, the leading province in the conversion of agricultural land into shrimp farms is East Java. Along the northern coast of the province, especially within the regencies of Situbondo and Banyuwangi, the coastal landscape is fast resembling the Pingtung coast of Taiwan. In this area, which certainly must have the densest concentration of shrimp hatcheries and intensive shrimp farms in Indonesia, the trend clearly is towards the use of agricultural lands rather than improvement or rehabilitation of traditional tambak areas.
As shown in Table 2, Situbondo and Banyuwangi have the most number and largest areas registered for shrimp farm development among the regencies of East Java, with 86 and 165 respectively totalling some 3,166 hectares, or more than 50% of the total area of intensive farms registered in the whole province. While traveling within the province of East Java it can also be immediately noticed that while shrimp farms in most areas, whether already operational or still under development, are traditional tambak areas, in Situbondo and Banyuwangi many are clearly being developed out of existing rice fields or other agricultural fields such as sugar cane. For this reason, this paper has concentrated on the two regencies.
Using the land use maps for Situbondo and Banyuwangi from the national land agency (BPN), it is possible to determine the type of land selected for each particular farm site as shown in Figure 1. The result is Table 3. It will be noted that in the said table in each Kecamatan the areas of the registered shrimp farms consist of two figures. The figures classified as coming from the land use map means that the particular shrimp farm that was in the District Fishery Office list was also found indicated clearly on the BPN land use map. The figures indicated as “inferred” refers to those shrimp farms in the District Fisheries lists which have not yet been entered by the BPN in their file maps. The type of land were therefore merely inferred from the land-use map based upon their respective location (name of Desa), and the type of coastal land available within the said Desa.
In some cases a shrimp farm actually straddles across two or three types of land. In the case of Situbondo, the land use map which had a scale of 1:25,000, was very clear and the boundaries of most of the farms clearly indicated. It was therefore possible to estimate the proportion of each type of land by measuring the map area with a planimeter. The land-use map obtained for Banyuwangi had not yet been updated to the same extent, it is for this reason that it has a greater number of farms with their land types merely inferred.
With that as a background, a perusal of Table 3 shows that the clear preference of most of the investors for agricultural lands over tambak lands. Tambak land represents only 34 percent of the total. Paddy fields top the list with 44 percent and dry land farm or tegalan, 9.9 percent. Such dry land areas consist of either sugar cane or corn fields. The land classified as others were either grassland or barren land (tanah tandus).
3.2 Implication of the Present Trend
There is no doubt that the rapid rise of the intensive shrimp farms contributed to the strengthening of Indonesia's position as a major shrimp supplier, which in 1988 was estimated to have earned US$ 500 million. However the recent trend towards the use of new, unconventional areas for shrimp farming, mainly productive farmlands including riceland, warrants a close look. While very attractive in the short term in terms of increasing shrimp export in a short time, there are reasons to believe that in the long term it might be considered regrettable.
In the history of mankind, the tendency has always been directed towards pushing back the sea. This is done by reclaiming shallow areas of the sea through filling or by simply building a system of dikes and windmills to keep out the seawater as exemplified by the Netherlands. Any entry of saline water into the terrestrial domain is always viewed with alarm since it affects agricultural crops and potable water resource. The conversion of agricultural land for shrimp farming involves the deliberate salinization of a tract of land which cannot normally be flooded by seawater. Such an act therefore can be considered as contrary to conventional human reaction towards the sea and seawater, since it is in effect “pulling in the sea”. In fact there appears to be no precedent and therefore no literature to guide us as to what its possible consequence might be.
The absence of available information however should not prevent us from speculating on its possible consequences. An earthen pond bottom, no matter how good the soil quality, is to a certain extent porous. Part of the seawater can therefore be expected to percolate through the top soil. The effect of saline water on agricultural crops and drinking water need not be elaborated on. In fact this is no longer mere speculation. Already there have been reports of shallow wells getting saline due to the presence of shrimp culture activities. In Takalar, South Sulawesi, an intensive shrimp farm has to pay the neighboring riceland for the loss of the rice crop due to seepage of seawater from his farm.
Another point to consider is the alternate use of such shrimp farm sites if ever the shrimp market crashes as it did already this year. A conventional intertidal pond has the option of going semi-intensive or even all the way to extensive in order to reduce the production costs. An above ground pond designed for intensive culture can reduce their culture density to a limited extent. For extensive and semi-intensive culture, an intertidal area is still more suitable, (Apud, 1985).
Should the market crash to a point of nonrecovery, many investors will probably opt to clear out. An above-ground intensive shrimp farm, obviously, can no longer be reconverted back to agricultural activity without first leaching out the accumulated salts -- which may take time and may be expensive. Because of its elevated nature it also will not be financially viable to use such farms for milkfish farming. The only viable alternative will be a high value species which can be cultured with almost the same financial return as the black tiger shrimps. At present such an alternate species is not yet in sight. Thus an abandoned intensive shrimp farm will likely remain unproductive in the foreseeable future.
This dilemma is by no means limited to Indonesia. The same phenomenon is happening in the Philippines. In Negros Occidental huge tracts of sugar cane plantations were converted to shrimp farms when the sugar prices fell. This practice has reportedly also spread to other provinces such as Iloilo and Pangasinan. Already environmental groups have raised their concern on such a practice.
While an outright moratorium on the development of new farms is probably too drastic it is perhaps well at this stage to initiate certain steps in order to manage the situation. First and foremost is to obtain a clear inventory of the extent of conversion of agricultural land to shrimp farming. The data presented in this paper is by no means complete or precise, it should be considered only indicative. Short of an actual farm by farm site survey, there is no way of determining how many of the shrimp farms registered have actually been built and how many are still on paper. It appears that the shrimp farms in the list can be classified into four categories: permit not yet released, permit already released but pond not yet developed, permit already released and ponds already built and/or operating, permit not yet released but ponds already built and/or operating.
Another step is to actually monitor the water quality from wells within the vicinity of the shrimp farm and to monitor effects on the remaining agricultural farms around it, if any. This will require an inter-agency team probably consisting of Fisheries, Food Crops and Public Works offices. The result of such a study can serve as a basis for defining definite criteria for approving or disapproving any application for conversion of agricultural land to shrimp farm or an outright ban for such a practice altogether. In this manner possible conflict with neighboring, unconverted agricultural land can be minimized if not avoided altogether and shrimp farm development can be rationalized.
Significantly enough, the local government of Jepara has already shown an example with a local ordinance regulating the development of small-scale hatcheries. Beyond a pre-defined boundary close to the Jepara coastal line no shrimp hatcheries will be allowed. Any shrimp hatchery that has been erected inland outside the allowable zone will have to be demolished after an appropriate grace period. This is a highly commendable move which demonstrates concern for long term effects to the town's ground water supply rather than for short-term gains.
While any move to regulate the development of intensive shrimp farms might be decried by some as unnecessarily imposing additional obstacles to fisheries development and export promotion, regulating it is the only way to minimize possible adverse longterm impact on the environment. The lesson of the global consequences of massive deforestation as a result of unbridled logging activities in the name of generating export revenues should not be lost in us.
Apud, F.D.1984. Extensive and semi-intesive culture of prawns and shrimps in the Philippines. In Proceedings of the First International Conference on the Culture of Penaeid Prawns/Shrimps, 4–7 Dec. 1984, Iloilo City, Philippines. Aquaculture Dept, SEAFDEC, Iloilo, Philippines. pp. 105–114.
Badan Pertanahan Nasional (National Land Agency) Land-use maps, (working copies) for Situbondo and Banyuwangi, East Java.
Delmendo, M.N. and H.R. Rabanal. 1956. Cultivation of sugpo (jumbo tiger shrimp), Penaeus monodon Fabricius in the Philippines. In Proceedings, Indo-Pacific Fisheries Council, 6(2–3):424–431.
Dinas Perikanan Daerah (District Fisheries Office), Situbondo, East Java. List of intensive shrimp farms as of October, 1988, unpublished.
Dinas Perikanan Daerah, Banyuwangi, East Java. List of intensive shrimp farms as of October, 1988. unpublished.
Dinas Perikanan Propinsi (Provincial Fisheries Office), East Java. Fisheries Statistics Report, East Java 1986.
Rabanal, H.R. 1977. Forest conservation and aquaculture development of mangrove areas. In Proceedings of the
This paper was made possible only with the assistance of the following: Dr. Nyan Taw, UNV/Algal Culturist and Mr. Budiono Martosudarmo, National Pond Culturist, both of Project INS/85/009 in collecting the most recent data from the District Fishery Offices of Situbondo and Banyuwangi, East Java; and the staff of the Badan Pertanahan Nasional (National Land Agency) Surabaya Office in providing the land-use maps. To all of them, a heart-felt Terima Kasih!
Table 1. Comparison between conventional brackishwater pond sites and dry coastal flat lands when used for intensive shrimp culture.
|Tensive||Conventional tambak sites for milkfish and/or shrimps||New areas for intensive shrimp farming|
|1. Land Type||Mangrove swamps.||Dry coastal flat land.|
|2. Elevation||Intertidal, between lower high tide to average lower low tides.||Supratidal, above high tide level but generally less than 2 m above high water line.|
|3. Land Cost||Low for new undeveloped areas, usually public lands available for long-term lease from government||Relatively higher. Land normally already privately
owned or claimed and cultivated.
|4. Clearing and Grubbing||Costly for forested area.||Minimal.|
|5. Development Cost||Low for conventional design involving large shallow ponds,||Variable. Ponds with earthen ponds only can be low cost, but cost can increase drastically if concrete is used for dikes.|
|6. Development Time||Long, has to rely largely on manual labor. Mechanization limited.||Short. All earthworks can be fully mechanized.|
|7. Pre-production||May take several months if||Can be operational immediately|
|conditioning||acid-sulfate condition exists.||after construction.|
|8. Energy Cost for Water||None to minimal for extensive or semi-intensive culture.||Relies totally on pumps regardless of culture density.|
|9. Operational Problem||Entry of extraneous organisms difficult to prevent.||Entry of extraneous organisms easier to control.|
|10. Harvesting||Depends on tide condition.||Anytime.|
|11. Pond Preparation||Difficult to dry pond bottom. Seepages common especially in old ponds.||Complete drying easy.|
|12. Environmental Impact||Destruction of rich mangrove environment.||Salinization of land and possibly ground water resource.|
Table 2. Area of traditional tambak and intensive shrimp farms registered in East Java broken down by Kabupaten (Regencies).
|Kabupaten:|| Gross area
|Intensive shrimp farms|
|4. Kodya Surabaya||5,815||n.d.||n.d.|
Notes: Gross area of tambak from 1987 provincial statistics. Number and area of intensive shrimp farms based on 1989 data from respective District Fishery Offices. Gross areas of tambak and registered shrimp farms are not mutually exclusive, since some of the intensive farms may use traditional tambak areas.
Table 3. Intensive shrimp farms, either proposed or already developed, broken down by Kecamatan and by land type, Situbondo and Banyuwangi, East Java, Indonesia. (Based on data from District Fishery Offices, Situbondo and Banyuwangi and land-use map from the National Land Agency, Surabaya office, 1989.)
|No. Total farms area||(ha)||A r e a b y l a n d t y p e (h a)|
|TOTAL SITUBONDO & BANYUWANGI||251||3,166||1,027||1,394||314||431|
|Percent of Total (%):||100||32.4||44.0||9.9||13.6|
|(fr. BPN map)||4||37||15||7||0||15|
|(fr. BPN map)||13||181||39||102||0||41|
|(fr. BPN map)||1||5||5||0||0||0|
|(fr. BPN map)||15||292||159||90||22||21|
|(fr. BPN map)||10||64||0||60||4||0|
|(fr. BPN map)||5||83||0||77||6||0|
|(fr. BPN map)||4||85||0||54||31||0|
|(fr. BPN map)||10||176||0||111||65||0|
|(fr. BPN map)||6||107||5||102|
|(fr. BPN map)||4||56||56|
|(fr. BPN map)||5||76||31||45|
|(fr. BPN map)||6||150||15||135|
|(fr. BPN map)||5||0|
|(fr. BPN map)||0|
|(fr. BPN map)||3||403||78||325[b]|
[a] Includes one farm in BPN map not found in District Fisheries Office statistics.
[b] New site reported to have been selected in another Kecamatan since originalsite applied for is forest reserve area.
Figure 1. Typical portion of land-use map used for determining original land use of planned or developed intensive shrimp farms in Situbondo and Banyuwangi, East Java, showing boundaries of shrimp farm sites superimposed. (Source: National Land Agency, Indonesia)