Production of crustaceans by various forms of aquaculture increased by nearly 325% to nearly 1million t between 1984 and 1992. In 1992, 90% (884000metric tonnes) of this production was marine shrimp (FAO 1995). The major species cultured are Penaeus monodon (44%) and Penaeus chinensis (23%) in Asia, and Penaeus vannamei (13%) in Latin America. Associated with the rapid increase in the production of crustaceans there has been large scale conversion of mangrove to shrimp ponds. Many of these endeavours have proven non-sustainable with the result that large areas of ponds have been left idle and disused, some have been abandoned and new sites are being developed in an effort to maintain production. This presents a major challenge for both coastal resource managers and pond owners who will have to address the question of what to do with disused ponds. There are three basic options. The first is to rehabilitate the pond sites so that they can be put back into shrimp production. The second is to rehabilitate the pond sites so that they can be put to some alternative, sustainable use such as salt production. The third option is to restore the environmental conditions within the pond site and the surrounding area, and to re-establish a productive mangrove ecosystem. Each of these options is heavily influenced by the causes of failure of the pond operations and the conditions which remain in the pond after disuse. This paper briefly explores the issues that must be faced if rehabilitation and restoration are to be successful.
The Scale of the Problem
A recent issue of Aquaculture Asia includes several articles which refer to the problem of disused ponds. These include Fegan (1996), who suggests that unsuccessful shrimp farmers may continue shrimp culture until finances run out, before either selling or abandoning the farm, and Macintosh (1996b), who states that many disused ponds now lie unproductive in Thailand, and finally Hambrey (1996), who states that there has been significant abandonment of ponds where disease and water quality problems have become chronic. It is noteworthy however, that none of these authors have been able to quantify the scale of the problem or substantiate its significance. This is not unusual. Why?
Accurate estimates of pond disuse are difficult to obtain since land tenure records are often unreliable and out of date, and assessments using remote sensing and GIS technologies are hampered by the inability to remotely discern between productive and disused ponds. Attempts to quantify the scale of the problem have been marred by the belief that an admission of pond abandonment on a significant scale, is tantamount to an admission of management failure. It is not surprising therefore that, with the exception of Thailand, there is little quantification of pond disuse or abandonment in the literature.
However, unofficial estimates of pond disuse have suggested that the percentage of ponds left idle after a period in production can be as high as 70%, although it would appear that some disused shrimp ponds in Thailand (and probably elsewhere) are subsequently converted to other uses, such as redevelopment into factory or housing estates (Macintosh 1996b).
There have been several reports in Thailand which have included estimates of the scale of pond disuse and/or abandonment (both in mangrove and non mangrove); Briggs and Funge Smith (1994) reported that an area of 40,000-45,000 ha south of Bangkok became derelict after shrimp production collapsed in 1989/90. A report produced by NACA (Network of Aquaculture Centres in Asia- Pacific) details that in 1989 about 62% of farms were operating ìunder capacityî and another 22% of farms were ìabandonedî in Samut Sakhon province (OEPP 1994). Enright (J.Enright personal communication. 1996) estimates that currently 70-80% of ponds are ìabandonedî in Prachuap Khiri Khan, and that a similar figure can be estimated for the provinces of Songkhla and SiThammarat. However, it must be remembered that estimates of disuse are only pertinent to the time they were formulated; the situation changes rapidly from month to month and ponds are frequently converted to other uses and shrimp production can be recommenced at any time.
Pond disuse has been reported elsewhere, but not quantified. For instance, reported in India, as a result of white spot disease (Sammut and Mohan 1996); in Sri Lanka, as a result of improper site selection and poor water quality (Jayasinghe 1995); in Vietnam, as a result of acid sulphate soils (V. D. Quynh personal communication. 1996; and Tuan 1996); in Cambodia, as a result of acid sulphate soils and poor water circulation (Sreng 1996); in the Philippines, as a result of disease problems (Ogburn and Ogburn 1994); in Taiwan, as a result of disease (C. Chin Chen personal communication.1996) and in Indonesia as a result of disease and water quality problems (Matondang personal communication.1996). It is also thought that there are significant areas of disused shrimp ponds in Peru, Bangladesh, China, Malaysia and Colombia.
Environmental Problems Associated with Pond Development.
The environmental problems which may be associated with shrimp farming are well documented (Macintosh and Phillips 1992; Landesmann 1994; Pullin et al. 1993; Phillips 1995a and 1995b) and include mangrove deforestation; reduction of habitat; reductions in shoreline protection; increased coastal erosion; coastal water pollution; nutrient enrichment; depletion of wild prawn and fish larvae stocks; land subsidence; salinisation of soils, agricultural land and ground water, activation of acid sulphate soils; loss of agricultural lands; introduction of exotic species and the discharge of undesirable chemicals. Whilst these problems have been identified, quantification of them is difficult.
For instance, few estimates of mangrove deforestation for shrimp culture exist on a country by country basis, however, there are a few notable exceptions. In the Philippines it is estimated that 50% (approximately 141,000 ha) of mangrove loss can be traced to brackish water pond construction (Primavera and Agbayani 1996). In Vietnam over 102,000 ha of shrimp ponds were constructed in mangrove between 1983 and 1987 (Tuan 1996). There are, however, several regional estimates of the extent of mangrove destruction for shrimp pond construction.
For instance in the Gulf of Fonseca, Honduras, 4307 ha of shrimp farms were constructed in formerly dense mangrove areas between 1973 and 1992, amounting to a 22% loss of mangrove between this period directly due to shrimp pond construction. (DeWalt et al. 1996). According to Potaros (1995), 38% of the reduction of mangrove lands in Thailand are attributed to conversion to shrimp farms. A similar figure is given by Sahavacharin (1995) who states that 34% of mangrove areas in Thailand are cleared for shrimp farms. In some regions of Thailand, as much as 80% of shrimp farms are located in previously mangrove forested areas, for example, the provinces of Chantaburi and Trat, (NACA 1996). In Vietnam, of an estimated original 250,000 ha of mangrove in the Mekong delta, by 1988 there only remained an approximate 93,502 ha, and of the 156,498 ha lost, approximately 60,000 ha of these were cleared between 1985 and 1988 for shrimp farm development (Thoc 1995).
However, it is important to appreciate two important points. Firstly, that mangrove deforestation is also caused by conversion to agriculture, salt ponds, industrial uses, urbanisation, or mining activity and although substantial areas of mangrove are converted into shrimp ponds, shrimp farming is not always the main culprit. Secondly, shrimp ponds are also constructed in a wide variety of habitats and land-use types including marshlands, mangrove, rice paddies, coconut plantations, fish ponds, sugar cane fields, salt pans and salt flats, bare land, (Phillips et al. 1993) orchards, and degraded lands (Potaros 1995). The loss of these forms of productive land is also of concern and more research should be centred on investigating means of rehabilitating disused ponds located in all land-use types, not just those constructed in mangrove.
Causes of Disuse and Abandonment of Shrimp Ponds
It is thought that declining environmental suitability results in an increased incidence of disease and may ultimately lead to the failure of the shrimp crop (Chanratchakool et al. 1995) There appears to be a clear linkage between environmental conditions and disease, although the precise nature of the relationship is complex and is yet to be established (Chanratchakool et al. 1995). Disease has been widely cited as a cause of production failure, and the shrimp industry has seen the development of a variety of diseases which have spread from one shrimp culturing nation to another. The production collapse of 1988 in Taiwan was associated with widespread disease, although the disease problems were the result of a variety of pathogens including viral, bacterial and protozoan infections as opposed to the occurrence of one single disease (Lin 1989).
During 1989/1990 Monodon Baculovirus (MBV) was regarded as having been responsible for massive shrimp mortality in Thailand, (Briggs 1994). By 1992, MBV had been supplanted by the Yellow Head Baculovirus (YHDBV), which was estimated as being responsible for a loss of production amounting to £21 million in 1992 (Briggs 1994). However, by 1996 this too had been supplanted by the spread of white spot disease, which has been estimated as being responsible for losses of 50-80% production in Thailand, in 1996 alone, (S. Funge-Smith 1996 personal communication.) Currently it is estimated that in some areas of Thailand only 20% of farms are operational as a result of white spot disease, and that in some small producing areas 100% of ponds may be inoperative (S. Funge-Smith 1996 personal communication.)
Aside from disease, the development of acid sulphate soils has been cited as either a direct or indirect cause of production failure (Abbot 1994; Hamilton and Snedaker 1984; Jayasinghe 1995; Sammut and Mohan 1996, and Simpson and Pedini 1985). These soils exist as 'potential acid sulphate soils' (P.A.S.S.) in many mangrove areas, and as a result of the excavation and construction of shrimp ponds P.A.S.S. may become ‘actual acid sulphate soils’ (A.A.S.S.). Upon wetting, these soils release large quantities of acid into pond water and also toxic levels of iron and aluminium. Research in South East Asia reveals that acid, iron and aluminium are directly responsible for fish and prawn losses and general low productivity, (Simpson and Pedini 1985). Acid conditions (and poor water quality in general) may indirectly cause production failure by increasing physiological stresses, and decreasing the immune system response.
Although the technology exists to enable shrimp culture in acid sulphate soils (by the addition of lime and increased water exchanges), the technology is not always readily available. In India and Vietnam, for instance acid sulphate soils have been identified as the primary reason for crop failures. It is significant to note that during shrimp culture the effects can be ameliorated and managed by additions of lime. However, after disuse such efforts cease.
Ponds may also be disused for economic and political reasons as well as environmental deterioration. For instance, a small number of ponds are abandoned due to government intervention, such as by the revoking of shrimp farming licences (as occurred in Bali). It must be noted that it is being increasingly suggested that some farmers have had a deliberate policy of constructing farms, as cheaply and quickly as possible, with no intention of achieving sustainability, and with the explicit aim of abandoning the site after only a few crop cycles (M. Beveridge 1996 personal communication., and D.Brown 1996 personal communication.) Flaherty and Karnjanakesorn (1995) report that although industry estimates of pond viability have increased to between 7 and 15 years as a result of improved management practices, in practice shrimp producers often abandon ponds whenever yields or profits drop.
Environmental Conditions Following Abandonment.
The environmental conditions remaining after abandonment may be more important than the causes of failure, and may be unrelated to the cause(s) of failure. For instance in Karnataka, India many hundreds of ponds are said to be disused as a result of white spot disease (J. Sammut personal communication. 1996), however, the major obstacle to the redevelopment of these ponds is not the prevalence of disease, but the remediation of acid sulphate soils which may persist for many years after abandonment.
The ecological effects of acid sulphate soils have only recently begun to be clearly identified. The acidic water which results from acid sulphate soils destroys food resources, displaces biota, releases toxic levels of aluminium, precipitates iron which smothers vegetation and microhabitat and alters the physical and chemical properties of the water (Sammut et al. in press). Persistent alterations to pH have been shown to lead to changes in the flora and fauna in an area, by favouring acid tolerant plants and animals. This could have implications for attempts to restore the mangrove ecosystem and its resident flora and fauna. Insufficient research has been carried out to assess the number of shrimp ponds which are located in A.S.S., however, the figure may be significant, for instance in the provinces of Chantaburi and Trat in Thailand, 49% of shrimp farms are reported to be located on A.S.S. (NACA 1996). The presence of A.S.S. may pose a problem if ponds become disused and unmanaged in the future.
In addition to the activation of acid sulphate soils a further consideration, unrelated to the causes of failure, are the alterations to soils which are likely to occur as a result of the clearance of mangrove, shrimp cultivation and abandonment. Ignoring the changes from shrimp culture which are likely to vary with farming method and culture intensity, the effects of clearance and abandonment may include accelerated soil erosion due to increased surface run off and subsurface flow; decrease in soil water storage capacity; reduction in biodiversity of soil fauna; transport of sediments, dissolved inorganic and organic constituents and principal nutrients; and depletion of soil organic matter through leaching and mineralisation.
As neatly summarised by Flaherty and Karnjanakesorn (1995) ìthe abandoning of aquaculture developments represents a significant challenge to the productive use of coastal areas in the future because of the limited land use prospects for vast areas of former rice fields and mangrove forest. The rehabilitation of these areas is complicated by the fact that many of the environmental conditions that once fostered the growth of mangrove forests have been removed or severely altered.î When considering options for redeveloping disused ponds, it is important that the environmental parameters remaining in a pond are identified. To date little work has been conducted to elucidate the conditions found in disused ponds, or to identify what implications these may have for future uses. Disused ponds are likely to be unstable and actively deteriorating and may represent a risk to neighbouring habitats, and unless managed may become progressively more difficult to rehabilitate or restore. Means of assessing such risks should be developed.
Options for Rehabilitation and Restoration
However, despite these potential difficulties there are examples of conversions to other uses, some of which may be regarded as successful. In Samut Sakhon large tracts of abandoned shrimp ponds are being converted to non-agricultural land use, such as for housing estate and industrial developments. This is supported by Buntoon Srethasirote (1996 personal communication.) who states that in Samut Sakhon, shrimp farms "are left idle for opportunities such as factory or housing development, and some abandoned shrimp farms have been converted to salt farms or fish culture operations and shrimp farms located near main roads have sold top soil for construction projects."
Table 1. Use of disused shrimp
ponds in Samut Sakhon, Thailand
|converted to traditional system shrimp culture||
|converted to salt farming||
|converted to coconut plantations||
|top soil sold for construction purposes||
In Samut Sakhon of the 22,220 rai (3,555 ha) of abandoned farm area, 33% (see table 1) of this area was converted to traditional low input, low output (extensive) system shrimp culture, 20% converted to salt farming, 7% converted to coconut plantations, 7% sold top soil for construction purposes and 6% of the area was idle. (OEPP 1994). It is important to examine whether conversions are sustainable in themselves, and some means of evaluation should be developed to facilitate identification of 'best use' scenarios for disused ponds.
Other possibilities for rehabilitation include aquasilviculture (Baconguis 1991; Primavera and Agbayani 1996), which appears to have significant potential. "Aquasilviculture is a multiple use system that promotes a harmonious co-existence between fishery species and mangrove tree species in a semi-enclosed system while providing coastal protection and maintenance to the ecosystem" (Baconguis 1991). It is suggested that the technology could be used to facilitate the reversion of abandoned/unproductive or denuded open areas to their productive condition (Baconguis 1991). In the Philippines disused fish ponds, particularly those which have been left idle due to A.S.S., are suggested as being suitable for the development of aquasilviculture (Baconguis 1991).
The concept of aquasilviculture is that mangrove is planted in a shallow central area covering 70-80% of the selected site, and the remaining 20-30% is used to create a deeper pond along the dikes, and is devoted to brackishwater aquaculture (fish, shrimp or crab). The expected forestry/mangrove development and the expected fisheries production over time are outlined in Baconguis (1991) such that the first 5 years rely on fish production to repay construction costs and main activities undertaken are the maintenance of dikes, fertilisation for fish production, protection of plants from pests and diseases, control of predators and replanting. During the next 5 -10 years pruning and thinning of the mangrove can be conducted to provide a harvestable crop for firewood, low-cost housing materials, and cattle fodder. Pruning is also required to enable sunlight to reach the ponds since this is necessary for fish production (Baconguis 1991).
One of the advantages of using aquasilviculture for this purpose is that it involves the flooding of the pond. If the pond contains P.A.S.S. flooding will prevent further creation of A.A.S.S., and will reduce the potential for further environmental decline. While not experimentally tested, it is likely however, that ponds with A.A.S.S. will have to undergo some form of remediation activity to remove the acid and toxic leachates prior to planting. In addition it is suggested that these areas should be used to provide a source of propagules for further replantation schemes in the vicinity, thereby reducing replantation costs by providing a local supply of propagules, and minimising transport costs.
There has also been some success in the Philippines with using disused shrimp ponds for Grouper spat culture (R. Johannes personal communication. 1996), and there have been some suggestions of using disused shrimp ponds for polychaete culture (P. Cadman, S. Rees, and V.D. Quynh personal communication. 1996). Other suggestions have included integrated salt-artemia production or shrimp - salt production systems (Rasowo 1992). Integrated shrimp culture of some sort may be the most attractive option to those farmers who have ceased shrimp production but still own or have the lease rights to the ponds. An important consideration for the rehabilitation of disused shrimp ponds must be the intentions and motivations of the farmers themselves, who in most cases will be keen to take up only those activities which are going to result in, at least, mid-term financial gain, so that outstanding loans or other financial obligations can be paid off. This is exemplified by the results obtained from a survey conducted in Thailand (see table 2) which showed that the primary aim of farmers in possession of unproductive ponds is to recommence shrimp culture, and who are no doubt motivated by the perceived economic benefits of shrimp farming.
An interesting venture that may still be an attractive option to shrimp farmers is crab culture or renting the land to other culturists, thereby avoiding the risk of monetary loss should the culture fail. Buntoon Srethasirote (1996 personal communication.) reports that he has visited a crab culture farm in Samut Sakhon that had once been a disused shrimp farm. The land owner rented land to crab culturists at a price of 300,000 Baht/Yr./100 rai.(£443/yr/ha) It seems that the crab were imported from Bangladesh, and grown in a pond for a period of only 15 days, and were then exported to Taiwan. In the opinion of Buntoon Srethasirote the business was quite successful.
Table 2: Intentions of farmers in possession of unproductive ponds.
|Return to shrimp culture||
|Culture fish (Seabass)||
|Make salt farm||
|Leave the land||
|Other (Agriculture land, housing and land for rent)||
Constraints on Sustainable Rehabilitation and Restoration
The remediation of acid sulphate soils presents many problems, not the least of which are the time required and the costs involved. One treatment method involves the deliberate oxidation of the acid producing material (pyrite) and the washing out of the acid by means of brackish water or fresh water flushing. Simpson and Pedini (1985) calculated that 150 flushes would be required to flush out acid and potential acid from a culture pond, however, enhanced removal rates have been achieved by drying and tilling the pond bottom in between flushes. Whilst this method solves the problem of soil acidity, it does not take into account the ecological effects of the acid leachate, and in areas with poor tidal flushing the use of this method would be inadvisable.
In addition to the activation of acid sulphate soils, a further consideration, is the alteration to soils which is likely to occur as a result of the clearance of mangrove, shrimp cultivation and abandonment. Changes from shrimp culture are likely to vary with farming method and culture intensity, however, the effects of clearance and abandonment may include accelerated soil erosion due to increased surface run-off and subsoil surface flow; decrease in soil water storage capacity; reduction in biodiversity of soil fauna; transport of sediments, dissolved inorganic and organic constituents and principal nutrients; and depletion of soil organic matter through leaching and mineralisation.
Alterations such as these have significant implications for the potential future uses and for restoration efforts such as replanting schemes, which have had variable success in areas degraded by other activities. The likelihood of replanting success in disused shrimp ponds may be severely limited by the reduction in the soil quality as a result of increased erosion, the activation of acid sulphate soils, and the addition of chemicals. This potential problem was noted as long ago as 1982 when Ong (1982) stated that it is not known if mangroves will recolonise ponds abandoned as a result of A.S.S.. The answers unfortunately, are no more clear today. To date, variable success rates (from very high to very low) have been achieved with different species in small scale experiments to plant mangroves in disused shrimp ponds and in shrimp sludge cake (MacIntosh1996a, and 1996b and Tantipukanond 1993). Unfortunately replanting success is rarely evaluated.
Furthermore, many other constraints to redevelopment exist in addition to the biophysical constraints mentioned previously. These includes a general lack of technical expertise in restoring habitats, and an even greater lack of experience of restoring disused shrimp ponds. The preferences of the pond operators themselves must be taken into account, market forces must be considered when identifying potential alternative uses, and economic feasibility must be assessed. Finally, there is the considerable question of who bears the costs of redevelopment? If the decision is taken to restore disused shrimp ponds to a productive managed mangrove ecosystem, then the costs of such activity are likely to be very high, particularly if propagules are in short supply and if it is deemed necessary to create a nursery in which to grow seedlings.
Tri et al. (1996) quantified the economic benefits of mangrove rehabilitation (taking the costs of rehabilitation into account), and found that mangrove rehabilitation can be desirable from an economic perspective based entirely on the direct use benefits by local communities, and that when indirect use benefits are incorporated into the analysis, the benefits of rehabilitation far out weigh the costs. It may be possible to create financial disincentives to habitat destruction, such that the costs of destroying or denuding a habitat might be raised to a level that matches or exceeds the cost of keeping or replacing it. However, this would not speed the redevelopment of ponds which have been left idle historically, and consequently other means of catalysing and funding rehabilitation or restoration activities must be found.
Given the importance of mangrove, there is a strong case for either restoration of disused shrimp ponds or the rehabilitation of them to alternative uses. The rehabilitation of ponds back to an economically productive, but sustainable use may serve to reduce development pressure on neighbouring mangrove areas. For this to be possible, the views and preferences of the pond owners must be considered, and options which meet both the management objectives of coastal resource mangers and those of the pond owners should be pursued. When considering restoration, it is important to assess the potential for restoration, and this requires that the environmental problems associated with shrimp culture, the causes of failure and the resulting conditions in the disused shrimp ponds must be taken into account. More research should be conducted into quantifying the environmental conditions which remain in ponds after disuse, and more effort should be made into identifying and testing measures which may address these conditions.
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