The Potential of Mangroves in the Treatment of Shrimp Aquaculture Effluent on the Eastern Coast of Thailand
7
Nina Fancy
B. Sc. (Horn), Queen's University, 1999 A Thesis Submitted in Partial Fulfillment of the
Requirements for the Degree of MASTER OF SCIENCE in the Department of Geography
O
Nina Fancy, 2004 University of VictoriaAII rights reserved. This thesis may not be reproduced in whole or in part, by photocopy or other means, without the permission of the author.
Supervisor: Dr. Mark Flaherty
ABSTRACT
This thesis examines the potential of low-cost, low-maintenance mangroves in the treatment of nutrient-rich effluent originating from a shnmp f m on the coast of Thailand's Chanthaburi province. The objective of this thesis is to identify the environmental impact of shnmp aquaculture effluent and to determine if mangrove wetlands can be used as effective biofiltration areas to remove significant quantities of nitrate, ammonia and nitrite from shrimp wastewater. The study mangrove was found to remove an average of 44.5% of nitrate, 46.6% of ammonia and 59.0% of nitrite from shrimp effluent. The ratio of mangrove treatment area to shrimp f m required to adequately treat daily effluxes of wastewater from shrimp f m s was calculated to
.
be 1 : 14. This ratio is significantly less spatially demanding than ratios calculated by.-:
P 0previous researchers and reveals the potential of mangroves to be used as large-scale wastewater treatment areas in shrimp-producing nations.
TABLE OF CONTENTS
..
ABSTRACT...
11...
TABLE OF CONTENTS...
UI...
LIST OF TABLES v i.
.
LIST OF FIGURES...
WI...
ACKNOWLEDGEMENTS...
WIN CHAPTER 1...
1 INTRODUCTION...
11.1 NATURE OF THE PROBLEM ... 1
... 1.2 PURPOSE OF THE STUDY 4 ... 1.3 THESIS OUTLINE - 5 CHAPTER 2
...
7...
BACKGROUND 7...
2.1 GLOBAL AQUACULTURE TRENDS 7 2.2.1 Geographical ShiJi ... 13 2.2.2System%iJi
... 16...
2.3 MANAGEMENT PRACTICES 17...
2.3.1 Site Selection 17 2.3.2 Pond Preparation and Stocking ... 192.3.3 Feeding
...
202.3.4 Aeration and Water Exchange ... 22
2.3.5 Harvesting ... 23 ... 2.4 ENVIRONMENTAL IMPACTS 23 2.4.1 Land Requirements ... 23 2.4.2 Water Supplies ... 26 2.4.3 Chemical Discharge ... 26 2.4.4 Organic Outputs
...
272.5 BEST MANAGEMENT PRACTICES
...
302.5.1 Planning and Management ... 31
2.5.2 Physical Techniques
...
3 1 2.5.3 Feed Related Practices ... 33...
2.5.4 Policy Options 34 2.5.5 Biological Practices ... 35 2.6 SUMMARY ... 38 CHAPTER 3...
40...
MANGROVE ECOLOGY..
...
40 ... 3.1 DEFINITION 40 3.2 GEOGRAPHICAL DISTRIBUTION ... 41 ... 3.3 BIOL~GY OF MANGROVES 42 3.3.1 Anatomy ... 42 3.3.2 Environmental Adaptations...
44 3.3.3 Nutrient Dynamics ... 473.3.4 Associated Hora and Fauna
...
493.3.5 Natural and Anthropogenic Impacts ... 52
3.4 USES OF MANGROVES ... 54 ... 3.4.1 Local Subsistence 54 3.4.2 Coastal Function
...
55 3.4.3 Tourism ... - 5 6 3.4.4 Wastewater Filtration...
563.5 CONSTRUCTED AND MODIFIED MANGROVES ... 60
3.5.1 Design ... 6 0 3.5.2 Can Mangroves be Constructed? A Question of Environmental Feasibility ... 61
3.5.3 Economic Feasibility
...
643.6 SvMMARY ... 66
CHAP'lXR 4
...
67STUDY AREA
AND
METHODOLOGY...
674.1 REGIONAL CONTEXT ... 67
4.2 FARM AREA ... -70
4.3 MANGROVE DESCRIPTION ... 74
4.4 EXPERTMENTAL DESIGN
...
764.5 DATA COLLECTION OVERVIEW
...
774.5.1 Pre-TreatPnent Samples: Mondays ... 77
4.5.2 Post-Treatment Samples: Wednesdays ... 78
4.6 INSTRUMENTATION AND LABORATORY ANALYSES ... 79
4.7 LIMITATIONS ... 80
CHAPTER 5
...
82RESULTS
AND
DISCUSSION...
825.1 SAMPLE I D ~ C A T I O N
...
825.2 PH AND RAWFALLDATA ... 83
5.3 BOD ANALYSIS ... 86
...
5.4 PJUSE ANALYSIS 89 5.5PRE-TREATMENT
DATA: EFFLUENT SAMPLES...
-925.6
PRE-TREATMENT
DATA: MANGROVE ... 975.7 POST-TREATMENT DATA
...
985.8 COMPARISON OF PRE-TREATMENT AND POST-~XI~ATMENT NITROGEN DATA
..
1035.8.1 Nitrate ... 104
5.8.2 Ammonia
...
I07 5.8.3 Nitrite...
1 0 8 5.9PERCENT
REMOVAL OF NUTRIENTS ... 1095.10 NITROGEN LOADING OF THE COAST OF CHANTHABURI ... 111
5.1 1 L ~ A T I O N S AND
FUTURE
RESEARCH NEEDS ... 1135.12 MANAGEMENT IMPLICATIONS ... 116
5.12.1. Mangrove to Shrimp Pond Ratio ... 116
5.12.2 Recirculation
...
I21 5.12.3 Global Impacts ... 1225.13 SVMMARY ... 123
CONCLUSION
...
125 ...6.1
SUMMARY
125...
6.2 MAJOR RESEARCH FINDINGS 126
...
6.3
FUTURE
RESEARCH DIRECTIONS 127LIST OF TABLES
Table 2.1 : Comparison of extensive, semi-intensive and intensive shrimp farms in
... ...
Thailand.. , .18
Table 2.2: Common chemical and biological compounds used in intensive shrimp ...
culture in Thailand. .2 1
Table 2.3: Optimum water quality parameters for culture of
P.
monodon.....
.24 Table 2.4: Concentrations of key nutrients in shrimp farm effluent from an...
intensively managed shrimp pond in Thailand.. .29
Table 2.5: Regulations for shrimp farming issued by the Thai government,
...
November 1991.. -36
Table 3.1 : Examples of mangrove loss in Asia and Oceania..
...
.43 Table 4.1 : Land use practices along the coast of Chanthaburi in 1991.. ... .70 Table 5.1: Mann-Whitney tests to detect differences between nitrate, ammonia...
and nitrite concentrations in the 2 distinct phases 91 Table 5.2: Pretreatment mean concentrations of nitrate, ammonia and nitrite in
...
effluent. -93
Table 5.3: Mean concentrations of nitrate, ammonia and nitrite during weeks ...
1-4 and weeks 5-8.. ..93
Table 5.4: Mann-Whitney test to detect if differences in nitrate, nitrite and
ammonia concentrations exist between weeks 1-4 and 5-8..
...
.93 Table 5.5: Comparison of mean nitrate, ammonia and nitrite concentrations...
in eMuent in this study (averaged over 8 weeks). .96 Table 5.6: Post-treatment mean concentrations of nitrate, ammonia and nitrite
...
in the mangrove.. -100
Table 5.7: Concentration of nitrate, ammonia and nitrite in well samples.. ... .lo0 Table 5.8: Comparison of pre-treatment values of nitrate, ammonia and nitrite
compared to the upper bound of the 95% confidence interval of
...
vii
LIST OF FIGURES
... Figure 2.1 : Aquaculture production of shrimp (by weight) in 1 998 10
...
Figure 2.2. Aquaculture production (by weight) of P.monodon in Thailand 15 ... Figure 3.1. Photo of Avicennia marina pneumatophores 46 Figure 3.2. Simplified nitrogen cycle in mangrove ecosystems
...
.SO... Figure 4.1 : Map of Chanthaburi with approximate location of the study site 68 Figure 4.2. Average monthly temperature in Chanthaburi, Thailand ... 69 ... Figure 4.3. Average monthly rainfall in Chanthaburi, Thailand 69 Figure 4.4. Layout of the shrimp farm and mangrove study site ... 72 Figure 4.5: Magnified view of the modified mangrove area with approximate
location of the submersible pump ... 75 Figure 5.1. Field schedule with sample week and day identification ... 84 Figure 5.2. pH values for pre and post-treatment samples ... 85
... Figure 5.3. BOD2 data for pre-treatment effluent samples 87 Figure 5.4: Residual Dissolved Oxygen concentration after 2-day incubation
...
period 87
Figure 5.5. Average nitrate concentrations for effluent and mangrove samples ... 99 Figure 5.6. Average ammonia concentrations for effluent and mangrove samples .... 99 Figure 5.7. Average nitrite concentrations for effluent and mangrove samples ... 99 Figure 5.8: Concentration values for all pre-treatment samples
-
plotted in...
ascending order -105
Figure 5.9: Percent of nitrate, ammonia and nitrite removed from effluent by the ...
...
V l l l
ACKNOWLEDGEMENTS
There are a number of people I would like to thank for making this research possible. Thank you to CIDA for providing the funding for the field research portion of this thesis. My warmest thanks also goes to my supervisor, Dr. Mark Flaherty whose support throughout this degree has been invaluable. To my committee, your knowledge and insight enabled me to delve further into my results to uncover the true value of my research.
There are a few people without whom both my sanity and smile would have been lost long ago. Michey, your endless laughter, your overwhelming understanding of all my quirks, your cleverness and your adoption into the Fancy Family has made you the better halfmanx I never thought I'd find. Without you Operation Man-Grove and Operation My-Probe would have been a disastra rather than a pleasure. I can't thank you enough for getting me through this! Krissy - you always know how to make the whole world seem right again, even when I don't know whaaahappened and the walskis crumble, thanks sweets! Curls, ahhh, sweet Curls, the autumn leaves blow in
the wind and I wonder what we all would have done without the brains of the operation! Billy, my friend through thick and thm, armed with bevvies and a huge smile at every turn of life - cheers to the next chapter. And to J-cks, my muse for everything un-academic and my reminder that magic truly exists. To my family - no words exist for the thanks I'd like to express. You have all been my rock when I wavered in the wind, you've shown me the paths and walked beside me as I choose my way, even when I make wrong turns. I thank you not just for your support
throughout this degree, but for your unfaltering knowledge, wisdom and love. Thanks for always believing in me and making me realize I can move mountains - I just have to put a little heart into it!
CHAPTER 1
INTRODUCTION
1.1 Nature of the Problem
Aquaculture has traditionally been viewed as a low-impact technique for rearing aquatic species with little alteration to their natural environments (Csavas 1993). The ever-increasing demand for aquatic protein on international markets, coupled with declining global finfish and shellfish stocks, however, has transformed aquaculture from a low-intensity farming practice, capable of sustaining small rural communities, into a thriving industry contributing over 27% to world fisheries production ( FA0 2002). The economic success of the recent boom in aquaculture has brought much needed foreign exchange and employment opportunities into previously impoverished areas (Flaherty and Kamajanakesom 1995). Unfortunately, as with many industries that expand at such a high rate, this so-called "Blue Revolution" does not come without some negative political, social and, of most concern, environmental repercussions (Moss et al. 2001).
Yields obtained from low-impact, extensive practices are minimal and farm expansion is neither spatially nor financially feasible (Hopkins et al. 1995b, Nunes and Parsons 1999). In order to increase production, given land scarcity and monetary constraints, intensive high-input farming techniques were developed. However, the degree of nahual system manipulation necessary for intensively rearing aquatic species renders this farming technique unsuitable for many animals sensitive to environmental fluctuations (Folke and Kautsky 1992, Boyd 2001). Of the aquatic
species harvested for human consumption, shrimp appear to be the most resilient to variations in environmental conditions and are also a highly profitable commodity,
making them one of the most commonly intensively farmed aquatic animals (FA0 2002).
The widespread adoption of intensive shrimp farming has provided financial benefits to numerous communities. However, when environmental degradation is considered, the true costs of such high input practices are generally much greater than their monetary benefits (Folke and Kautsky 1992, Naylor et aZ. 2000). The increased popularity of intensive shrimp aquaculture, along with a lack of environmental regulations in most of the producer nations, allowed for the establishment of high input f m s adjacent to sensitive tropical coastlines and estuarine waters (De Silva 1998). Although many of the effects of intensive practices were initially unknown, the environmental impacts of shrimp farming have since been extensively researched and include mangrove destruction, the introduction of exotic species and land
salinisation as common side-effects of these techniques (Pbz-Osuna 2000, Dierberg and Kiattisimkul 1996). These impacts all degrade the ecological integrity of
surrounding ecosystems to varying degrees. The most serious of these result from the large effluxes of untreated nutrient-rich waters released into neighbouring waters (Dierberg and Kiattisimkul 1996).
Shrimp farm effluent contains high concentrations of organic compounds
containing nitrogen and phosphorus (Funge-Smith and Briggs 1998). The continuous draining of such eutrophic waste into surrounding irrigation canals and coastal waters is extremely handid to the health of proximal ecosystems. Eutrophication of
receiving waters is of primary concern where shrimp farms are densely situated along coastlines that serve as nursery and breeding grounds for numerous marine species. This is most prevalent in Thailand, the largest producer of farm-reared black tiger shrimp (Penaeus monodon) in the world (Kongkeo 1994, FA0 2002).
The environmental effects of the unmonitored disposal of organically rich effluent into coastal regions of Thailand has recently received widespread attention (Funge- Smith and Briggs 1998). The excessive discharge of polluting effluent onto
Thailand's coastlines has resulted in toxic algal blooms and anoxic waters unable to sustain healthy species assemblages leading to a detrimental decrease in floral and faunal diversity (Paez-Osuna 2001).
The large-scale impacts of coastal eutrophication are predicted to reach all levels of the food chain thereby disrupting the dynamics of many aquatic and terrestrial ecosystems (Paez-Osuna 2001). The gravity of the situation has resulted in the creation of environmental regulations aimed at increasing the quality of aquaculture effluent as well as financial penalties for farmers not adhering to healthy effluent standards (Kongkeo 1997). Unfortunately, these attempts to improve effluent quantity and quality have been unsuccessll thereby revealing the need for research focusing on biological treatment of shrimp f m wastewater.
Studies have been conducted exploring the potential use of bivalves, seaweed and other aquatic flora and fauna in reducing the harmful impacts of shrimp effluent (e.g. Macintosh and Phillips 1992b; Jones et al. 2001). Although promising results have been obtained from such research, these biological treatments are often seen as practical solutions only for small subsistence aquaculture farms. Large-scale
operations require extremely efficient and rapid nutrient cycling areas such as those of wetlands. Until recently, little attention was given to wetland wastewater treatment (Redding et al. 1997). However, the employment of constructed wetland systems for the treatment of eutrophic waste has received tremendous support owing to the high nutrient assimilative capacities of many wetlands as well as the minimal financial and labour inputs required to maintain these areas (Schwartz and Boyd 1995, Wong et al.
1997). The wastewater treatment capacities of natural and constructed freshwater wetlands have been examined with respect to industrial and agricultural wastes as well as domestic sewage, but little research has been conducted on the potential of
mangroves in the treatment of saline and brackish aquaculture effluent (Schwartz and Boyd 1995).
Mangroves are renowned for their efficient nutrient cycling dynamics and have the ability to transform leaf-litter and other organic matter influxes into biologically available forms imperative for the growth and survival of a myriad of aquatic and intertidal species (Ellison 2002, Jennerjahn and Ittekkot 2002). Pilot studies examining the potential of mangroves in the treatment of eutrophic effluent have produced encouraging results, but further research is required to determine the specific benefits of mangrove biofiltration (Rivera-Monroy et al. 1999, Tilley et al. 2002).
Low-impact, inexpensive treatment of aquaculture effluent is of vital concern as more nations throughout the world adopt intensive shrimp aquaculture practices and attempt to reap their financial benefits. Environmentally sustainable effluent treatment systems may be the only solution to ensuring a healthy future for many of the world's coastal regions.
1.2 Purpose of the Study
The purpose of this thesis is to investigate the efficiency of modified mangroves in the treatment of aquaculture effluent. This is accomplished through an experiment in which effluent concentrations of ammonia, nitrate and nitrite are measured before and after exposure to a modified mangrove. The specific objectives of this study are:
To review recent trends in shrimp aquaculture so as to identifj the main environmental impacts of farming practices, and to document the need for effluent treatments
To investigate the potential of natural, constructed and modified mangroves as effluent filtration areas, with focus on their nutrient assimilative capacities To determine if one modified mangrove can effectively reduce BOD, ammonia, nitrate and nitrite concentrations in effluent originating from one shrimp farm off the coast of Chanthaburi province in Thailand
To quantify the amount protein one modified mangrove can process into ammonia over a 48-hour period
To quantify the ratio of mangrove to shrimp pond required to effectively treat effluent
To determine the average nitrogen loading along the coast of Chanthaburi originating from shrimp farm effluent and the number of hectares of mangrove required to treat efnuent in this region
1.3 Thesis Outline
The thesis has seven chapters. Chapter 2 provides background information on shrimp aquaculture outlining fanning practices in Thailand and their negative environmental impacts. A review of best management practices that have attempted to reduce the harmful effects of shrimp aquaculture is presented and the need for more sustainable effluent treatment is identified. Chapter 3 provides a synopsis of general mangrove ecology and investigates the potential of mangroves as wastewater filters. The economic and environmental costs and benefits of natural and constructed
effluent treatment areas. Chapter 4 gives a description of the study site and the procedures followed throughout the duration of the field component. Methods of data analysis and limitations of the research are also outlined. The results of the
experiment are presented in Chapter 5 with focus on ammonia, nitrate and nitrite. The local and regional implications of the findings are discussed in Chapter 6. Chapter 7 summarises important conclusions, addresses issues of feasibility and identifies future
CHAPTER 2
BACKGROUND
This chapter provides a broad overview of global aquaculture trends and then focuses on the development of shrimp farming in Thailand. The management practices of intensive shrimp farming are discussed along with the environmental impacts of such techniques and current attempts at minimising these eff'ects. The need for hrther research exploring environmentally friendly solutions is expressed and constructed mangroves are introduced as one possible option.
2.1 Global Aquaculture Trends
Throughout history rural peoples have relied on aquatic species for both their nutrition and livelihoods, depending on wild catch and extensive aquaculture practices to meet their needs. Today, the FA0 (2000) estimates that aquatic species supply over one billion people with their primary source of protein, providing 16% of the animal protein consumed globally. In addition to their nutritional value, fish and aquatic invertebrates are also economically and socially important, trading at levels of approximately US $50 billion per year and providing income to over 230 million people (FA0 2000). Although most of the benefits from aquatic food sources are gained from wild capture marine fisheries, the demand for aquatic protein has far exceeded the sustainable yields available from natural waters. Exploitative harvesting has led to a severe depletion of wild stocks and has created a niche for aquaculture to supply the ever-growing need for aquatic species (Phillips and Macintosh 1996). Aquaculture is differentiated from wild catch fisheries in that stocks are owned and the production cycle of the species of interest is intentionally altered or controlled
(Meade 1989). The movement towards the rearing of aquatic species is aimed at raising nutritional standards, providing employment and income and improving food security, especially amongst developing nations (FA0 2002).
The expansion of aquaculture has proceeded at the highest rate of any food- producing industry in the world. Between 1987 and 1997 the weight and value of farmed species supplied to the global market doubled (FA0 2000). Worldwide, more than 220 species of finfish and shellfish are farmed in marine, brackish and fresh waters with Asia contributing approximately 90% to total global aquaculture production (Naylor et al. 2000). Most aquatic products are used for human consumption, but it should be recognised that a small portion is harvested for aquariums and pharmaceutical purposes (Bezard and Maigret 1990).
AquacuIture has proven beneficial on both national and regional scales, providing a significant source of foreign exchange and bringing wealth into areas previously experiencing extreme poverty (Funge-Smith and Briggs, 1998). Although the Einancial successes of aquaculture endeavours have been outstanding, reaching levels of over US $50 billion in 1997, as is the case with many industries that intensify rapidly, the boom has not arisen without its share of negative impacts, both environmental and social (FA0 1999; Folke and Kautsky 1992).
Traditional aquaculture activities depend on natural processes to rear a number of finfish and shellfish in so called extensive systems. Tidal action and naturally
occurring phytoplankton blooms provide the necessary water exchange and feed required to cultivate aquaculture crops (Csavas 1993). With the rising demand for f m e d species, techniques to increase productivity were developed. These primarily involved larger inputs of feed, fertiliser, chemicals and skilled labour as well as increased frequencies of water exchange (Hopkins et al. 1995b). The intensification
of farming practices has brought with it much wealth, but the rapid shrift from minimal to high input processes has resulted in degradation of coastal and public water supplies, destruction of marine habitats (specifically mangrove forests), salinisation of soils on neighbouring lands and spread of bacterial and viral diseases (Naylor et al. 2000; Bevendge et al. 1994; Boyd 2001). These impacts not only cause significant social strife among rural peoples, but also negatively affect the aquaculture industry itself (Macintosh and Phillips 1992a).
The most controversial of all the marine aquaculture industries is that of shrimp farming. The demand for shrimp on the international market has skyrocketed since the late 1970's. h l 9 8 O the annual global production of all farmed species of shrimp was estimated at 200 000 T. This number rose to 744 000 T by 1993 and by 1998 was calculated at 840 200 T (Rosenbeny 1998 in Paez-Osuna 2001; Briggs and Funge- Smith 1994). Using this data Rosenbeny (1 998 in Paez-Osuna 2001) projects that the global yield of fanned shrimp will reach approximately 2.1 million T by 2005. The estimated maximum sustainable yield of wild catches of shrimp is 1.6 - 2.2 million T revealing that by 2005, the shrimp aquaculture industry may be producing more shrimp than available from wild catch (Paez-Osuna 2001). Intensive crustacean culture, specifically of the black tiger prawn, Penaeus monodon, is extremely
prevalent in Southeast Asia where over 50% of the world's production, by weight, are produced. Since 1994, Thailand has been the world's largest producer of cultured shrimp (Figure 2.1).
Thailand Indonesia Philipppines Malaysia Countries
Figure 2.1 : Aquaculture production of shrimp (by weight) in 1998 (Rosenbeny 1998 in Paez-Osuna 2001)
2.2 Shrimv A~uaculture in Thailand
Prior to 1985 Thailand had a small niche in the international shrimp market. By the mid-19807s, however, Thailand had replaced Taiwan as the largest exporter of shrimp in the world (FA0 1999). Numerous natural and economic factors made Taiwanese farmers unable to produce more than one shrimp crop a year. Taiwan had long supplied to Japan, the largest consumer of shrimp, but with rising electricity costs, Japan was no long able to feasibly cold store Taiwanese shrimp to ensure year round availability (Kongkeo 1994). In addition, Taiwanese crops suffered high mortality and losses in 1987 due to bacterial and viral shrimp pathogens causing their billion dollar industry to almost come to a halt (Csavas 1993; Kongkeo 1997). As a result, Japan started to encourage suppliers from other countries to boost production, forcing the price of shrimp to rise from US $2.50 to US $8-10 per kg (Kongkeo 1994). Many Southeast Asian nations, specifically Indonesia, Taiwan, Vietnam and the Philippines, were potential rivals for Thailand. However, Thailand's political, economic, social and environmental climates were ideally suited for the development of shrimp aquaculture. Recognising the potentially enormous economic benefits of leading the world's shrimp exports, the Thai government offered loans and grants to provide capital for new aquaculture establishments, in addition to encouraging large businesses to invest in upcoming shrimp farms (Kongkeo 1994). Thailand soon became the largest exporter of farmed shrimp contributing over 30% to global farmed shrimp production in 1998 (FA0 1999).
Among the factors leading to Thailand's dominance in the global shrimp market are the climate, geology and biology of the country's coastlines as well as the
availability of materials and technical expertise in the field of aquaculture. Typhoons and cyclones are extremely rare along the coast and the water temperature is non-
fluctuating making the region ideal for shrimp farming (Kongkeo 1994). The high clay content of many soils allows for minimal seepage and is therefore a suitable material for grow-out ponds (Hambrey and Lin 1998). With respect to infrastructure, Thailand is more advanced that its neighbouring competitors. Electricity is widely available and comparatively reliable (Kongkeo 1994). This is important for daily farm operations such as aeration, pumps and lighting and is imperative for freezing and packaging procedures (Tookwinas 1994). Telephones are also common in most areas allowing for rapid communication and consultation should a problem arise during a culture cycle. Equally important is Thailand's extensive network of paved roads (Kongkeo 1994). This facilitates the transport of materials (e.g. construction goods, feed, chemicals) as well as providing easy access for trucks transporting shrimp to packaging and freezing plants.
Shrimp farms can not survive as isolated entities and therefore rely on numerous support industries for their success. Prior to the shrimp "gold rush" in the 19807s, Thailand was already producing and exporting fish and poultry products (Csavas
1993). When the demand for shrimp increased, Thai farmers were able to obtain formulated feed from previously established chicken feed suppliers. This not only provided low cost feed but reduced the risk of toxicity associated with imported feed which may have expired or been stored incorrectly in humid conditions (Kongkeo 1994). Preexisting fish processing plants were also quickly and cheaply modified to package and freeze shrimp. In addition, equipment, such as pumps, tanks and heavy machinery required for pond construction, was readily available (Harnbrey and Lin
1998). Compared to developed nations, skilled labour in Thailand is relatively cheap allowing for profitable export of a variety of shrimp: head-o$ head-on, peeled and specialty (Kongkeo 1994).
Despite all these conditions conducive to success in the shnmp aquaculture industry, Thai farmers experienced mass shrimp mortalities resulting fiom poor site selection. This resulted in frequent geographic shifts of shrimp farming areas. Large-
scale crop losses were most often attributed to farm establishment on acidic coastal soils and self-pollution in areas densely populated by shrimp farms.
2.2.1 Geoaaphical Shift
Extensive shrimp aquaculture has been practiced on Thailand's coasts for centuries. The sheltered calm seas along the 2700 lcm of coastline provide an ideal setting for aquaculture operations as does the abundance of natural seed in these temperate waters (Tookwinas 1996). Historically, Penaeus merguensis were harvested in the dry season and Metapenaeus species were farmed in the wet season (Tookwinas 1994). This was achieved through the construction of dykes on the perimeter of rice fields allowing wild shrimp to enter though sluice gates that
subsequently retained the animals until they reached maturity (Tookwinas 1994). The surge in popularity of shrimp products awakened the need for techniques that would increase production yields. As a result, supplementary feeds were introduced in the 1970's while the 1980's gave rise to technological advancements enabling intensive systems to come into fruition. The most significant of these new technologies was the Department of Fisheries' (DOF) success in hatchery producing Penaeus monodon or black tiger shrimp at commercially demanded quantities (NACA 1996). This species of shrimp is the most widely cultured owing to its rapid growth rate and high tolerance to temperature and salinity fluxes (Chanratchakool et al. 1995).
Intensive shrimp fanning began in the upper Gulf of Thailand replacing salt pans and extensive aquaculture sites (Flaherty and Vandergeest 1998). Low yields were
frequently observed in the upper Gulf as these cultivation activities require large amounts of high quality water, which is rarely available in these areas
(Chanratchakool et al. 1995). Water supply canals are narrow and muddy and subject
to pollution from upstream industries, agricultural lands, domestic sewage and other
shrimp farms. These conditions have resulted in severe losses of shrimp from disease and unexplained mortality (Hambrey and Lin 1998). The upper Gulf also experienced
an industrial boom causing land prices to soar thereby pushing shrimp fanners out of the region and, either out of the industry altogether, or onto the east coast.
Farmers achieved varied success along the eastern coastline due to poor soil quality (acidic with high concentrations of iron and aluminum, characteristic of many mangrove soils), variable salinity and pesticide runoff from neighbouring fruit plantations (Hambrey and Lin 1998).
In the early 1990's the hghest concentration of shrimp aquaculture operations could be found in the southeast (NACA 1996). Shrimp farmers were quite successful in this area as soils are more suitable for cultivation and the coastline is straight with deep waters of stable salinity. As industry had not yet inundated this area, there was little upstream pollution. Another possible reason for the success of intensive farms in the southeast is that farmers gained experience and knowledge from their previous attempts further to the north and improved upon existing culture practices and techniques (NACA 1996).
Despite the near optimum conditions in the east, Thailand was unable t o escape the same fate as Taiwan and suffered a crash in production in 1996 due to disease outbreaks (Figure 2.2). Thailand's shrimp indust~y did not completely collapse, however, as many farmers had already begun to move from diseased, polluted coastal farms to inland areas owing to the recent discovery of low salinity fanning.
Year
Figure 2.2: Aquaculture production (by weight) ofP.monodon in Thailand (FA0 2001)
2.2.2 Svstem ShiR
In addition to a geographical shift, there has also been a noticeable movement fiom open to semi-closed and occasionally closed intensive culture systems. These methods of shrimp farming primarily differ by the amount and frequency of water exchange through a culture cycle. Open farming systems exchange 20-30% of pond water per day during the initial stages of the culture cycle but increase these rates to 50% in the final month (Hambrey and Lin 1998). Farmers discovered that with such high water exchange rates, effluent from upstream aquaculture operations could easily pollute and even spread disease to their ponds (Dierberg and Kiattisirnkul1996). The most popular solution to this was the adoption of semi-closed systems characterised by limited water exchange throughout the 90-120 day grow out period (Funge-Smith and Briggs 1998). This practice, however, did not receive unanimous support as farmers must construct settling or biological treatment ponds for both incoming and recycled water in order to maintain water quality at optimum levels, (Dierberg and Kiattisimkul 1996). Although the yields of semi-closed intensive systems are similar to that of open systems, treatment ponds occupy space that could be used a s grow out ponds and many farmers are unwilling to sacrifice this potentially productive land.
Closed system farms have not been readily adopted by farmers in Thailand due to the capital and skill required f%r efficient operation. Closed water systems d o not release any effluent but do require water input as some is lost to evaporation and seepage during a production cycle (Funge-Smith and Briggs 1998). As this technique uses recycled water, solids must be continuously removed and nutrients kept at suitable concentrations. This can be achieved with polyculture areas, water treatment or settling ponds, and through the use of filters (Dierberg and Kiattisirnkul 1996).
2.3 Management Practices
There are three types of farming practices, extensive, semi-intensive and intensive systems. These differ by feeding and water exchange practices, stocking densities and production potential (Table 2.1). The largest contributors to commercial shrimp supplies, and thus the operations of greatest concern, both economically and
environmentally, are intensive farms. Accordingly, the culture practices of intensive establishments are discussed from the time of site selection to pond preparation, through stocking, rearing and finally harvesting.
2.3.1 Site Selection
The first and arguably most important step in establishing a shrimp aquaculture operation is selecting an appropriate site. Location must be considered with respect to water quality and availability as well as soil composition and infrastructure. Supply water should contain minimal amounts of suspended solids and pollutants originating from upstream activities as these substances create a stressful environment for shrimp (Boyd and Tucker 1998). In addition, to achieve optimal growth and survival rates, pH levels should range from 7.5 to 8.5 and salinity should be maintained between 10- 30 ppt. (Chanratchakool et al. 1995).
With respect to soil, clay or loamy substrates with a pH higher than 5 are ideal. Sandy soils are less desirable as they have a low organic content making it difficult to start and sustain phytoplankton blooms. Ponds established in sandy areas also
experience high seepage rates and therefore require constant filling to ensure water remains at optimum levels. This high demand for water is not of great concern in coastal areas but poses a problem
in
inland areas where water is often a limitedSize 2-20 ha 1-5 ha 0.1-1 ha
Management Minimal attention Continuous and Continuous and
required semi-skilled skilled
Water exchange Tidal Tidal and pump Pwnp and aeration
Feed Natural Natural and Formulated diet
supplement
Stocking density 0.1-1.0 P L / ~ ~ 1-5 P L / ~ ~ 15-100 pvm2
FCR 0 4 . 5 1.5-2.0
Yield
1
4000-15000 k a/Table 2.1 : Comparison of extensive, semi-intensive and intensive shrimp fanns in Thailand (modified from Phil et al. 1993; Thongrak et al. 1997;Chanratchakool et al. 1995; Lorenzen et al. 1997)
Note: FCR refers to feed conversion ratio (a measure of shrimp biomass per kg of food supplied)
resource (Bangkok Post 1999). Acid sulphate soils, such as those present in mangrove forests, are also sub-optimal as they cause water pH levels to decrease and are not suitable for phytoplankton blooms (Tookwinas 1994).
When selecting a farm site, access is of vital concern. In order to decrease post- larval stress and ensure high quality shrimp for export, the distance from hatchery to grow out pond should not exceed 6 hours while that between the farm and processing plant should remain less than 10 hours (Chanratchakool et al. 1995).
2.3.2 Pond Preparation and Stocking
The average size of grow out ponds in Thailand is between 0.16 and 1.0 ha
(NACA 1996). Each of these ponds must be thoroughly cleaned between culture cycles to ensure optimum water quality and decrease the risk of disease outbreaks in subsequent crops (NACA 1996). This process begins on the day of harvest. Once the pond has been drained and the shrimp harvested, a layer of sludge weighing between
185-199 tomes dry weightha remains on the pond bottom (Briggs and Funge-Smith 1994). This fouled layer is composed of shrimp faeces, uneaten food, dead
phytoplankton and solids originating from pond wall erosion (Funge-Smith and Briggs 1998). In the dry season this sediment can be removed by bulldozers or excavators and packed on pond banks to dry. In the rainy season, heavy machinery cannot be operated so sludge is often flushed out by high pressure hoses (Kongkeo 1997). Despite government efforts to ensure that sediment is retained in settlement ponds so as to prevent water contamination, in practice, most farmers dispose of this fouled substrate directly into public water canals (Tiensongrusmee and Phillips 1994 in Dierberg and Kiattisimkul 1996). Once the sludge layer has been removed, ponds are left to dry for approximately one month to oxidise toxic gases such as methane and
ammonia (Macintosh and Phillips 1992). The bottom is then ploughed to expose deeper layers of soil and eliminate more toxic gases. Prior to stocking, the pond bottom is often treated with a barrage of chemicals to neutralise acidic soils as well as
disinfect and eliminate predators (Table 2.2).
Once the pond bottom has been treated, a small amount of water is added which, when stimulated by organic and inorganic fertilisers, results in the formation of a phytoplankton bloom (Funge-Smith and Briggs 1998). Phytoplankton provide oxygen and uptake excess nutrients in the water column. They also provide a shaded pond environment that deters harmful benthic algae and reduces flues in water temperature (Chanratchakool et al. 1995). Once the phytoplankton population is stable, a total of 120-15Ocm of water is allowed to enter the pond and hatchery reared 15-20 day old post-larvae (PLIS -PL20) are stocked at a density of 30-100 P L / ~ ~ (NACA 1996). These juvenile shrimp must be acclimatised to conditions in the grow-out pond before stocking. This is achieved by putting larvae in a tank near the pond with an equal mixture of their hatchery water and pond water, or by placing the plastic bag in which they are transported into pond water for an hour before releasing them into the grow out area (Chanratchakool et al. 1995).
2.3.3 Feeding;
Shrimp grown under intensive conditions require inputs of commercial pellet diets high in nitrogen and phosphorus, These pellets contain attractants to encourage food uptake as well as prophylactic doses of antibiotics to decrease the risk of disease outbreaks (Tookwinas 1996). Schedules vary from farm to farm but feeding usually
occurs between 4 to 6 times over a 24-hour period (NACA 1996). As shrimp are a nocturnal species, more feed is distributed at night than during daylight hours. The
Treatment Type Compounds Commonly Used
_.___________.__.__.__ ... _.___________.__.__.__...________________.___________.__.__.__..._______________ .... ... ... ,-- ... -- ... ..-...-...- ... ---.--.
Soil and water treatments Lime Dolomite Zeolite Chlorine
Microbial preparations Laundry detergent Pesticides, piscicides and molluscicides
Chemotherapeutants to prevent disease
Teaseed cake (saponin) Derris root extract (rotenone) Calcium hypochlorite Ammonium sulphate Malachite green Formalin Chloramphenicol Oxytetracycline Tetracycline Plankton growth stimulators N-P-K fertilizers
Table 2.2: Common chemical and biological compounds used in intensive shrimp culture in Thailand (adapted from NACA 1996; Macintosh and Phillips l992b; Prirnavera 1993)
feed conversion ratio (FCR) in intensive ponds should not be higher than 2 although levels exceeding 5.5 have been documented, resulting in mass accumulations of waste (Macintosh and Phillips 1992b). Feed is commonly scattered by hand with attempts to disperse the majority of feed in areas where the pond bottom has been swept clean by aerator-induced currents (Chanratchakool et al. 1995). This concentrates shrimp in areas with minimal waste buildup thereby reducing exposure to infectious sediment dwelling pathogens. The amount of feed supplied to ponds is determined by placing feed in nets, submersing them in ponds and estimating the rate of consumption (Chanratchakool et al. 1995; NACA 1996).
2.3.4 Aeration and Water Exchange
High stocking densities and large feed inputs result in increased concentrations of phytoplankton and a high biological oxygen demand (Csavas 1993). In order to maintain Dissolved Oxygen concentrations at suitable levels, both in the water column and at the sediment surface, and to facilitate the decomposition and mineralisation of organic debris, aerators are used to produce water currents (Boyd and Tucker 1998). Aeration has proven to be very effective in concentrating wastes in the centre of the pond, keeping most of the pond bottom clean and increasing oxygen levels in the water column. However, it has also been shown to aggravate pond wall erosion resulting in high levels of suspended solids (Dierberg and Kiattisimkul 1996). As aeration is most important when Dissolved Oxygen levels decrease and wastes accumulate, aerator use increases as grow out progresses and are operated 24 hours a day in the final stages of production (Chanratchakool et 01. 1995).
As previously mentioned, water exchange rates vary depending on the fanning system in use (open, semi-closed or closed). The rate and amount of water exchange is
dependent on the tabulated characteristics of pH, salinity, Dissolved Oxygen, visibility, hydrogen sulphide and unionised ammonia along with visual observations of water colour and the presence of bubbles or foam on the pond surface (Table 2.3) (Boyd and Tucker 1998).
2.3.5 Harvesting
The average grow-out period for P. monodon is 100-120 days after which time environmental conditions become stressful resulting in slower growth and higher probability of disease (NACA 1996 and Funge-Smith and Briggs 1998). At the time of harvest, shrimp weigh approximately 30g (Chanratchakool et al. 1995). The two most common methods of harvesting are manually netting shrimp by wading into the pond or draining the pond and collecting shrimp in a bag net attached to an outlet pipe. Any shrimp remaining in the sediment are collected by hand (NACA 1996).
Shrimp should be harvested as quickly as possible, graded according to size and placed on ice in packing boxes and transferred rapidly to processing plants.
2.4 Environmental Imvacts
It cannot be refuted that shrimp farming has been an astronomical success for many rural and urban peoples. Along with this "gold rush',, however, comes an array of negative impacts. The most well documented of these focus on the degradation of coastal ecosystems, inland areas and ground and surface waters.
2.4.1 Land Requirements
Shrimp aquaculture has arisen in many geographic regions of Thailand owing to the conversion of agriculture fields, salt-pans, rice paddies and mangroves into shrimp
Water Parameter Optimum Level
PH
7.5-8.5 Salinity 10-30 ppt DissolvedOxygen
5-6 ppm Secchi Disk 30-40 cm H2s <0.03 ppm0
Table 2.3: Optimum water quality parameters for culture of P. monodon (adaptedfarms. By 1997 productive shrimp farms occupied roughly 70 000 ha, although this figure does not account for many of the recently established inland farms or numerous
abandoned sites (Tookwinas and Songsangjinda 1999). The majority of these farms are situated on converted rice fields and mangroves.
Shrimp farmers have long been labelled as destroyers of mangroves, but such a claim is often unfounded. Although shrimp aquaculture can be observed along a large portion of Thailand's coastline where mangroves used to be abundant, most of these farms occupy land previously cleared for charcoal and timber production (Primavera 1998). This is supported by statistics describing the increase in shrimp production areas by 37 760 ha between 1986-1989 while the corresponding decrease in mangroves was less than half this value (15 867 ha) (Kongkeo 1994). Although shrimp farms may not be the sole cause of the decrease in mangrove coverage, they have significantly contributed to the overall decline of rnangal abundance in Thailand.
Increased awareness regarding the detrimental effects of mangrove removal has resulted in the creation of policies preventing further mangrove destruction and
banning the establishment of shrimp farms in most mangrove areas. These regulations
coupled with increased shrimp disease prevalence in coastal regions have led to the creation of inland shrimp farms. Inland aquaculture techniques have received little attention as the widespread adoption of low salinity practices has only occurred in the last 5 to 7 years (Flaherty et al. 1999). Shrimp fanns are being established in
agricultural areas the majority of which are amongst rice fields and fruit orchards. At present, there is not a shortage of viable land for agricultural use, but the
establishment of aquaculture operations in these areas has raised concern, specifically with regard to salinisation of inland areas (Flaherty et al. 1999).
2.4.2 Water Supplies
Coastal aquaculture operations primarily draw water from the sea Owing to the abundance of source water, water use on coastal farms can be considered negligible. With the emergence of inland shrimp farms, however, attention has been directed towards the potential impacts on availability and quality of surface and groundwater used for domestic and agricultural purposes (Flaherty et al. 1999). Thailand's central plain has an extensive irrigation network and has long enjoyed an abundance of fresh water and therefore has no regulations controlling water use. Few studies have assessed the water requirement for shrimp farms but it is estimated that 1 kg rice uses 5m3 of water whereas an equal amount of shrimp (assuming 3.75TIha) uses 8.8m3 water (International Rice Research Institute 1998 in Flaherty et al. 1999; Flaherty et
al. 1999). In addition, it is important to recognise that the creation of ponds exposes a large surface area of water to the atmosphere causing evaporation rates to increase from those in canals (Beveridge et al. 1994). With the increased occurrence of
drought conditions and a heightened urban and agricultural demand for water in recent years, it is not unreasonable to hypothesise that freshwater supplies will be insufficient to meet future requirements. At present, however, the water supply is still sufficient to meet the needs of its users thus concern is focused on the composition and quality of output water.
2.4.3 Chemical Dischawe
Semi-intensive and intensive aquaculture operations manipulate natural systems in such a way that chemical additions are often necessary to ensure the health of cultured species. Fertilisers are commonly used to promote phytoplankton growth while pesticides and piscicides are applied to prevent unwanted species from competing
with cultured shrimp. Previous research suggests that standard chemotherapeutant applications are lethal to many marine species and may also have deleterious effects on overall ecosystem health (Macintosh and Phillips 1992b).
The recent popularity of highly intensive aquaculture farms has led to greater demands on water resources, higher effluent loads and a necessity for extremely frequent water exchange. These practices have degraded source and pond water resulting in weakened shrimp immune systems and providing an optimum environment in which shrimp pathogens thrive (Kongkeo 1994, Flaherty and
Vandergeest 1998). Farmers have responded to diseases by applying antibiotics, both as a prophylactic present in most commercially available feed products and in
concentrated doses during times of acute disease outbreaks (Flaherty and Vandergeest 1998). The effects of unregulated antibiotic seepage into surrounding ecosystems are poorly understood, but evidence suggests that widespread use of such medications may encourage the emergence of antibiotic resistant strain bacteria. This has the potential to harm natural ecosystem processes, fisheries and may even pose a risk to human health (NACA/FAO 2000).
2.4.4 Organic Outputs
Shrimp aquaculture requires significant organic inputs in order to h c t i o n at optimum levels. These organic materials are added to the system in the form of fertilisers and feed and are either incorporated into shrimp body mass or remain in the pond as uneaten feed or shrimp excreta (Briggs and Funge-Smith 1994). The amount of uneaten feed in a pond depends on the individual practices of each farmer.
Intensive practices, specifically those with high FCRs, severely degrade pond water quality and necessitate frequent water exchange. Daily water exchange and seepage
result in discharges of 1 050 to 23 188 m3/ha depending on the production stage (Briggs and Funge-Smith 1994). The concentrations of key nutrients present in this discharge are described in Table 2.4.
Shrimp effluent is not as toxic as waste from other sources such as sewage, when compared in terms of milligrams of toxic substance per litre of effluent (Macintosh and Phillips 1992b). It is, however, extremely hazardous as it is typically discharged in large volumes due to the synchronized harvesting cycles of f m s within a region or province (Macintosh and Phillips 1992b). This leads to large effluent discharges into neighbowing ecosystems at key points during the shrimp culture cycle. The most common way to assess the polluting potential of effluent is to monitor nitrogen and phosphorus concentrations in wastewater. Of specific concern are nitrate, nitrite and ammonia. At low levels, nitrate is not h a d to ecosystems and is in fact a major contributor to growth and survival of a number of species. At high concentrations, however, this nutrient is responsible for disrupting ecosystem dynamics by
encouraging rapid growth of microorganisms that increase primary productivity (Moriarty 1986 in Phil et a1. 1993). This in turn alters community structure and can lead to harrml/toxic phytoplankton blooms. The increase in respiring organisms, along with the breakdown of organic material and waste products in effluent, disturbs dynamics at the very bottom of marine food chains in addition to reducing Dissolved Oxygen in canals and estuaries (Phil et al. 1993). The resulting anaerobic
environment can suffocate and severely diminish, if not completely destroy, fauna and flora present in nearby habitats (Nunes and Parsons 1998). Nitrite and ammonia are
also of concern as they are toxic to many marine species even when present at very low concentrations.
...--- ~ . - ~ ~ ~
Parameter Concentration in Effluent
.
it rite-nitrogen-
0.04*
0.06 Nitrate-nitrogen 0.11*
0.19 Total Nitrogen 3.45 =t 1.69Table 2.4: Concentrations of key nutrients in shrimp farm effluent from an intensively managed shrimp pond in Thailand (stocking density of 80 to 100 P L I ~ ~ ) (Briggs and Funge-Smith 1994)
Of particular importance is the potential degradation of sensitive estuarine, coral reef and seagrass habitats that are home to a plethora of marine life, many of which are commercially important. The large sediment loads of effluent discharged at harvest not only lead to eutrophication in receiving water ways, but can also cause severe blockage in neighbouring canals resulting in decreased water supplies, flooding and lethal anaerobic conditions (Paez-Osuna 2001). Eutrophication and sedimentation of neighbouring waters are also of concern to aquaculturalists. As shrimp farms are usually established in close proximity to each other, wastewater discharge from one fann often serves as inflow water for a neighbowing farm. If effluent quality is extremely poor owing to high nutrient content and minimal or non- existent waste treatment facilities, the potential for a farm to pollute all other farms in the vicinity is extremely high Self-pollution has been responsible for low harvest yields and a rapid spread of disease in many coastal areas. (Dierberg and Kiattisimkul
1996)
2.5 Best Management Practices
The seemingly endless array of potentially negative impacts of intensive shrimp farming have given rise to a remarkable number of mitigation options and suggested best management practices. Unfortunately each of these options comes with a price, either financial, temporal or spatial, and have therefore not been widely accepted. Despite this, many are worthy of consideration and a combination of a number of practices may prove to be the most viable and effective solution for ensuring long- term sustainability of these operations.
2.5.1 Planning and Management
One of the most imperative steps in reducing the negative impacts of intensive shrimp aquaculture is proper site selection. Farmers must be educated to conduct risk analysis prior to establishing shrimp farms. It is essential that they view their farm as part of a larger, dynamic whole as opposed to an isolated entity. By considering the suitability of a site, with respect to soil and water quality and the potential impacts both on and from the surrounding environment, farmers will be able to decrease the frequency of site abandonment and disease outbreaks (Macintosh and Phillips 1992a). Such a holistic approach could have helped prevent the destruction, degradation and subsequent abandonment of many lands (Phillips et al. 1993). For example, through careful assessment regarding the suitability of mangrove forests for shrimp culture, farmers would have soon realised that the pH of mangrove soils is too low to sustain shrimp culture, and therefore that mangroves are not ideal areas in which to establish ponds (Chanratchakool et al. 1995).
2.5.2 Physical Techniques
As a significant amount of solids enter ponds through soil erosion aggravated by aerators, separating the pond bottom from the water column is an effective way to decrease the amount of the accumulated sediment (Funge-Smith and Briggs 1998). This can be achieved through the use of bitumen impregnated geotextile or PVC pond liners (Chanratchakool et al. 1995). This practice reduces the amount of solids remaining on the pond bottom after harvest, while minimally affecting the total nutrient load, as the majority of organic matter originates fiom feed inputs. As a result, the remaining liquid contains high levels of organic matter and can be sold as
In addition to decreasing sediment outputs, liners can halt the seepage of pond water thereby eliminating the risk of chemicals and salt leaching into agricultural lands and underground aquifers (Macintosh and Phillips 1992b). The separation of pond bottom from water also allows farms to be situated in sub-optimal areas with porous, acidic soils (Dierberg and Kiattisimkul 1996).
Although pond liners are quite costly, the initial investment is negligible when the benefits are considered (Dierberg and Kiattisimkul 1996). The average time to clean and treat an earthen pond varies from four to eight weeks depending on weather, disease, soil condition etc. This time is decreased to ten days when using lined ponds thereby reducing costs by up to 50%. It is estimated that the cost of liners, which are guaranteed for ten years, can be recovered within five to eight years of installation (Macintosh and Phillips 1992b).
Unfortunately there a few problems associated with PVC and geotextile liners. Firstly, organic phosphorous concentrations tend to increase in lined ponds. This is attributed to the blocking of potential phosphorus binding sites in pond soil (Dierberg and Kiattisimkul 1996). Secondly, cannibalism and high FCRs have been reported. The smoothness of the substrate and aerator induced water circulation facilitate the buildup large concentrations of feed in the centre of the pond. This makes feed inaccessible to shrimp. The relationship between pond soil and shrimp nutrition is not well understood but it is thought that pond liners could potentially hinder nutrient uptake (Funge-Smith and Briggs 1998).
Liners themselves also have physical drawbacks as they are sensitive to sunlight and extended exposure between crops can dramatically reduce the length of their service life (Funge-Smith and Briggs 1998). If pond lining becomes standard management practice, the disposal of used materials will become an important
environmental issue. No effective pond liner recycling programme exists in Thailand and many fanners may simply opt to bum them, which would release noxious gases into the atmosphere, while others may dump them or bury them on site.
2.5.3 Feed Related Practices
Most best management practices revolve around remediating, minimising and even terminating the impact of effluent on surrounding lands and water sources. Feed is the source for the largest input of nutrients into shrimp ponds. Commercial pellet diets account for 92% of nitrogen and 51 % of phosphorus input in pond water (Briggs and Funge-Smith 1994). As these nutrients can cause the disruption of neighbouring ecosystem structures when drained into irrigation canals, many management strategies that are concerned with reducing environmental effects of effluent, focus on ways to decrease the impact of feed. A number of researchers have recommended the
production of highly digestible, low pollution diets (e.g. Thongrak et al. 1997; Burford and Williams 2001). Although the cost of such feed would be higher than diets currently available, the FCR will decrease thereby reducing both the quantity of feed required and subsequent wastage. Unfortunately, owing to the reluctance of feed manufacturers to cooperate with government and academic researchers, it is difficult to determine the current ingredients in feed and therefore to suggest more suitable formulas (Kongkeo 1 994).
Thai shrimp farmers have been widely criticized for the extremely haphazard ways in which they feed their crops resulting in frequent overfeeding and high wastage (Funge-Smith and Briggs 1998). Nunes and Parsons (1 998) suggest the development of a feeding model based on shrimp behaviour and physiology. They propose that feeding frequency, distribution and feed quantity should be adjusted according to an
animal's varied requirements dependent on rates of ingestion and digestion as well as size, age, life stage, light intensity and spatid patterns.
2.5.4 Policv Options
Along with the variety of physical and biological management options, there are a number of policies that can aid in decreasing the environmental impact of intensive shnmp cultivation. In November 1991 the Thai government established a number of rules and regulations in an attempt to control the imminent spread and impact of shrimp farms (Table 2.5). Unfortunately, the government does not have the money, the manpower and as if ofien speculated, the desire, to effectively enforce these regulations. Thongkrak et al. (1997) suggested some additional policies, some of which do not require such intensive use of government resources. Many farmers overstock grow-out ponds simply because of a belief that more initial stock results in higher yields. In reality, shrimp yields are approximately equal due to lower survival in higher stocked ponds as a result of competition and decreased water quality. High stocking rates also increase organic waste fiom farms. One option, then, will be to impose a tax on fry which would result in more appropriate stocking densities while minimally affecting production. Taxation, however, is likely to be extremely
unpopular among farmers. It was also suggested that the government distribute loans and grants to farmers interested in adopting environmentally friendly practices (Thongkrak et al. 1997). This type of funding would require close monitoring to ensure funds are being used appropriately and for designated purposes. Thongrak et al. 's (1997) suggestion of communal water treatment areas is also worthy of
consideration. This involves separating water source and discharge canals and ensuring that effluent passes through communal sedimentation and treatment ponds.
This idea is of interest as it decreases the amount of land an individual farmer must sacrifice to establish treatment areas and gives farmers access to high quality source water. However, all farmers in an area must be willing to participate in this practice in order for treatment to be effective. As an additional incentive to adopting
environmentally sustainable practices, shrimp reared on farms following such guidelines could be certified and labelled as "eco-friendly" and therefore warrant higher price tags.
2.5.5 Biological Practices
If pond liners are not used, suspended solids in influent and effluent waters can be rapidly removed in settling ponds (Corea et al. 1995). A 1 ha pond can effectively settle-out solids in 1000m3 of effluent per day (Kongkeo 1994; Teichert-Coddington er al. 1999, in Paez-Osuna 2001). The unavoidable drawback to this form of
treatment is that shrimp effluent contains large amounts of phytoplankton which are naturally buoyant and will therefore not easily settle (Macintosh and Phillips 1992b). For this reason, settling ponds are often used in conjunction with biological treatment ponds which filter solids and improve overall water quality.
Biofiltration experiments in Thailand revealed that Perna viridis (green mussels) are capable of reducing biological oxygen demand, organic solids and phytoplankton levels (Macintosh and Phillips 1992b). Gracilaria, a common seaweed, has also been effective in removing dissolved nutrients and sea cucumbers and macroalgae
efficiently uptake settled particulate matter (Lin et al. 1991 in Macintosh and Phillips 1992a). Similarly, oysters are capable of removing fine organic particles in
Regulation Problems with Enforcement
No new mangrove destruction
-
all existing farms in mangrove areas must leave by 1994Shrimp farmers must register with the DOF
Exporters may only buy shrimp from =* Lack of manpower and h d s
registered farms
Shrimp farms over 8 ha must have wastewater treatment areas not less than 10% the size of production area
Effluent released into receiving waters must have
BOD
less than 1 Omg/lMud and sedimmt is not permitted to =Waste water generally drained at be released into natural water sources night --> difficult to monitor or public areas
S& water must not be drained into neighbouring freshwater or farming areas
Table 2.5: Regulations for shrimp farming issued by the Thai government, November 1991 (modified fiom Kongkeo 1994)
(Jones et al. 2001). An added benefit to adopting biological treatment techniques is the secondary use of these marine species as food for humans and other organisms.
The main drawback to biological treatments is that the organisms used require extremely specific environmental conditions for optimal functioning and survival. For example, unlike P. monodon which are tolerant to a wide variety of salinities, green mussels cannot survive with even minimal fluxes. In addition, bivalves have low sediment assimilation capacities and are therefore sensitive to heavy loads of sediment present in shrimp farm effluent (Jones et al. 2001). Another area of significant
concern to shrimp farmers is ammonia, a harmM by-product of biological treatment. Although bivalves absorb some dissolved nutrients and suspended solids,
approximately 27% of absorbed nitrogen is excreted as ammonia, a compound that is toxic to shrimp. If, however, bivalve treatment is used in conjunction with macroalgal absorption, ammonia can be reduced to non-toxic levels (Jones et al. 2001).
Artificial bacterial suspensions are also an avenue of increasing interest. Microbes have proven successful in treating sewage and industrial wastes but have achieved minimal success in fish and shrimp ponds. This has been attributed to a lack of understanding of microbial processes and their specific environmental requirements (NACA 1996). As a result, their potential benefit in treating shrimp fann effluent has not yet been disregarded.
Salt-tolerant plants, or halophytes, are another possible source of water treatment. The water discharged from 1 ha of intensive shrimp farms can irrigate 18 ha of halophytes for one week, assuming water is only discharged at harvest (Brown and Glenn 1999 in Paez-Osuna 2001). Salicornia bigelovii, Atriplex bardyana and Suaeda esteroa are efficient biofilters capable of removing a significant amount of nitrogen and phosphorus from intensive shrimp f m effluent as well as providing forage straw
for ruminants. The hypersaline, nutrient poor water end product of halophyte processes can serve as a habitat for the growth ofArtemia, brine shrimp, or be manipulated to produce salt (Brown and Glenn 1999 in Paez-Osuna 2001).
Wetlands also serve as effective nutrient sinks and filtration sites and have been used in a number of temperate regions to treat an array of domestic, agricultural and industrial wastes (Gopal 1999, Summerfelt et al. 1999, Nzengy'a and Wishitemi 2001). Most wetland treatment areas consist of salt-intolerant reeds and are therefore unsuitable as biofilters for aquaculture effluent (Summerfelt et al. 1999). Of
increasing interest in saline and brackish wastewater treatment is the use of mangrove wetlands.
Mangroves are one of the most productive ecosystems in the world capable of recycling large influxes of organic matter from both anthropogenic and natural sources (Tam 1998). The dynamics of mangal environments are complex, but
preliminary examinations of mangrove effluent treatment have produced encouraging results that demand further research to unveil the true potential of these intertidal forests as biofilters (Wong et al. 1997, Rivera-Monroy et al. 1999). If mangroves prove to be efficient at removing large quantities of nutrients from shrimp f m effluent, without being negatively affected by influxes of organically rich wastewater, they may become a primary source of effluent treatment for coastal farms around the world.
2.6 Summary
Shrimp aquaculture is an ever-expanding industry driven by the global demand for marine species and has not yet been curbed by government policies, social strife or environmental degradation. Thailand is the leading producer of shrimp worldwide
