Nutrient budgets in intensive shrimp ponds: implications for sustainability

Aquaculture 164 Ž1998. 117–133 Nutrient budgets in intensive shrimp ponds: implications for sustainability Simon J. Funge-Smith ) , Matthew R.P. Briggs Stirling Aquaculture (Asia), P.O. Box 32, Kao Seng, Songkhla 90001, Thailand Abstract Serious production losses have occurred in shrimp producing countries around the world, principally due to poor rearing environments and pathogenic disease. In response to this, shrimp farmers are changing their culture methods. To understand the source and sink of nutrients which affect pondwater quality and effluent impact, the nitrogen, phosphorus and solids budget have been constructed for water exchange systems. These budgets reveal the contribution of the pond bottom soil to the accumulation of sediment and phosphorus and its potential contribution of nitrogen to the pond system. A survey of shrimp farm water quality and management practices in southern Thailand has also been completed. This reveals a high proportion of farms using low water exchange methods of shrimp culture but without the ability to maintain suitable water quality in the production ponds. Shrimp production in these systems is variable due to high incidences of disease and slow growth rates. The pond processes that might cause this are suggested and potential methods for their amelioration are discussed. Alternative culture systems such as lined ponds, low salinity rearing and recirculation farms are described in relation to their potential for remediating problems within the shrimp culture industry. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Shrimp; Production systems; Solids and nutrient budgets 1. Introduction The rapid increase in world cultured shrimp production and its equally rapid decline in some countries like Ecuador, China and Indonesia ŽShrimp News International, 199. has left environmental, social and financial problems in its wake. This has not prevented ) C o rresp o n d in g au th o r. aquacon@t-rex.hatyai.inet.co.th. T el.: q 6 6 -7 4 -3 2 4 -4 7 5 ; fax : 0044-8486r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 4 4 - 8 4 8 6 Ž 9 8 . 0 0 1 8 1 - 1 q 6 6 -7 4 -3 2 4 -4 7 5 ; e-m ail: 118 S.J. Funge-Smith, M.R.P. Briggsr Aquaculture 164 (1998) 117–133 the development of new stretches of coastline for shrimp culture and the further intensification of existing areas. Shrimp farming has the capacity to dramatically transform coastal areas. Extensive farms have an enormous requirement for land and the development of intensive culture practices increases nutrient impacts on the local coastal environment. Alongside environmental changes such as eutrophication, salination and land use changes, are attendant social transformations ŽLin, 1989; Chua et al., 1989; Csavas, 1993; Liao, 1990; Macintosh and Phillips, 1992; Panvisavas et al., 1991; Primavera, 1991, 1992, 1993, 1995.. These can be both positive and negative; the increased income into traditionally poor coastal areas must be balanced against loss of job diversity, loss of independence, rising prices and growing inequity between farmers and non-farmers ŽChong, 1990; Masae and Rakkheaw, 1992; Nuruzzaman, 1996; Primavera, 1993, 1995.. Once changes have occurred Žfrequently irreversibly., it is often important to maintain the industry in some form. Collapse of shrimp farms leaves nothing for the inhabitants of an area since agricultural land and mangroves are degraded and often, former farmers are left with considerable debt. This leads to loss of land and an inability to return to their original lifestyle. There is a pressing need for the development and dissemination of a range of shrimp culture systems that are both environmentally and economically sustainable. Typically the pattern of production from a shrimp farm is that of an initial ‘honeymoon period,’ characterized by success and good production followed by gradual decrease in yields over successive crops. Depending upon a wide range of factors, decreased yields are manifested as reduced growth, higher FCR, and disease outbreaks that require emergency harvesting. The worst case is that of complete mortality of stock and this is being encountered more frequently with the increasing incidence of extremely pathogenic viral diseases. Although there are now several primary pathogens Žviral. of shrimp, the majority of shrimp disease is caused by secondary pathogens that are able to invade shrimp already stressed and weakened by a poor quality rearing environment. A shrimp that is stressed does not grow rapidly, principally due to reduced feeding and delayed moulting. It is important to maintain a healthy rearing environment for the shrimp to maximize production potential and minimize the risk of opportunistic disease. The viral type diseases such as ‘yellowhead’ ŽYBV. and ‘white spot’ ŽSEMBV. disease appear to be transmitted via influent water and intermediate crustacean host, respectively. There is now some evidence that ‘white spot’ can also be transmitted through the post-larvae ŽFlegel et al., 1996.. These diseases cannot necessarily be prevented by the provision of a good quality rearing environment, although a poor environment certainly will increase susceptibility. Yellowhead disease already appears to be decreasing in pathogenicity from that of a primary to opportunist type pathogen. In 1993, only 0.05% of the tiger shrimp Penaeus monodon carrying the virus were asymptomatic, now this figure has increased to 60% ŽFlegel et al., 1996.. Therefore, environmental quality will have a more significant effect than before. The response of Thai farmers to these two viral diseases, has been to reduce water exchange in an attempt to prevent transmission between farms and ponds. This practice has generated its own problems which will be discussed later. S.J. Funge-Smith, M.R.P. Briggsr Aquaculture 164 (1998) 117–133 119 In order to better understand the link between environmental quality and production, it is important to understand some of the underlying relationships between the two. This knowledge allows the significant processes occurring in different culture systems to be identified and suitable management strategies to be derived. 2. Environmental processes in shrimp culture systems with water exchange The two significant components of the pond environment are the pond water and sediments which interact continuously to influence the culture environment. Pond sediments can be further divided into the pond soil component Žthe pond bottom and walls. and the accumulated sediment component Žthe sludge that accumulates on the pond bottom during culture. ŽBriggs and Funge-Smith, 1994.. Pond management activities are a third external factor which influence the culture environment. Management activities include feeding, use of aerators, water exchange and liming ŽFig. 1.. The original method of intensive shrimp culture in Thailand involved relatively high stocking densities Ž50–100 my2 ., high production Ž6–12 t hay1 cropy1 ., high feeding rates ŽFCR 1.8–) 2.0. and high rates of water exchange Žup to 5–10% per day towards harvest time.. Water quality management was achieved by a combination of flushing the pond with clean seawater and management of the phytoplankton bloom by assessment of pond colour. Water exchanges were frequent, especially in the latter half of production. Accumulated sediment was known to be undesirable and was removed between cycles. Experience has taught farmers that inadequate sediment removal would cause water quality problems early in the subsequent crop. Due to the high flushing rates and naturally high levels in seawater, the alkalinity of the pondwater was not considered important and no attempt was made to control pH. Oxygenation of the pond during the Fig. 1. Water quality interactions and management activities in intensive shrimp ponds. 120 S.J. Funge-Smith, M.R.P. Briggsr Aquaculture 164 (1998) 117–133 day was largely achieved by the phytoplankton bloom giving rise to supersaturated oxygen Ž130–150%. and high pH Ž8.3–9.5. by the afternoon. This oxygen concentration would gradually diminish overnight and aeration would be started during the very early morning to prevent critically low levels. Aeration was usually suspended during the day to save power costs and in the belief that it was unnecessary. Feed management in Thai intensive ponds is achieved through the use of lift trays which are checked regularly to assess if shrimp are feeding. On larger farms, shrimp growth was recorded to cross check the feeding rates and also to estimate potential production. 3. Nutrient and solids budgets A study of this ‘open’ type system was performed in southern Thailand using shrimp ponds constructed on clay soils ŽBriggs and Funge-Smith, 1994.. Budgets were derived for solids, particulate organic matter, nitrogen, and phosphorus. Three different types of pond were investigated: one-year and two-year old ponds Žstocking density 50–60 my2 . and one-year old ponds with higher stocking density Ž80–100 my2 .. The solids budget ŽFig. 2. shows that erosion of pond soil was the major source of both solids Ž88–93%. and organic matter Ž40–60%. in the pond. The feed applied to the pond was a significant source of organic matter Ž31–50%. but contributed little solids Ž4–7%. to the system. This is important since the feed component is also an indication Fig. 2. Total and organic solids budgets for intensive Thai shrimp ponds ŽFunge-Smith and Briggs, unpublished data.. S.J. Funge-Smith, M.R.P. Briggsr Aquaculture 164 (1998) 117–133 121 of the faecal contribution by the shrimp. Influent water is a major source of sediments in extensive systems ŽBoyd, 1992., but contributes only 2–3% of solids in intensive systems because stirring generates more endogenous solids. The organic contribution of the influent water was significant Ž7–13%., but less so than feed and soil erosion. Shrimp culture ponds are effective sedimentation areas ŽBoyd, 1992. such that the bulk of the solids introduced to the system endogenously and allocthonously remain in the pond as accumulated sediment Ž91–94%.. This accumulated sediment was the sink for 58–70% of the organic matter in the system. Routine water exchange accounted for 4% of solids discharge and pond drainage for a further 3%. The organic content of these discharged solids were 13% and 9% respectively. The shrimp were a relatively minor component in the whole system, accounting for just 0.7% of the solids and 6.1% of organic matter. The important consideration that emerges from these budgets is that the character of the pond soil will have a significant effect on the water quality and production of a shrimp pond. Depending upon initial compaction and organic content, the soil will affect the organic matter and solids of the system. In the case of mangrove soils, organic content can be two or three times that of clay soils Že.g., rice paddy.. Sandy soils are the opposite containing little organic matter. It is frequently observed that starting phytoplankton blooms in these ponds is difficult and they are liable to crash frequently. Ponds with sandy soils also suffer from high seepage rates causing problems as organic material is drawn into the soil matrix where anaerobic decomposition can occur. After one or two crops, production losses occur due to seriously deteriorated pond bottom conditions ŽFunge-Smith and Stewart, 1996.. The organic component of the accumulated sediment is a mixture of pond soil organic content and detrital material. This detrital material is composed of sedimented organic material from plankton, shrimp faeces and uneaten feed. The character of the accumulated sediment is therefore dependent upon culture intensity, pond soil organic content, and water exchange practices. Problems associated with the pond bottom and accumulated sediment occur when excessive organic material builds up causing release of ammonia, organic sulphur compounds, and, in cases of extremely high organic matter and acidic soils, hydrogen sulphide ŽAvnimelech, 1996; Lin, 1989; Fast and Lannan, 1992; Wang and Fast, 1992.. Inadequate cleaning of the pond bottom between crops leaves organically enriched sediment across the pond bottom. Due to its non-compacted nature, this sediment is easily suspended by the action of aerators during the next production cycle. The organic matter released from this sediment tends to stimulate very heavy phytoplankton blooms in the first month of production. Because of this, old ponds in Thailand are rarely fertilized to stimulate phytoplankton bloom. Non-removal and respreading of the accumulated sediment over the pond bottom has been proposed since the increase in organic content can be slight. However, experience of poorly cleaned intensive ponds suggests this is unwise. Non-removal of sediment might be a viable alternative if ponds were farmed less intensively, fallowed for longer periods, and the sediment compacted. The solids discharged from the ponds form a minor but significant part of total solids in the system. The total discharge of organic matter in the effluent was 4.8 t hay1 and the total solids approximately 12.6 t hay1 . Trapping these solids as they are discharged 122 S.J. Funge-Smith, M.R.P. Briggsr Aquaculture 164 (1998) 117–133 represents a major problem in reducing shrimp farm impact on the surrounding environment. The suspended solids in a shrimp pond have a different character from the sedimentable solids. Frequent cessation of aeration during the production cycle allows the heavier soil particles to settle and form the accumulated sediment. Phytoplankton, bacteria and the organic particulate fraction are not easily sedimented due to natural buoyancy, or having a specific gravity close to that of seawater ŽRubel and Hager, 1979.. In trials on shrimp pond waters one hour settlement achieved 22–44% settlement of suspended solids. There was no significant difference between settling times of 1, 2 and 3 h ŽFig. 3.. Thus, the use of settling ponds for removal of this fraction is bound to be ineffective unless enhanced sedimentation is practiced Že.g., flocculation.. Harvest effluents are more easily settled because they are a mixture of resuspended accumulated sediment and the suspended solid fraction of the water. The actual amount of nutrients assimilated into shrimp biomass is a small fraction of the total applied as feed ŽTable 1.. Only 18–27% of nitrogen and 6–11% of carbon applied to the pond was assimilated thus there is considerable wastage as nutrients are either incorporated into plankton biomass, volatilized or trapped in the sediments. The nitrogen and phosphorus budgets reveal in more detail, the sources and sinks of the organic components in an intensive shrimp pond ŽFig. 4 Fig. 5. ŽBriggs and Funge-Smith, 1994.. Applied feed accounted for 78% of the input of N to the ponds ŽFig. 4.. Erosion of the pond soils, whilst a major contributor of solids, accounted for only 16% of N added to the system. Other minor contributions were influent water Ž4%. and fertilizer, rainfall and postlarvae Ž2%.. The sinks for nitrogen were the sediments Ž24%., harvested shrimp Ž18%., and discharged water Ž27%.. This leaves approximately Fig. 3. Settlement of suspended solids from shrimp pond waters ŽFunge-Smith and Briggs, unpublished data.. S.J. Funge-Smith, M.R.P. Briggsr Aquaculture 164 (1998) 117–133 123 Table 1 Composition of shrimp and feed showing assimilation and loss to environment ŽFunge-Smith and Stewart, 1996. Nutrient Protein Lipid Ash Fibre Carbohydrate Dry matter Nitrogen Phosphorus Carbon a Assimilation at FCR 1.65–2.40 a Proximate analysis Ž% dry weight. Composition of 1 kg dry Feed Shrimp feed Žg kgy1 dry feed. grams kgy1 assimilated % non-assimilated 45.4"2.6 6.1"0.5 12.8"0.8 3.1"0.4 23.0"2.4 90.3"1.1 7.08"0.59 1.34"0.20 43.16"1.71 61.2–89.4 5.5–8.1 21.8–31.9 2.6–3.8 21.8–31.9 y 13.0y19.0 1.3–2.0 46.5–67.9 80.3–86.5 86.7–90.9 75.1–83.0 87.8–91.6 86.2–90.5 y 73.2–81.6 85.3–90.0 84.3–89.2 54.2"2.5 4.9"0.5 19.3"0.8 2.3"0.2 19.3"1.5 24.6"1.2 11.50"0.18 1.19"0.15 41.2"1.3 454 61 128 31 23 y 70.8 13.4 43.16 Ž1 kg of dry feed at FCR 1.65–2.40 produces 113–165 g dry weight shrimp.. 30% of the nitrogen unaccounted for which is assumed to be N lost to the atmosphere as N2 or ammonia. This volatilization of nitrogen emphasizes the significance of microbial decomposition processes in ponds, especially bacterial conversion of nitrogen compounds Žprincipally nitrate. to N2 ŽFry, 1987.. The high loading of nitrogen in the pond effluent highlights its potential impact on receiving waters. This loss of nitrogen is also a Fig. 4. Nitrogen budget for Thai intensive shrimp ponds using water exchange Žadapted from Briggs and Funge-Smith, 1994.. 124 S.J. Funge-Smith, M.R.P. Briggsr Aquaculture 164 (1998) 117–133 Fig. 5. Phosphorus budget for Thai intensive shrimp ponds using water exchange Žadapted from Briggs and Funge-Smith, 1994.. wasted resource that could be incorporated into the growth of other organisms. The site of this bacterial activity is unknown—does nitrate conversion occur principally in the sediments or in anaerobic microzones in the suspended solids fraction? This question is of considerable importance in the management of bacterial flora in shrimp ponds. The principal source of phosphorus in this system was the applied feed Ž51%. ŽFig. 5.. The 26% shortfall in inputs was assumed to be the eroded pond bottom. None of these ponds were new and previous cleaning had left old uncompacted sediment on the surface, which is easily eroded during the next cycle contributing phosphorus to the system. Effluent water still constituted 10% of P loss in the budget and this is mostly bound in the suspended solid fraction. Again, trapping of the suspended solid fraction is important to minimize impact. 4. Potential solutions Common problems in the open water exchange system include phytoplankton crashes, deteriorated pond bottoms and bacterial diseases. A phytoplankton crash causes a significant increase in ammonia in the water, a decrease in dissolved oxygen and a rise in organic material. This stressful situation, together with increased bacterial concentrations, often leads to outbreaks of vibriosis, Zoothamnion infections, and luminescent Vibrio in the ponds. Traditionally, the only method for ameliorating this problem was high levels of water exchange ŽChanratchakool et al., 1995.. S.J. Funge-Smith, M.R.P. Briggsr Aquaculture 164 (1998) 117–133 125 Table 2 Monthly water exchange rate Ž%. in open, semi-closed and closed shrimp production systems in southern Thailand Ž1993–1995. Month Ranod–Hua Sai Ž1993. a Open system Chantaburi Ž1994. b Semi-closed Southern Thailand Ž1994–1995. c Semi-closed Closed 1 2 3 4 12 86 128 168 13 30 43 57 6 54 97 107 1 5 7 7 a NACA Ž1994.. Marsden Ž1994.. c Funge-Smith, 1996. b Due to deterioration of estuarine and coastal water bodies, many shrimp culture areas no longer have the clean seawater required for this method of culture. This has led to decreasing amounts of water exchange. One reason for the widescale adoption of low water exchange systems in Thailand has been the perception that water exchange triggered disease outbreaks. There is now substantial evidence that the viral disease yellowhead is transmissible in water and that white spot disease is introduced via crustacean intermediates during water exchange. The assumption that separating the production pond from external water inputs may prevent the introduction of viral disease has given rise to the ‘closed’ and ‘semi-closed’ culture systems in Thailand ŽTable 2.. 4.1. Closed and semi-closed systems These systems operate by filling a pond and using a biocide to kill any potential vectors of Žviral. disease Žmysid shrimp, white shrimp, swimming crabs, zooplankton.. The usual biocide used is calcium hypochlorite Ž60% wrw applied at 300 kg hay1 ., but there is increasing interest in the use of more specific organo-phosphate pesticides due to the effect of chlorine on the phytoplankton as well. After chlorination the pond is limed and aerated to disperse residual chlorine and stimulate the development of phytoplankton bloom. There is no water exchange in the first two months after stocking in these systems, although filling of the pond is necessary towards the end of the second month. Depending upon season and rainfall, evaporative loss can cause salinity to rise to an unacceptably high level. To counteract this, freshwater is pumped where available although this has very serious environmental and social impacts if aquifer water is used ŽLiao, 1990; Primavera, 1991; Csavas, 1993.. During the monsoon seasons, rainfall is sufficient to prevent high salinities. In the semi-closed system, limited water exchange commences during the second month of production. In the fully closed system, the farmers attempt not to change any water at all and merely add water if necessary. Total effluent loadings from systems employing any form of water exchange are not significantly different when compared with fully closed systems. The loadings in Table 3 represent those discharged as a result of water exchange, and do not account for 126 S.J. Funge-Smith, M.R.P. Briggsr Aquaculture 164 (1998) 117–133 Table 3 Nutrient loadings as a result of water exchange activities ŽFunge-Smith, 1996. Nutrient Total ammonia–nitrogen Nitrite–nitrogen Nitrate–nitrogen Total phosphorus Dissolved reactive phosphorus Chlorophyll a Chemical oxygen demand Total suspended solids Organic suspended solids Total effluent loadings as a result of water exchange Žkg cropy1 . Open system lined pond Open system clay soil Open system mangrove soil Semi-closed system Closed system 50.5 8.8 9.7 34.4 1.13 5630.1 456.4 4352.4 2236.7 50.6 1.6 3.8 19.0 1.49 7126.2 n.d. 5053.5 2719.0 95.7 3.8 5.7 25.9 0.38 7092.6 432.8 4250.6 1836.6 53.9 7.2 7.6 13.1 0.82 4261.2 244.1 3555.6 1889.1 6.7 0.8 0.6 1.2 0.12 312.3 21.1 336.3 155.5 accumulation within the system. In the case of fully closed systems, impacts are relatively small with respect to effluents, but the actual rearing environment is stressful as a result of high organic loadings within the ponds. Low water exchange systems such as these are complete sinks for nutrients and thus there is no outlet for wastes during production except for discharge at harvest ŽTable 3.. The nitrogen budget above shows that even in an open system water exchange does not result to a major loss of N Ž17%. ŽFig. 4.. The lack of water exchange has one significant management problem, over-blooming phytoplankton. This has always been a problem since the phytoplankton eventually crashes and causes severe stress to the shrimp. One of the management methods currently employed is the killing of the bloom by the application of biocides Žusually BKC, formalin, glutaraldehyde, calcium hypochlorite.. This is applied in the corners where phytoplankton density is greatest. Aeration is suspended during treatment followed by vigorous aeration to disperse the chemicals and reduce the concentration to a harmless level. The dead phytoplankton releases a great deal of ammonia and decomposes, forming a thick stable foam on the surface of the pond or sinks to be incorporated into the sediment. The floating foam is removed from the corners of the pond where it collects. By virtue of their higher organic loadings, closed and semi-closed systems appear to encourage dinoflagellate and blue–green algae Žcyanobacteria. blooms Že.g., Gymnodinium, Peridinium, Gonyaulax, Coscinodiscus, Anabena, Oscillatoria.. The dark brown, red or purple–black appearance of a pond is typical of such blooms and are considered to be stressful to the shrimp and undesirable in shrimp ponds. Due to the high concentrations of ammonia often encountered in these two systems Ž; 3.0 mg ly1 ., it is vital to control of pH. Daily pH checks are often performed and an attempt is made to maintain pH between 7.8 and 8.4 during the day time. Alkalinity Žphenolphthalein method. is also checked regularly. If alkalinity is considered too low dolomite is applied. If pH rises too high during the day some attempt is made to reduce the phytoplankton concentration. If alkalinity is too high, organic acids such as acetic S.J. Funge-Smith, M.R.P. Briggsr Aquaculture 164 (1998) 117–133 127 acid are sometimes added to neutralize some of the carbonate. The requirement for CO 2 as a nutrient by the phytoplankton is well known, but the loss of bicarbonate as a result of acidification due to bacterial denitrification is rarely considered. From the nitrogen budget ŽFig. 4., approximately 30% of nitrogen appeared to leave the pond in gaseous form, therefore the bacterial activity required to achieve this must be considerable. Due to their low solubility, the addition of lime and dolomites to increase buffering capacity of pond water may not always be effective in controlling water buffering capacity. Use of formalin has increased significantly due to its biocidal effect, acidity, and capacity to combine with ammonia. Treatment of ponds with 10 ppm formalin not only kills phytoplankton when applied, but removes ammonia when dispersed. Formalin also contains formic acid so that pH is not elevated during treatment. The use of BKC in this respect has declined because of its high pH ŽChalor Limsuwan, Department of Fisheries, Thailand, personal comm... The use of bacterial remediation treatments is also commonplace in these systems in the belief that regular addition will help maintain low ammonia concentrations, reduce organic matter concentration, and improve the quality of accumulated sediment in the pond. Since monitoring of these two parameters is rarely performed, there is little evidence of any effect ŽFunge-Smith and Stewart, 1996.. The addition of carbon sources to production ponds has yielded interesting results in terms of modification of microbial communities in ponds. Additional carbon sources Žsugar, molasses, cane sugar, etc.. appear to increase activity of heterotrophic bacteria in ammonia removal, but in what form the ammonia is removed and whether this removal is sustained are unknown. Ideally ammonia would be de-nitrified to nitrate, subsequently converted to nitrogen and volatilized. This function is not performed by the bacteria usually present in bacterial remediation products. It appears that if sugar is not applied regularly the bacterial community is not sustained and ammonia can re-appear Ži.e., a bacterial bloom crash.. Applications of sugar are approximately 3–5 kg hay1 per time and this may last for several days to a week. The carrying capacity of a closed or semi-closed system appears to be exceeded at approximately 100–120 days post stocking resulting in stressful conditions, slow growth and disease outbreaks. Growth rates are slower in closed systems than open, with harvest sizes of approximately 40–50 pieces kgy1 . Harvesting is usually done when shrimp stop feeding or when farmers perceive that the shrimp are no longer growing. The slower growth rates and eventual overloading of the closed and semi-closed systems can be attributed to several causes. The build up of waste sediment releases increasing amounts of ammonia and organic matter. This in turn encourages heavy Vibrio and Zoothamnium numbers in the pond. The shrimp become stressed and therefore become more susceptible to these opportunist diseases. Phytoplankton bloom crashes are frequent in these systems and, whilst re-blooming is comparatively quick, there is still a period of stress to the shrimp. The control of accumulated waste in these systems is considered critical to the success of the closed system, as is the effective cleaning of the pond between cycles. This has resulted in very high water circulation Žusing aerators. in the production ponds to keep the feeding areas clear of detritus and mud. The solids budget above demonstrates that soil erosion is the most significant source of solids input to the system, thus 128 S.J. Funge-Smith, M.R.P. Briggsr Aquaculture 164 (1998) 117–133 heavy water circulation results in high sediment accumulation and increased suspended solids in the water. The heavy erosion of soil from the pond bottom and its combination with detritus and faeces is potentially a method by which organic material is removed from the pond system. By consolidating the highly organic detritus in a low organic sediment the rate of diffusion out of the sediment could be slowed down. Many farmers are all too aware of the problems with ammonia and disease they can cause if this sediment is disturbed Ži.e., when aerators are moved.. Many farms employing low water exchange systems are still unable to maintain clean feeding areas due to inadequate water circulation or poor aerator position. Deep ponds Ž) 1.5 m. are difficult to circulate using paddlewheels and airjet aerators have caused problems with erosion. Removal of accumulated sediment during production is an option ŽHopkins et al., 1995. that is unknown in Thailand, although it shows great potential for lowering organic loading in these systems. The problem with removal of the accumulated sediment is that once it has accumulated it loses its fluid properties, becomes gel-like and cannot be pumped out. A second problem is too frequent pumping would require addition of water to the pond. 4.2. Lined ponds One method by which many problems of earthen ponds Žespecially low water exchange systems. can be avoided is the use of pond lining. Full pond liners Žbitumen impregnated geotextile. have great potential in separating the pond bottom from the water column at the Tinsulanonda Songkhla Fisheries College. This eliminates soil erosion and reduces the accumulation of sediment in the centre of the pond, resulting in a larger clean feeding area for the shrimp and quick but efficient pond cleaning Žtypically 1–2 days with bulldozerrbackhoe for earth pond, 3 h with hose for lined pond.. Very little waste is left in the pond after harvest and the dryout time required for earth ponds is not necessary. The characteristic of the small volume of accumulated sediment found in lined ponds is completely different for that of earthen ponds. Accumulated sediment in lined ponds is not consolidated Ž83% water vs. 62% water in earth ponds., remains extremely liquid and can be easily pumped out of the pond. The lack of soil in the accumulated sediment of lined ponds causes it to have a higher organic content Ž36%. than in accumulated sediments of an earth pond wex-mangrove 13% ŽTable 4.x. This high organic content is also reflected in the higher levels of leachable nutrients in the accumulated waste of lined ponds. The higher levels of labile Table 4 Accumulated sediment analysis Žmeans"s.d.. ŽFunge-Smith, 1996. Parameter Geotextile liner Mangrove soil Total ammonia–nitrogen Žmg kgy1 dry wt.. Nitrite–nitrogen Žmg kgy1 dry wt.. Dissolved reactive phosphorus Žmg kgy1 dry wt.. Water content Ž%. Organic content Ž%. 67.5"27.7 0.51"0.44 6.8"5.9 85.1"12.7 35.9"9.1 50.8"38.4 1.08"1.04 2.6"2.5 48.7"19.3 13.3"5.0 S.J. Funge-Smith, M.R.P. Briggsr Aquaculture 164 (1998) 117–133 129 ammonia and dissolved reactive phosphorus in the lined pond waste have implications for sediment management since these nutrients can act as fertilizers and, if in excess, stimulate overblooming in the pond. On the other hand removal of these nutrients can destabilize the phytoplankton bloom resulting in high water transparency. The former situation is becoming usual at the Fishery College. The high organic content of accumulated sediment from lined shrimp ponds can also be used as fertilizer after desalting ŽBergheim et al., 1993. Despite the promise that lined and partially lined ponds can improve water quality, production from these systems has not been fully convincing, possibly because new management techniques are required. One reason for the observed cannibalism and occasional high FCR in lined ponds, is the tendency for feeds to be rapidly carried to the centre of the pond with water circulation. The contribution of the pond soil and detrital feeding to shrimp nutrition and growth is uncertain in intensive ponds, lined ponds might limit the availability of some nutrients to the shrimps. By forming a barrier between the pond soil and the water, anaerobic conditions develop beneath the liner. Adequate drainage must be provided to allow exit of seepage water. Gas formation below the liner can cause it to float so weighting is also required Žtypically concrete strips or fencing posts.. If pond soil is potential acid sulphate, the conditions below the liner can become extremely toxic. In such situations, sand can be applied between the soil and liner to facilitate drainage. Care must be taken to avoid seepage from adjacent unlined ponds and canals. The principal drawback of liners in shrimp culture ponds is their high cost and relatively short service life. The lining material for ponds can cost between $1–3 my2 , and service life can be as short as 2 crops. More expensive liners last longer, but care must be taken to avoid deterioration from exposure to sunlight if there is extended time between crops. Ponds should be full of water even if there is no stock. Disposal of old lining materials will become a problem if widescale adoption occurs, since burning them appears to be the most likely method. 4.3. Recirculation and integration Recirculating farms exist in Thailand but appear to have production problems associated with salinity and disease transmission within the farm. In fully recycled farms, water is recirculated through production ponds and into settlement, treatment and storage reservoirs. If disease enters this system, it is very difficult to isolate and treat parts of the system. The trend now is to use a limited water exchange system for the production ponds, replacing water loss from treated reservoirs. This allows individual ponds to be controlled if there is a disease problem. The large water area required for these farms causes high salinity in the dry season, such that recirculated farms are filled during periods of low salinity. Salinity can then increase through the dry season until harvest. Second cropping is difficult since low salinity water is not available, but losses from the system can be replaced with full strength seawater, and by the end of the second crop lower salinity water is available. Systems such as this can only be used when the farm is situated in estuaries or where river water is available. 130 S.J. Funge-Smith, M.R.P. Briggsr Aquaculture 164 (1998) 117–133 Integration of other species such as fish, molluscs and seaweeds in the recirculating system is not possible until the problem of heavy solids loading in effluent water can be resolved. There are additional engineering problems when the farm cycle of four to five months is not sufficient to produce a crop from the integrated species. Tilapia’s nest building activity can disturb sediments, and the pseudofaeces from bivalves can cause self-fouling and severe sediment problems in areas where they are cultured. Plankton grazing fish species can be used to control phytoplankton. Mangrove oysters require a diurnal tidal rhythm to grow effectively and cannot be flooded 24 h a day. Seaweeds need cleaning every other day by shaking off the adhering sediments. Both bivalves and seaweeds are extremely effective at removing solids from the water, bivalves by filtration and seaweeds by trapping solids in the mucilage on the thallus surface. It appears that the obstacles to successful integration lies in the solution of some engineering problems and finding the potential markets for these low value species. It is worth noting that if pond lining becomes widespread the lower inorganic solids loading may enable successful integration of other species. 4.4. Low salinity Recently, low salinity farms have increased in number dramatically due to the establishment of closed system culture techniques. Clay ponds inland are filled to one third of its capacity with sea water that is transported from the coast. The ponds are then filled with fresh water and the resulting salinity is approximately 5–10‰. Alternatively farms on inland estuaries that have access to very low salinity water can also now farm shrimp. The problems of these farms are similar to the closed system ponds with respect to sediment accumulation and heavy phytoplankton blooms Žalthough the higher salinities favour dinoflagellates and low salinities tend to give rise to blue–green algae.. Low salinity ponds do not have a problem with increasing salinity, but if culture takes place during a season when there is significant amount of rainfall, salinity will decrease in the ponds and give rise to problems. If shrimp of appropriate age, then they can tolerate salinities as low as 2‰ but this is not advisable for small animals. Seasalt placed in bags in front of the aerators can slightly increase the salinity if necessary. Buffering in this system is low due to low salinity and limes or bicarbonate can be used to boost this. Generally growth rates are very fast in these systems and FCR can be as low as 1.3–1.4 ŽPornlerd Chanratchakool, Thai Department of Fisheries, personal comm... However, production cannot usually be extended beyond 90–100 days, so shrimp are still harvested when they are still quite small Ž50 pieces kgy1 .. The occurrence of blue–green algae may result to bad smell or poor taste of the shrimp but this rarely happens. Middlemen often complain about these to be able to buy at lower price. 4.5. The future? The range of shrimp farming systems and techniques currently available offer solutions to many of the problems within the shrimp industry. These solutions can improve the environment both within and outside the rearing pond environment. The biggest threat facing the shrimp culture industry is its size and the density of farms in S.J. Funge-Smith, M.R.P. Briggsr Aquaculture 164 (1998) 117–133 131 shrimp culture areas. The high intensity production and high concentration of animals offer the ideal environment for the evolution of new diseases Žespecially viruses.. Although we may be better able to control the environment within a farm, there is still constant threat of disease introduction from external sources. Viral disease transmission via postlarval transfer is currently acknowledged to be a possible route of introduction to farms and between shrimp producing countries. It is important therefore, to control broodstock and postlarval transfer between countries. Research on captive breeding of P. monodon will allow genetic improvement of stocks and also the production of disease freerhigh health stocks. The reduction of pathogenicity of viral diseases is likely to be more rapid than our ability to produce disease-resistant shrimp. However, screening postlarvae and water to prevent entry of disease can be an effective method in limiting transmission. The production of healthy postlarvae is of paramount importance in many countries due to broodstock shortages and poor post-larval quality and in itself is still a major economic and production constraint. Within areas with established shrimp farms, it is important to ration the flow of influent and effluent water ŽNACA, 1994.. This is essential if disease is to be excluded from farms and good environmental quality maintained. Rationing can take the form of shared inlet and outlet canals, effluent settlement and storage and settlement of harvest effluent. Waste removal from ponds, though largely innocuous Žalthough saline., can seriously degrade water bodies if dumped on land. Since most shrimp diseases have a strong environmental component in their expression it is important that shrimp farmers are made aware of the effect of their actions on the pond environment and on the environment surrounding their farms. This is of great importance to the industry since regulation through policing is rarely effective due to lack of resources and farmer resistance. The dissemination of reliable information to shrimp farmers is vital since the principal source of information regarding culture methods are the retailers of feed and chemicals. The vested interest of these commercial organizations often results in inappropriate culture practices. Aquaculture production methods that rely heavily on chemical intervention can be viewed as stressful and therefore undesirable. Intensive systems utilizing high rates of feed application are often undesirable due to the requirement for close control of the pond environment. This is rarely achieved and thus the system is wasteful and often susceptible to disease. The development of semi-intensive shrimp farms is often seen as the most sustainable method of shrimp farming. However, the land requirement of this system can result in the disruption of huge areas of coastal zone. In some countries, a more limited, but more intensive form of shrimp culture might be appropriate. Unfortunately, the investment in such systems is often prohibitive and the skills required to run such farms are lacking. Shrimp farming is now a feature of many tropical coastal environments and affects all communities dwelling in the vicinity. Due to the dramatic effect of shrimp farming on coastal economies, collapse of farms in an area can seriously disrupt local economies. This is often due to the loss of existing livelihoods that cannot be replaced after shrimp farms are developed. Whilst it is often desirable to generate income in coastal areas, this has often been at the expense of the inhabitants of that area. The labour requirement of shrimp farms and processing plants is only useful if the demand is sustained. To prevent 132 S.J. Funge-Smith, M.R.P. 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