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3. Coastal aquaculture and the environment: The context

1. Aquaculture interacts with the environment. It utilizes resources and causes environmental changes. Most interactions have beneficial effects. In this section a brief overview follows on the benefits and on the potential adverse effects of coastal aquaculture (see sub-sections 3.1 and 3.2). Coastal pollution implications for coastal aquaculture are then considered (3.3). Last, social implications of coastal aquaculture development are highlighted (3.4).

3.1 Benefits of Coastal aquaculture

2. Generally, the socio-economic benefits arising from aquaculture expansion include the provision of food, contributing to improved nutrition and health, the generation of income and employment, the diversification of primary production, and, increasingly important for developing countries, foreign exchange earnings through export of high-value products (UNDP/Norway/FAO, 1987; Schmidt, 1982).

3. Aquaculture is also being promoted for its potential to compensate for the low growth rate of capture fisheries. Stocking and release of hatchery-reared organisms into inland and coastal waters support culture-based fisheries (Larkin, 1991).

4. Sustainable development of aquaculture can contribute to the prevention and control of aquatic pollution since it relies essentially on good-quality water resources.

5. Culture of molluscs and seaweeds may in certain cases counteract processes of nutrient and organic enrichment in eutrophic waters. Conversely, productivity of oligotrophic waters may be enhanced due to the nutrient and organic wastes released from aquaculture farms.

6. Aquaculture can contribute to rehabilitation of rural areas through re-use of degraded land.

3.2 Potential Adverse Effects of Coastal aquaculture

7. After a short description of the key areas of ecological concern (GESAMP, 1991c; Weston, 1991; Gowen et al., 1990; UNEP, 1990a; Pullin, 1989; Rosenthal et al., 1988), the main potential adverse effects of the farming of seaweed, shellfish and fish are addressed (Braaten and Hektoen, 1991; Pillay, 1990; Chua et al., 1989; Baluyut, 1989; Iwama, 1991).

3.2.1 Key areas of ecological concern

Nutrient and organic enrichment

8. Many aquaculture operations invariably result in the release of metabolic waste products (faeces, pseudo-faeces and excreta) and uneaten food into the aquatic environment. In general, the recipient for soluble waste is the water column and the recipient for the organic waste is the sediment.

9. The release of soluble inorganic nutrients (nitrogen and phosphorus) has the potential to cause nutrient enrichment (hypernutrification) possibly followed by eutrophication (increase of primary production) of a waterbody. Related changes in phytoplankton ecology may result in algal blooms, which can be harmful to wild and farmed organisms. However, there is no evidence that algal blooms have been caused by coastal aquaculture.

10. The largest proportion of solid wastes released, which is predominantly organic carbon and nitrogen, settles to the seabed in the immediate vicinity of the farm. Organic enrichment of the benthic ecosystem may result in increased oxygen consumption by the sediment and formation of anoxic sediments, with, in extreme cases, outgassing of carbon dioxide, methane and hydrogen sulphide; enhanced remineralization of organic nitrogen and reduction in macrofauna biomass, abundance and species composition.

11. There is evidence of very localized effects of reduced concentrations of dissolved oxygen in bottom and surface waters close to farm sites which are due to the considerable biochemical oxygen demand of released organic wastes and the respiratory demands of the cultured stock.

Degradation of wetland habitats

12. Coastal wetlands such as mangrove swamps are amongst the most productive ecosystems sustaining the ecological integrity and productivity of adjacent coastal waters, and are important breeding and nursery grounds for many commercially exploited fish and shellfish species. Several tropical countries have lost extensive mangrove areas due to clearing and conversion to fish and shrimp ponds, often accompanied by salinization and acidification of soils and aquifers.

Use of chemicals

13. A variety of chemicals are used in coastal aquaculture. These include: therapeutants, disinfectants, anaesthetics, biocides, hormones and growth promoters to control predators, prevent and control diseases and parasites and to alter sex, productive viability and growth of cultured organisms. Current concerns centre on: the longevity of bioactive compounds in animal tissues, the fate and effect of these compounds or their residues in the aquatic environment (e.g., toxicity to non-target organisms) and the stimulation of antibiotic resistance in microbial communities.

Biological interactions

14. The introduction and transfer of species and breeds for aquaculture purposes may alter or impoverish the biodiversity and genetic resources of the marine ecosystem through interbreeding, predation, competition, habitat destruction and, possibly, through the transmission of parasites and diseases.

15. Large-scale cultivation of bivalves in coastal embayments can interact with the marine food web by substantial removal of phytoplankton and organic detritus, as well as by competing with other planktonic herbivores.

16. Diseases may occur since many aquaculture practices and conditions around aquaculture operations can be stressful to the farmed stock. Stress increases susceptibility and predisposition to infectious diseases. Certain water quality conditions enhance virulence of potential pathogens. In the presence of stress and the appropriate pathogen, disease outbreaks can ensue. It has however been particularly difficult to show clearly that pathogens have been transmitted between cultured and wild organisms (Iwama, 1991; Baluyut, 1991).

3.2.2 Potential ecological effects of farming systems

17. It is useful to distinguish between extensive, semi-intensive and intensive farming systems when considering environmental effects of particular aquaculture operations. In extensive systems, cultured organisms are kept at low densities and may occasionally receive additional nutrition through fertilization. In semi-intensive aquaculture, cultured organisms are kept at higher densities than in extensive systems. The culture media are often fertilized and supplementary feed may be provided. In intensive aquaculture, cultured organisms are kept at high densities and feeding is regular, usually in the form of specially prepared/manufactured feeds. Figure 1 illustrates the relative contribution of natural food organisms and artificial feeds in the nutritional budget of fish and shrimp within extensive, semi-intensive and intensive farming systems (Tacon, in press).

Figure 1: The relative contribution of natural food organisms and artificial feeds in the nutritional budget of fish and shrimp within extensive, semi-intensive and intensive farming systems (from Tacon, in press).

Seaweed culture

18. Seaweeds, being autotrophic organisms, are at the lowest trophic level of the aquatic ecosystem; they remove nutrients from the water and produce dissolved oxygen in the daytime, but consume oxygen during the night. Seaweeds are farmed (Trono, 1986, 1990; Wu, 1990; Liana, 1991; Santelices and Doty, 1989; Bird, 1989; Tseng, 1987) in shallow nearshore waters using lines, nets which are either fixed to the bottom or attached to floating rafts and buoys. Increasingly, some seaweeds are also being farmed in land-based ponds, tanks and raceways.

19. The main potential effects of sea-based systems (see Tables 1 and 2) probably stem from the large surface areas required for viable seaweed culture, possibly affecting benthic communities and primary productivity in the water column (Phillips, 1990). Sedimentation of organic matter from off-bottom culture units may also result in changes in benthic communities, particularly where water current velocity has been decreasing.

20. So far, there is no reported evidence on ecological impacts of chemicals being used to remove fouling organisms and predators and to control diseases, such as for example p-aminobenzene sulphonic acid, chlorox or paraquat applied in Porphyra and Gracilaria culture (Chen, 1991). Inorganic fertilizers and, occasionally, organic manure, are applied to compensate for nutrient depletion. Fertilization of Laminaria culture in China has resulted in enhanced production of phytoplankton and invertebrates in the culture zones (UNDP/FAO, 1989).

21. There is also concern on the negative effects of transportation of seaweed species from one region to another. Inadvertently, Sargassum muticum has been introduced from Japan to Europe, where it has spread to the extent of causing significant problems to navigation in some areas (Rueness, 1989).

Bivalve culture

22. Bivalves, being sedentary organisms, require substrate for spat settlement and subsequent growth, during which time they filter-feed on phytoplankton, detritus, protozoans and bacteria. Culture of oysters, mussels, clams, cockles, scallops relies on naturally available phytoplankton and requires considerable acreage of intertidal areas and nearshore waters. Bivalves are cultured both on and off the bottom using a variety of substrates such as shells, tiles, stakes, ropes, trays, floating racks, rafts, etc. (Nash, 1991; Dore, 1991; Perez Camacho et al., 1991; Angell, 1991; Lovatelli, 1990; Cai and Li, 1990; Chew, 1989; Sitoy, 1988).

23. Large-scale cultivation of bivalves will consume substantial quantities of phytoplankton, particularly when there is a high density of culture units over a large area. This may show the effect of reduction of primary productivity in coastal embayments. In Japan, the culture of 50 000-60 000 oysters reduced the amount of seston (predominantly phytoplankton) by 76-95%. Suspended culture of green mussels in New Zealand was shown to remove up to 60% of the available food as the water flows through the farm, so primary production from an area many times that of the actual farm may be required to sustain maximum growth of farmed mussels (Hickmann, 1989). A mussel culture raft in the Spanish rias removes 35-40% of plankton and detritus (Figueras, 1989), whereby 30% of the carbon. 42% of the nitrogen, and 60% of the chlorophyll a of the paniculate organic matter present in the water is retained (Perez Camacho et al., 1991). However, it has also been suggested that primary productivity may be stimulated by an increase in the rate of nutrient cycling (Rosenthal et al., 1988) although field evidence of increased primary production in the vicinity of farms is lacking. Bivalve culture competes with other planktonic herbivores which has been shown for the Spanish Ria de Arousa where suspended mussel culture replaced copepods as the main pelagic grazing organism.

24. Bivalve culture structures modify current velocity and direction of water movements. In turn, these changes may alter patterns of erosion and sedimentation of paniculate matter. Reduced water flow may result in decrease of natural erosion by wave action, which in turn is followed by siltation and accumulation of suspended matter in cultured areas.

Table 1: Potential physical issues associated with seaweed culture indicating their potential positive and negative effects (from Phillips, 1990).




Cleaning and preparation of culture areas
* > * *

Improved production and management

Potential loss of native species and habitat diversity

Routine management (weeding, harvesting)
* > * *

As above

As above

Shading by growing seaweed
* > * *

Reduced competition

Reduced water column and benthic production

Attenuation of waves and water currents
* > * * *

Shelter for sensitive species

Increased sedimentation

Aesthetic issues
* > * * *

Enhanced coastal productivity in degraded ecosystems

User conflicts
Loss of resource value

* > * * *

Enhanced productivity of barren or degraded ecosystems

User conflicts (e.g., with fishermen)

Substrate area and volume
* > * * *

Enhanced productivity of barren or degraded ecosystems

Ecosystem changes

* = minimal effects
* * = potential for significant effects

Table 2: Ecological issues and seaweed culture, indicating their potential positive and negative effects (from Phillips, 1990).




Water quality
* > * *

Enhanced oxygen, removal of nutrients, seaweed production

Reduced coastal phytoplankton

Nutrient cycling


Fertilization and chemical treatments
* > * *

Seaweed production

Product quality

Enhanced polyculture production

Water quality changes

* > * *

Enhanced polyculture production (e.g., with mollusc)

Changes in benthic species and production

Water column productivity
* > * * *

Enhanced production of invertebrates and finfish

Changes in community structure

Shelter of fish fry


* = minimal effects
* * = potential for significant effects

25. In addition to these physical effects, bivalves produce pseudofaeces and faeces (termed biodeposition) which constitute organic-rich particulate waste from bivalve culture. For example, in Hiroshima Bay a raft holding 420 000 oysters generates 16 metric tons of faeces and pseudofaeces over a 9-month growing season, which may - with about 1 000 rafts in operation -have a major impact on sediment deposition in the bay. Culture of oysters on tables in France recorded outputs of 7.6-99 g C/m2/day, depending on season and plankton production. In Sweden, sediment deposition beneath a 100-t mussel operation was estimated to be 7 kg dry matter/m2 (equivalent to 1 kg C/m2) during a farming period of 1.5-2 years which was due to a sedimentation rate of up to 3 g C/m2/day (Dahlback and Gunnarson, 1981). Thus, for a farm covering an area of 1 500 m2, the sedimentation of dry matter would amount to about 10 t, and sediment under the rafts would accumulate to about 10 cm per farming season (Sweden, 1983). Data on waste production, sedimentation and sediment accumulation associated with mussel farming are summarized (NCC, 1989) in Tables 3 and 4. About 30% of the oyster and mussel farms of France face problems of sedimentation, forcing occasional relocation and abandonment of the beds (Weston, 1991).

26. The deposition of particulate organic wastes can result in physico-chemical changes of the substrate, particularly in the immediate vicinity of the culture site. The enrichment of the sediment with organic material stimulates microbial activity resulting in desoxygenation of the substrate and bottom waters due to reduced interstitial oxygen concentrations and increased oxygen consumption, increased sulphate reduction, increased denitrification, and increased release of inorganic nutrients such as nitrate, nitrite, ammonium, silicate and phosphate from mussel beds (Smaal, 1991). The regeneration of potentially limiting nutrients may increase primary productivity.

27. Benthic communities beneath suspended culture may be affected in various ways (Kaspar et al., 1985). Macrofauna may be lacking entirely in the area directly under the culture site. Around the culture site, species richness is reduced and opportunistic enrichment-tolerant species become predominant. Crabs changed their diet and became abundant as a response to mussel culture in northwest Spain (Freire et al., 1991), where transport of organic material derived from farms was shown to enhance benthic macrofauna biomass beyond the area of the coastal embayment. Based on a literature review, Gowen et al. (1988) summarized the possible ecological effects of mussel raft farming (see Figure 2).

28. Introductions of bivalves may have negative ecological effects (Chew, 1990), particularly when parasites and diseases are also introduced. The re-introduction of the European flat oyster (Ostrea edulis) to Europe from North America resulted in the spread of Bonamia ostreae, a blood-cell parasite on oysters, which devastated the European flat oyster industry. Two predators on bivalves, the Japanese oyster drill (Ceratoderma inornatum) and the oyster flatworm (Pseudostylochus ostreophagus) as well as the parasitic copepod Mytilicola orientalis, that can greatly affect condition of several bivalve species, were introduced together with the Pacific oyster (Crassostrea gigas) from Japan to North America. Also with introductions and transfers of mussel species, there is risk of spreading infectious diseases and parasites that are harmful to mussels and other bivalves (Bower and Figueras, 1989).

Shrimp culture

29. Most shrimp culture is carried out in earthen ponds, although pens and cages are also used in some cases (Tookwinas, 1990; Walford and Lam, 1987). Shrimp ponds may cover extensive coastal areas which have often required the conversion of rice paddies, salt pans, coconut and sugar plantations, abandoned lands, and mangrove forests (Chong, 1990a; Aitken, 1990).

30. Although there is no doubt that substantial areas of virgin mangrove swamps have been cleared for shrimp pond construction (Terchunian et al., 1986; Kapetsky, 1987a), it is important to recognize that mangrove ecosystems have also been utilized for other purposes, such as forestry, agriculture and fishpond culture (Andriawan and Jhamtani, 1989; Soemodihardjo and Soerianegara, 1989; Zamora, 1989; FAO, 1985a; Neal, 1984).

Table 3: Faecal waste production and sedimentation from bivalve farming (from NCC, 1989), References cited in NCC, 1989.

species and system

faecal production


Mytilus galloprovincialis

mg DW/individual/24h

Arakawa et al. (1971)

M. edulis
natural shore population

1.76 g DW/gDW mussel/yr
0.13 g C/gDW mussel/yr
0.0017 g N/gDW mussel/yr
0.00026 gP/gDW mussel/yr

Kautsky and Evans (1987)

M. edulis

9.5 kg carbon/m2/yr
1.1 kg nitrogen/m2/yr

Rodhouse et al (1985)

M. edulis

0.88 kg carbon/m2/yr

Rosenberg and Loo (1983)

M. edulis

27 g carbon/m2/24h

Cabanas et al (1979)

M. edulis

2.4-3.3 g carbon/m2/24h
1.7 (ref. Station)

Dahlback and Gunnarsson (1981)

M. edulis

0.5-2.5 g carbon/m2/24h

Tenore et al (1982)

Table 4: Sediment accumulation below bivalve farms (from NCC, 1989). References cited in NCC, 1989.

species and system


current velocity

sediment accumulation


M. edulis


"very weak"

7-30 cm

Weston (1986a)

M. edulis
long lines


~ 3 cm/s

10-15 cm

Dahlback and Gunnarsson (1981)

M. edulis

no data

>1 cm/yr

Misdorp et al (1984)

M. edulis


up to 200 cm/s

no sig. biodeposits mussel shells present

Rodhouse et al (1985)

M. edulis
2 long lines
3 raft sites

"strong currents"

no sig. biodeposits mussel shells present

Earll et al (1984)

Figure 2: Summary of the possible ecological effects of mussel raft farming. Note: some effects are contradictory, and not all effects will be seen at one site (redrawn from Gowen et al., 1988)

31. Removal of mangroves for pond culture can significantly affect shoreline configuration and coastal erosion patterns, generation and cycling of nutrients in coastal areas as well as habitats of many - also commercially important - species, which use the intertidal ecosystem as breeding, nursery and feeding grounds (Honculada Primavera, 1991; Saclauso, 1989; Matthes and Kapetsky, 1988). Aquatic production patterns may shift from detritus-based to plankton-based food paths, depending on the mangrove area lost and the amount of degradation (Kapetsky, 1987b).

32. Ponds reclaimed from mangrove swamps exhibit typically acid sulphate soils (Pedini, 1981; Simpson and Pedini, 1985, 1987). Oxidation of pyrite (FeS2) occurs during pond-bottom drying which results in the release of sulphuric acid into the pond water and adjacent water bodies causing acidification and the generation of highly toxic soluble aluminium phosphate (Chua et al., 1989) and gill-clogging ferric hydroxide (Singh, 1987). Serious problems of sedimentation and siltation can be expected when mangrove soil in ponds is replaced with soil from upland areas, and dumped into nearby mangrove forests as practised in some projects in Thailand (Chantadisai, 1989). Long-term application of lime to neutralize acid sulphate soils, coupled with fertilization practices, however, may harden pond-bottom soils, rendering them less suitable for shrimp pond culture.

33. Only rather extensive culture methods can be used appropriately in the intertidal mangrove zone, since they rely on tidal flow for supply and exchange of water. Intensive culture methods usually requiring more than 1 m of pondwater depth will also have to allow for pumping for complete drying during pond preparation if located in the intertidal mangrove zone. Poernomo (1990) compares economic and environmental suitability of intertidal and supratidal zones for the various culture systems (see Figure 3 and Tables 5 and 6).

34. Construction of channels for water supply and drainage and pumping of brackishwaters inland results in hydrological changes, siltation and saltwater intrusion (Mahmood, 1987; Cholik and Poernomo, 1987). Pumping and trucking of saltwater to inland backyard hatcheries may affect groundwater quality (Yap, 1990). Abstraction of groundwaters for freshwater supply of intensive pond culture may also result in the salinization of these emptied freshwater aquifers, and certainly had severe consequences of land subsidence in Taiwan (Liao, 1989).

35. It can be expected that the use of fertilizers and feeds (trash fish, mussel meat, shrimp heads and formulated diets) to increase production (Kontara, 1988) will alter water and sediment quality in ponds and adjacent waters. Their excessive use carries the risk of increased nutrient and organic matter loading (Akiyama and Chwang, 1989).

36. Fertilizers, such as triple-superphosphate, urea, cow dung, chicken manure and rice bran (Apud et al., 1983; Chamberlain, 1991; Sin et al., 1989) which are used to promote growth of shrimp food organisms, may contribute to the nutrient and organic load. Use of feeds also generates nutrient and organic loads in form of dissolved metabolites (ammonia, urea and carbon dioxide) and particulate matter (uneaten feeds and faeces), which may result in the build-up of anoxic sediments and over-population of algae in ponds. Up to 30-40% of pond water volume is pumped and drained per day in semi-intensive and intensive pond systems to supply oxygen and to remove excess nutrients and organic matter. Water requirements are, therefore, considerable; for example, 11 000-21 430 m3/t production are required for semi-intensive culture and 29 000-43 000 m3/t for intensive culture in Taiwan (Chien et al., 1988; Wickins, 1986; both cited in Phillips et al., 1991).

37. Effluents discharged will reflect water quality conditions in the pond. Wide ranges have been recorded (Phillips et al., in press) for effluent water quality parameters during a five-month grow-out period at an intensive shrimp farm in Thailand (Table 7). A very high organic and nutrient load can be expected in effluents during harvesting, draining and cleaning of ponds, because of the additional discharge of material previously bound to sediment particulate matter. Unfortunately, there is a general lack of field-data specific to the amount and quality of effluent loadings from shrimp ponds as well as to related ecological effects on receiving waterbodies. However, the potentially adverse effects of the discharge of effluents rich in nutrients and organic material may include increased sedimentation and siltation, hypoxia, hypernutrification, and alterations of productivity and community structures of benthic communities.

Figure 3: Coastal zones for shrimp culture. I. Intertidal Zone; Mangrove virgin forest (A); Secondary forest (B). II. Supratidal Zone: Rice field (C); Coconut plantation (D). (from Poernomo, 1990)

Table 5: Types of coastal zones for shrimp ponds (from Poernomo, 1990)

Coastal zone

Tidal range

Soil properties

Construction work and cost**

Pond productivity

Soil treatment

Application of culture technology




Virgin mangrove forest

Submerged at LHWL



+ + + +

Very low initially

Reclamation (drying and leaching) before use


Secondary mangrove forest (shrub)

Submerge at LHWL



+ + +

Very low initially

Reclamation before use

Extensive to semi intensive

Grassy swamp

Submerged at MHWL

pyrite/clay loam


+ +

Low initially

Reclamation before use

Extensive to semi intensive

Coconut plantation

Submerged at MHWL along the ditches

pyrite/clay loam


+ +

Relatively low initially

Reclamation before use

Extensive to semi intensive


Rice field/dry land crope

Above HHWL

Clay loam to loam



High from the beginning

No specific treatment

Intensive only

Coconut plantation

Above HHWL

Sandy clay loam



High from the beginning

No specific treatment

Intensive only

Grass and others

Above HHWL

Sandy loam Loamy Sand



High from the beginning

No specific treatment

Intensive only



Porous. No good for tambak




* Soil pH after drying
* * + Very easy, least cost
+ + Easy, low cost
+ + + Difficult, high cost
+ + + + Very difficult, very high cost

Table 6: Comparison of intertidal and supratidal zones when developed for shrimp tambaks (from Poernomo, 1990)

Parameters and stage of development

Intertidal zone

Supratidal zone

Land type

Mangrove and grassy swamps

Irrigated and dry flat land


Intertidal, between MHWL and MLWL

Above high tide, less than 2m above MHWL

Soil type

Mostly potential acid sulphate soils (contains high pyrite and peat to some extent)

Generally does not contain acid sulphate soil

Land cost

The undeveloped areas are government owned, can be leased with very low cost

Land is mostly privately owned, or claimed and cultivated, cost high to very high

Land clearing

Very costly for forested area

Very minimal

Construction coast

Low for extensive ponds with shallow and large sized ponds by very high for intensive ponds

Only for intensive ponds. Low for earthendike ponds, high for concrete dike ponds

Development work

Rely mainly on manual labor, mechanisation limited, time consuming

Largely mechanised. Manual only for little finishing work. Construction work can be completed in short time

Pre-production pond conditioning

Through reclamation. Several months are required if acid sulphate condition exists

Production operations can be started immediately after construction finished

Pond preparation

Pond bottom drying is a chronic problem. Becomes worse if dike quality is not good

Complete drying easy. Time for pond preparation much shorter

Pond filling

Through natural gravity during high tide. Minimal pumping in semi-intensive culture

Fully relies on pumps regardless of level of culture technology and stocking density applied

Pest control

Difficult to control the entry of noxious extraneous organisms

Much less entry unwanted extraneous organisms, and easier to control if any


Time consuming and very much depends on tide condition. It may affect product quality

Fast, can be done any time

Environmental impact

Destruction of mangrove swamp environment if not strictly controlled

Landward intrusion of sea water and salination of land and ground water resource

Table 7: The ranges of effluent water quality recorded at an intensive shrimp farm in Thailand during a five-month grow-out period; data from C.K. Lin (from Phillips et al., in press)

Pond size (ha)

0.48 - 0.56

Pond depth (m)

1.5 - 1.8

Salinity (ppt)

10 - 35

Temperature (°C)

22 - 31


7.5 - 8.9

Total phosphorus (mg/l)

0.05 - 0.4

Total nitrogen (mg/l)

0.50 - 3.4

Total ammonia (mg/l)

0.05 - 0.65

Dissolved oxygen (mg/l)

4.0 - 7.5

Chlorophyll a (m g/l)

20 - 250

Total suspended solids (mg/l)

30 - 190

Water exchange frequency (%/day)

5 - 40

38. A variety of chemicals is used in shrimp aquaculture (Sunaryanto, 1988; Baticados et al., 1990; Liu, 1989), Antibiotics such as chloramphenicol, oxytetracycline, furazolidone, streptomycin, nitrofurazone, are applied in shrimp hatcheries and grow-out ponds to treat and to prevent outbreaks of disease. Over-use of antibiotics (Brown, 1989) could be very dangerous due to the potential generation of drug-resistant shrimp pathogens (e.g., Vibrio) and the risk of transfer of drug resistance to human pathogens. Spread of drug-resistant strains is probable in shrimp culture areas with a high concentration of farms. Abuse of antibiotics in Taiwanese hatcheries resulted in poor resistance and survival of shrimp under grow-out conditions (Lin, 1989). Abuse of formalin and malachite green is known to be harmful to algae and zooplankton, including penaeid nauplii. Unwanted species are being eradicated by applying copper sulphate as algicide, plant-based biodegradable piscicides, such as tobacco dust (nicotine), tea seed cake (saponin), derris root extract (rotenone); as well as organo-pesticides as molluscicides, such as chlorinated hydrocarbons (DDT, endrin, aldrin, thiodan) and organotins (brestan and aquatin). These organo-pesticides are of particular concern due to their toxicity and persistence, and their potential implications for product quality and human health. No ecological effect has so far been reported for disinfectants and chemicals for water and soil treatment such as with sodium hypochlorite, benzalkonium chloride, calcium carbide, Na-EDTA, and zeolites.

39. There is growing evidence that some shrimp pathogens such as the infectious hypodermal haematopoietic necrosis virus (IHHNV) and monodonbacilovirus (MBV) have been disseminated through introduction and transfer of commercial shrimp species. Outbreaks of shrimp disease have been also attributed to excessive intensification and deterioration of water and sediment quality in shrimp ponds and adjacent waters due to waste overloading. Soft-shell syndrome, red disease and blue shrimps appear to be linked to the acid-sulphate soil problem (Baticados et al., 1990; Brock, 1991), although conclusive evidence is still lacking. Due to disease and pond management problems, resulting in production losses, farms have been abandoned and relocated. In this connection, Lin (1989) has summarized the causes for the collapse in 1988 of the shrimp industry in Taiwan (see Annex 1).

40. A further biological implication of shrimp culture refers to the required seed and feed inputs. Even though stocking of hatchery-reared postlarvae is increasing, wild-caught postlarvae are often preferred (Hirono and Van Eys, 1990), which may lead to overfishing of postlarvae, and, to the discarding of an estimated 10 kg of larvae and fry of other species for 1 kg of shrimp larvae harvested (Silas, 1987).

41. In 1988, shrimp farming in Asia absorbed 180 000 t of fish meal which is used for compound feeds; and about 1.1 million t of shrimp feed may be needed by the year 2000. This increasing demand may eventually encroach on species caught for human food as well as on juveniles of these species (New and Wijkstrom, 1990).

Finfish culture

42. Most coastal finfish culture is carried out in ponds, pens, cages, tanks and raceways (Chua, 1982; Padlan, 1982; Beveridge, 1987).

43. Extensive land degradation, especially of mangrove areas, has been experienced with the farming of herbivorous and detritus-feeding fish species such as milkfish and mullets in brackishwater ponds even before shrimp culture started to expand. Brackishwater milkfish/mullet culture also faces potential ecological problems related to acid-sulphate soils, deterioration of water and sediment quality, overfertilization and chemical usage (organotinsas molluscicides), which may severely affect adjacent mangrove ecosystems (Poernomo and Singh, 1982; Ti et al., 1982; Kapetsky, 1982).

44. However, potential ecological problems related to nutrient and organic enrichment within and outside the culture unit are more likely to be found with semi-intensive and, in particular, with intensive farming of carnivorous fishes where provision of feeds is required (Mok, 1982). Nutrient and organic wastes, in dissolved and paniculate forms, stemming from uneaten food and excreta, are generally characterized by an increase in suspended solids (SS), biochemical oxygen demand (BOD), chemical oxygen demand (COD), and content of carbon, nitrogen and phosphorus. Unfortunately, most of the available information on wastes released from fishfarms relates to temperate species.

45. The following are examples of effluent loads from land-based fishfarms. The yearly oxygen demand and nutrient release per ton of rainbow trout produced at a Danish pond farm was reported to be 300 kg BOD5/t/y, and 10 kg Total-P/t/y and 81 kg Total-N/t/y (Sweden, 1983). For tanks stocked with adult Atlantic salmon, a range of values was found (Bergheim and Forsberg, 1992) for effluent loadings expressed as g/kg fish/24 hours: 0.5-1.4 g suspended dry matter (SDM), 0.01-0.05 g total phosphorus (TP), 0.15-0.30 total nitrogen (TN) and 0.1-0.2 g total ammonia nitrogen (TAN), with a food conversion ratio of 1.0-1.2 dry feed per kilogramme fish weight gain. The results of this study are shown in Figure 4, where effluent loading is related to the amount of food given.

46. Rosenthal et al. (1988) found wide differences when summarizing estimated loads from various intensive trout farms using pond and tank systems (Table 8). More recent comparative analysis (Beveridge et al., 1991) on effluent quality in freshwater salmonid aquaculture (Table 9) confirms the variability of data for effluent loadings. It is concluded that there are many variables governing effluent quality, including species, size, method and intensity of culture, management, temperature, mode of discharge and the extent of dilution or treatment prior to discharge.

47. Regarding the release of wastes from marine cage farms, estimates of solid waste production from salmonid farms range from 0.3 to 0.7 dry weight of waste feed and faeces per kilogramme of fish produced (Weston, 1991). A typical Norwegian net pen farm with an annual salmon production of 200 t and well-controlled feeding techniques is said to provide an annual loading level of 2 t of phosphorus, 18 t nitrogen and 100 t oxygen consumption as BOD, (Seymour and Bergheim, 1991). Gowen and Bradbury (1987) predicted the generation of 19.4 t of organic carbon waste, 2.2 t of organic nitrogen waste and 4.0 t of soluble nitrogenous waste from a salmonid farm with an annual production of 50 t using 100 t of food per year (see Figures 5 and 6). Results from a field study indicated that up to 76% of the carbon and 76% of the nitrogen fed to cage-farmed salmon is released into the marine environment as solid and soluble waste, and, that the main factors controlling the level of enrichment of the benthos and water column are the size of the farm, husbandry and the hydrography at the site (Gowen et al., 1988).

48. It is evident that considerable water exchange rates are required for waste removal and oxygen supply in both land-based and sea-based fishfarms. The dilution/dispersal, areal distribution and sedimentation of the released waste and its potential ecological effects around fishfarms are determined by current velocities and depth of waterbodies receiving the waste load from pond/tank effluents and cages.

49. It has been argued that the discharge from fishfarms of dissolved inorganic nitrogen (ammonium, nitrate and nitrite), and, in the case of brackishwater environments, phosphorus, may affect the standing stock, species composition, or productivity of phytoplankton and macroalgae. Weston (1991), however, anticipated localized and measurable effects on microalgal communities only in cases of unusually dense culture activity in poorly flushed coastal embayments (see also Aure and Stigebrandt, 1990; Frid and Mercer, 1989; Turrel and Munro, 1989).

Figure 4: Example of effluent loadings from tanks stocked with adult Atlantic salmon (from Bergheim and Forsberg, 1992)

Table 8: Estimated loads from various intensive trout farms using pond and tank systems (from Rosenthal et al., 1988). References cited in Rosenthal et al., 1988.




Feeding rate
(%) bodyweight

Feed type

Net loading


Tanks and ponds

Brown trout
(biomass 2260 kg)

2.2-100 g


dry pellets manual + automatic

11.5 g COD/kg fish/24h
2.7 g BOD7/kg fish/24h
0.05 g Total-P/kg fish/24h
0.9 g SS/kg fish/24h

Bergheim et al. (1982)

Tanks and ponds

Brown trout
(biomass 7320 kg)

1.0-25.0 g + brood stock


dry + wet pellets, manual

75.3 g COD/kg/24h
83.3 g BOD7/kg/24h
0.43 g Total-P/kg/24h
0.24 g PO4-P/kg/24h
1.4-3.8 g Total-N/kg/24h

Bergheim et al. (1982)


Brown trout, rainbow trout
(biomass 2690 kg)

0.2-500 g


dry pellets manual

17.0g COD/kg/24h
7.1 g SS/kg/24h
0.45 g Total-N/kg/24h
0.08 g Total-P/kg/24h
0.05 g PO4-P/kg/24h

Bergheim et al. (1982)

Tanks and ponds

Brown trout
(biomass 5970 kg)

1-550 g


dry, automatic

3.1 g COD/kg/24h
1.6 g BOD7/kg/24h
1.2 g SS/kg/24h
0.13 g Total-N/kg/24h
0.05 g Total-P/kg/24h
0.03 g PO4-P/kg/24h

Bergheim et al. (1982)

Ponds 12 700m2

Rainbow trout

35-150 g
500-2000 g


dry pellets

0.4-0.8 g Total-N/kg/24h
0.05 g Total-P/kg/24h
1.6-4.6 g BOD7/kg/24h

Bergheim + Selmer - Olsen (1978)


Rainbow trout

2.0-300 g


dry pellets, wet feed

0.5-1.4 g Total-N/kg/24h
0.13-0.18 h Total-P/kg/24h
1.9-5.7 g BOD5/kg/24h

Markham (1978)

Table 9: Loadings of suspended solids (SS), nutrients and biochemical oxygen demand (BOD) from freshwater salmonid culture (from Beveridge et al., 1991). References cited in Beveridge et al., 1991.






Total N


Total P


(a) g kg fish-1 day-1





































(b) kg ton fish-1 annum-1



























(c) g kg feed-1




























aBergheim et al: (1982); bClark et al. (1985); cButz and Vens Cappell (1982); dKorzeniewski et al. (1982); eAlabaster (1982); fSolbe (1982); gWarrer-Hansen (1982); hButz (1988); iMakinen (1988).

Figure 5: The estimated flux of carbon and nitrogen through a salmonid farm with an annual production of 50 t (from Gowen and Bradbury, 1987)

Figure 6: The estimated average flux of carbon and nitrogen through a salmonid farm (from Gowen and Bradbury, 1987)

50. The particulate organic matter released settles in the vicinity of the farm if the settling velocity of the particles is higher than the water current velocity. The solids falling to the seabed are enriched in carbon, nitrogen and phosphorus relative to the natural sediments (Holmer, 1991), possibly causing physico-chemical changes in sediments below or adjacent to fishfarming operations, including increase in organic carbon content, followed by increased sediment oxygen consumption rates and decreased sediment redox potentials (Brown et al., 1987), generation of hydrogen sulphide and methane (Lumb, 1989), and increases in inorganic and organic nitrogen (Kaspar et al., 1988), phosphorus, silicon, calcium, copper and zinc (Rosenthal et al., 1988). However, these physical and chemical effects have been reported to be limited only to the immediate vicinity of fish farms.

51. The areal extent of ensuing ecological effects on macrobenthic communities underneath and around cage culture can be described (NCC, 1989) according to the following patterns (see also Table 10): (a) lack of macrobenthos (the azoic zone, if present at all, usually only below cages), (b) dominance of enrichment-tolerant species (the opportunistic zone, covering an area of up to 30 m from the site) and (c) a gradual return to background conditions, normally occurring within 30 m of the farm, although effects may extend occasionally to up to 100 m off the farm site. Similar effects on the benthic community in the vicinity of discharge pipes from land-based fish farms can be expected, although no specific field-data were available.

52. Magnitude and areal extent of enrichment effects will generally depend on a variety of factors such as farm production characteristics, depth, bottom topography, current velocity and exposure to increased water movements (e.g., storms) which will determine the lateral spread of settling particles, organic input per unit area, and scouring and redistribution of bottom wastes (see also Hakanson et al., 1988; Lauren-Maatta et al., 1991; Kupka Hansen et al., 1991). The ability of the indigenous communities to assimilate and mineralize organic wastes may also be important.

53. A wide range of chemicals is used in finfish culture. These include therapeutants and biocides, vaccines, hormones, vitamins, flesh pigments, anaesthetics, disinfectants, water treatment compounds and chemicals present in materials used in the fabrication of aquaculture units. The use of chemicals varies greatly with species, intensity of culture and location. Table 11 indicates the diversity of methods and purposes of chemical applications. The amount used and mode of application will determine the extent of the ecological effects.

54. With regard to antibiotics (Grave et al., 1990), which are usually administered in feed, there is evidence that only 20-30% are actually ingested by the fish; thus, approximately 70-80% reaches the environment (Samuelsen, 1989), notably from uneaten medicated food. Oxytetracycline, for example, may be deposited in sediments, where it may remain in concentrations capable of causing antibacterial effects for 12 weeks after cessation of treatment (Jacobsen and Berglind, 1988). Antibiotics may spread to the present benthic fauna or to the wild fish by direct feeding on farm wastes. Antibiotic resistant strains may develop in aerobic and facultative anaerobic bacteria groups, which may affect rates and patterns of bacterial mineralization and decomposition in sediments (Holmer, 1991; Bjorklund et al., 1990).

55. The use of the organophosphorus compounds trichlorfon (Neguvon) and dichlorvos (Nuvan) for treatment of salmon lice has caused concern because of their potential toxicity to non-target organisms such as lobsters, crabs and mussels (Egidius and Moster, 1987), and larvae of herring and lobster (McHenery et al., 1991). However, predictions of assumed toxic effects still require data on inputs, chemical behaviour, degradation, persistence and distribution of the toxicant.

56. Antifoulants, such as tributyltin (TBT), which have been widely used to treat submerged structures and nets in cage and pen culture, are now much less commonly used as fears about the accumulation of organotin- and copper-based compounds in farmed fish flesh have grown (Beveridge et al., 1991).

57. The consequences of introductions and transfers of marine fish may be widespread and irreversible. Primary concerns with the introduction of Pacific salmon species (Oncorhynchus kisutch, O. gorbuscha and O. Keta) into Atlantic waters have been on the possible competition with Atlantic salmon (Salmo salar) in spawning streams and nursery areas, as well as on importation of diseases such as infectious haemapoietic necrosis (IHN) caused by a virus, and bacterial kidney disease (BKD) caused by Renibacterium salmoninarum (Sindermann, 1986). By 1987, Atlantic salmon in about 30 Norwegian rivers had been infested by Gyrodactylus salaris, an ectoparasitic trematode, which found its way to Norway via salmon smolts imported from Sweden in the mid-1970s (Folke and Kautsky, 1989). The introduction of "Tilapia mossambica" (Oreochromis mossambicus) is considered a "real nightmare" (Juliano et al., 1989) for Philippine brackishwater farming, due to competition for food with farmed milkfish.

Table 10: A summary of studies of the effects of marine cage culture on the macrobenthos (from NCC, 1989). References cited in NCC, 1989.

Species and system



Coho salmon cages, Puget Sound

High abundance and low diversity directly below cages, dominated by Capitella capitata

Pease (1977)

Yellowtail, Uchiura Bay, Japan

Increase in opportunistic polychaetes and decrease in relative proportion of molluscs and crustaceans with increased organic inputs

Kitamori (1977)

Atlantic salmon cages, Ireland

Azoic below cages, zone around cages dominated by Capitella capitata

Stewart (1984)

Atlantic salmon cages, Norway

Three farms examined:
1. low diversity and community dominated by opportunistic species.
2. biostimulated below cages
3. "minimal" effect below cages

Ervik et al. (1985)

Atlantic salmon cages, Scotland

Azoic zone below some farms

Earll et al. (1984)

Atlantic salmon cages, Shetland

Azoic zones below some farms. Zones below and outside cages dominated by Capitella capitata. Effects restricted to 40m of cages

Dixon (1986)

Atlantic salmon cages, Scotland

Azoic zone below cages, surrounded by C. capitata and Scolelepis fuliginosa out to 8m from the site. [enriched zone up to 25m and clean from>25m]

Brown et al. (1987)

Table 11: Chemicals used in aquaculture, FW = Freshwater; SW = Saltwater. Application methods: B = Bath; A = Addition to the system; F = Flush; D = Dip; I = Injection; S = Spray; T = Treated food (from NCC, 1989).







Acetic acid




Use with CuSO4 in hard water areas





165-250 ppm up to 1 hour, 20 ppm 4 hours use in sea cages as bath - common

Malachite Green

ectoparasites and fungus



Eggs and fish, 100 ppm 30 sec. 4 ppm 1 hour common in fw, occasional use in cages as a dye marker

Acriflavin (or Proflavine hemisulphate)

ectoparasites fungus and bacteria



Mostly for surface bacteria, fish and eggs occasional use only

Nuvan (dichlorvos)

salmon lice



1ppm for 1 hour, canvas round sea cage





Occasional alternative to formalin

Buffered Iodine




Use to disinfect eggs 10 mins 1000 ppm





Antibiotic widely used for systemic disease

Oxolinic acid




Antibiotic widely used for systemic disease

Romet 30 (Sulfadimethoxine and orthomeprim)




Antibiotic for systemic disease

Tribrissen (Trimethoprim/sulphadiazine)




Third most widely used antibiotic

Hayamine 3500




Quaternary ammonium compound used for treating bacterial gill diseases

Benzalkonium Chloride




Surface antibacterial; 'Roccel' (similar to above)

Chloramine T




As above, also effective for some protozoa


Vibrio Anguillarum vaccine



Not widely used

Enteric Redmouth vaccine



Widely used in trout culture

Aeromonas Salmonicida Vibro



Not widely used

Anguillarum vaccine


MS222 (tricaine methane-sulfonate)



Widely used approx 1:10,000 dilution




Widely used, requires acetone to dissolve

Carbon dioxide



Sometimes used at harvest


Calcium hypochlorite



General disinfectant for tanks etc.

liquid lodophore e.g. FAM30



For equipment and footbaths

sodium hydroxide



Most commonly used for earth ponds





Used in earth ponds

Potassium permanganate



Oxidizer and detoxifier

Copper sulphate



Algidde and herbicide

58. In a general review, Baltz (1991) concludes that more than 120 species of marine and euryhaline fishes have been successfully introduced around the world. Most introductions either did not establish populations, did not achieve their objectives if introduced deliberately, or often had deleterious effects if the species became established. Biodiversity of coastal ecosystems may be threatened due to extinction of native species and disruption of structure and function in natural communities. The extent of genetic interactions between cultured and wild populations will depend principally on the probability of breeding between strains, races or species (e.g., extent of escapes, ecological or behavioural barriers to "interbreeding") and the relative size of the breeding population of escaped fish in comparison to the wild breeding population.

59. An increase of abundance, biomass and diversity of fish and other species in the vicinity of fish culture structures, in particular cages and pens, can be expected as the culture site is likely to represent an area of increased food availability (uneaten food, dense macrobenthic populations) for wild fish communities.

60. Farming of carnivorous fish relies on fishmeal-based feed inputs, and it has been estimated that 5.3 t of fish is required to support 1 t of harvested cage-farmed salmon (Folke and Kautsky, 1989; Folke, 1988). With increasing demand for fishmeal-based feeds, aquaculture may in certain cases indirectly interact with natural fish populations.

Negative feedback effects

61. It is important to recognize that it is often aquaculture itself which is affected by ecological changes deriving from farming practices. For example, water currents may be reduced significantly due to farm structures (cages, pens, rafts, etc.), which may lead to increased deposition and accumulation of organic wastes underneath or around the farming unit, increase in siltation and water quality deterioration (e.g., increase in turbidity due to high content of suspended matter). In addition, oxygen supply may be reduced, and outgassing of hydrogen sulphide and methane from bottom sediments may occur which will further affect growth performance and increase susceptibility to disease (Lumb, 1989).

62. Pond culture which relies on tidal flow or pumping for water exchange may also face a steady increase of water quality deterioration. For example, total water exchange requirements of Indonesian intensive shrimp pond systems will often exceed the flow rate of the tidal creek that serves as the supply canal and drainage ditch. The net result is that instead of replacing waste with clean water, these farms are very often recycling waste water (Chamberlain, 1991). Extensive culture systems relying on the natural productivity of waters used may reduce or deviate water flow through farming structures and heavy siltation, thereby reducing the availability of food and nutrients.

63. Chemicals used may also present a potential risk to cultured organisms (Ackefors et al., 1990), and may result in contamination of aquaculture products which reduces product quality and consumer acceptance. The development of drug-resistant pathogens, resident (and possibly dormant) both within and around the farming unit, may have serious negative feedback effects on farm productivity. The over-use of chemicals in hatcheries may result in reduced fitness, poor growth and decreased survival rates during the grow-out phase. Pond soils may be rendered less suitable by excessive chemical treatment.

64. Predators such as fish and birds are attracted to the cultured as well as natural population of fish in the area and the ready supply of fish feed. There is the possibility not only that these predators damage and consume valuable fish, but also enhance disease in the area by serving as intermediate hosts in the life cycle of parasites. Predators as well as grazers on the epiphyton of farm structures can also damage netting or other enclosure material, resulting in escapes.

65. The magnitude of negative ecological feedback effects of coastal aquaculture practices may increase with expansion and/or intensification. An increase in the acreage and/or number of farming units (ponds, racks, rafts, cages, etc.) and farms may be followed by deterioration of required environmental quality within and beyond the aquaculture area.

66. As a result of expansion of farming systems relying on naturally available food and nutrients, the natural productivity of waterbodies in coastal areas may be exhausted. Large-scale coverage and degradation of tidal habitats, including mangrove areas, may also affect wild seed supply. Clearly, aggregations of farms will exhibit cumulative effects of waste release and increased oxygen demand. Negative feedback effects of siltation, turbidity, build-up of organic-rich sediments, hypoxic or anoxic bottom waters, toxic outgassing, spread of diseases, etc., may then affect all farms in the area, particularly when located in sheltered and shallow coastal embayments with low water exchange rates. Land-based farming systems have faced similar problems, such as, for example, intensive shrimp culture in Taiwan and Thailand where farms tend to cluster on suitable sites which resulted in very serious self-pollution problems (Csavas, in press). Csavas states that "drained water of one farm is often re-used by its neighbours, making thus water exchange in the heavily stocked ponds a futile exercise".

III.3 Degradation of Coastal Environments and Potential Effects on Coastal aquaculture

67. The coastal zone as an economic entity provides sites for a wide range of activities, such as agriculture (e.g., rice, coco palm, bananas), forestry (e.g., mangrove, nypa palm), fisheries and aquaculture, human settlements, manufacturing and extractive industries (e.g., sand mining, oil, minerals), waste disposal, ports and marine transportation, land transportation infrastructure, water control and supply projects, shore protection works, tourism and recreation. The multiple resource uses or activities in coastal areas may produce a variety of changes in environmental or socio-economic conditions, which in turn may result in an impact of social concern. Annex 2 gives an overview on issues of potential impact resulting from multiple uses of coastal resources. Figure 7 illustrates various sources of pollution within the coastal zone. Readers further interested in aspects of coastal and marine pollution are referred to: Phillips and Tanabe (1989); Gomez et al. (1990); Sen Gupta et al. (1990); GESAMP (1990b); UNEP (1990b); GESAMP (1990a); GESAMP (1987); GESAMP (1980); Maclean (1989).

68. It is important to recognize that in many coastal areas, pollution and habitat modification stemming from human activities other than aquaculture are increasingly affecting resource use productivity of aquaculture as well as limiting success and development possibilities of the aquaculture industry (Stickney, 1990; Chua and Tech, 1990; Menasveta, 1987).

69. The following are examples of potential pollution threats to coastal aquaculture (Chua et al., 1989). High organic and microbial loading in sewage discharged from densely populated urban and resort areas can contaminate cultured shellfish thereby rendering this aquaculture produce unsuitable for humans, particularly if consumed raw or partially cooked. As an example, coliform counts in excess of 1 000/100 ml have been recorded in Manila Bay, which is one of the major oyster and mussel culture sites in the Philippines. In 1979, an outbreak of gastro-enteritis was experienced in Singapore, which was traced to oyster meat imported from the Philippines. As a result of this incident, seafood demand from the Philippines declined. Heavy organic pollution due to effluents from Malaysian piggeries and Thai sugar mills, characterized by high biochemical oxygen demand, seriously damaged cockle beds and other cultured organisms. Thai shrimp and oyster farms were severely affected by liquid waste from a distillery.

Figure 7: Various sources of pollution within the coastal zone (from Chua et al., 1989)

70. Heavy metals found in industrial effluents may be found in the animals cultured in the receiving waters. In Jakarta Bay (Indonesia), the analysis offish and shellfish samples showed that WHO standards for heavy metals were exceeded in 76% of the samples tested for cadmium; 51 % for copper; 44% for lead; 38% for mercury; and 2% for chromium. Serious oil spills can cause large-scale fish kills, and obvious effects on aquaculture include the contamination of farming structures and tainting of farmed organisms.

71. High levels of pesticides, stemming from agricultural run-off, can be lethal to cultured organisms, while lower doses are believed to produce sublethal effects such as pathological changes in various organs.

72. The release of inorganic and organic nutrients into marine ecosystems can cause hypernutrification, and possibly phytoplankton blooms. Cultured fish can be killed by algal blooms through sudden water quality deterioration (suffocation due to gill damage and/or oxygen depletion) after collapse and decomposition of a bloom. In particular, bivalve culture is facing serious problems associated with the increasing occurrence of toxic phytoplankton blooms caused by a relatively small number of algal species producing a range of toxins, the effects of which include mortality of cultured stocks, as well as human illness and even death after consumption of contaminated bivalves (Shumway, 1990, 1989).

III.4 Social Implications of Coastal aquaculture Developments

73. The environmental impacts of and on coastal aquaculture may have serious adverse socio-economic and human health implications (Huss, 1991; Bernoth, 1991a).

74. There is concern that large-scale mangrove conversion for shrimp and fish farming in Latin American and Asian countries has affected rural communities which traditionally depended on mangrove resources for their livelihood (Chua, in press; Mena Millar, 1989; Nath Roy, 1984). According to Bailey (1988), "the expansion of shrimp mariculture into mangrove habitat generally involves the transformation of a multi-use/multi-user coastal resource into a privately owned single-purpose resource. Moreover, the costs of coastal ecosystem disruption for society may include coastal erosion, saltwater intrusion into groundwater and agricultural fields, and a reduction in supply of a wide range of valuable goods and services produced from the resources available in mangrove forests or other coastal wetlands."

75. Likewise, Smith and Pestano-Smith (1985) stated that large-scale aquaculture enterprises frequently displace small-scale fishermen and aquaculturists. Unfortunately, competition for land and water resources also results in use conflicts (Shang, 1990; Stansell, 1992), sometimes with ensuing violence, as seen between rice and shrimp farmers in Thailand (New, 1991).

76. Several economic disasters due to significant aquaculture production losses have been attributed to self-pollution as well as to increasing coastal water pollution which fueled disease outbreaks and harmful phytoplankton blooms (Rosenberry, 1990; Chua, in press; New, 1990a; Okaichi, 1991; Maclean, 1991).

77. Consumption of raw and partially cooked shellfish grown in coastal waters receiving high organic and microbial loadings from urban sewage effluents can result in severe consequences for human health, including gastro-intestinal disorders, gastro-enteritis, infectious hepatitis, cholera and typhoid fever (Shuval, 1986). Heavy metal pollution originating mainly from industrial discharges carries the risk of seafood contamination and human poisoning as experienced at Minamata Bay, Japan, with industrial effluents containing methyl-mercury (Piotrowski and Inskip, 1981; Huss 1988). Various forms of shellfish poisoning in humans such as PSP (paralytic shellfish poisoning), NSP (neurotoxic shellfish poisoning), DSP (diarrhoeic shellfish poisoning), ASP (amnesic shellfish poisoning) are occurring worldwide due to consumption of shellfish which accumulated phycotoxins stemming from toxic algal blooms. Effects of poisoning include gastro-intestinal disorders, respiratory paralysis, memory loss and death (WHO, 1984).

78. In summarizing, the potential negative implications of ecological degradation affecting directly or indirectly the socio-economic conditions within the environment of coastal aquaculture would include the following:

- decline in quality and quantity of food fish both cultured and captured,

- increased human health risks and reduced nutritional status,

- reduced consumer confidence and decreasing fish marketability within local, national and international environments,

- increasing resource-user conflicts and growing competition for markets and credits,

- decline and failures (collapse) of aquaculture enterprises and/or other fishery practices (e.g., artisanal fisheries) including the post-harvest sector, and

- social disruption within the rural environment following:

- displacement of traditional community-based activities in agriculture, forestry and fisheries;

- decreasing employment opportunities; shift towards unskilled and seasonal labour;

- marginalization of resident resource-users and non-resource users due to increasing income distribution changes;

- migration towards urban centres.

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