NEGATIVE IMPACTS OF COASTAL TROPICAL AQUACULTURE DEVELOPMENTS
by
Dr. Louis Landesman. 1994. World Aquaculture 25(2): 12-17.

Introduction

The 1980s witnessed a remarkable growth in shrimp farming, particularly in tropical regions of the world. The practice of culturing shrimp in ponds with artificial stocking of shrimp seed (postlarvae), feeding with specially formulated feeds and harvest for export to foreign markets is expanding both in Latin America and Asia. Tropical fish farming is also growing, particularly cage culture of marine fish. As of 1991, 750,000 tons of aquacultured shrimp were produced worldwide making up 30% of all shrimp consumed. It is projected that this will increase to 50% of world shrimp consumption by the year 2000 (Weidner 1992).

A long tradition exists of coastal aquaculture in southeast Asia, based primarily on the culture of milkfish, (Chanos chanos) in hand excavated coastal ponds (Chua 1987). Farmers in India and China have cultured shrimp in tidal impoundments on an extensive basis. This tradition of extensive mariculture depended on natural recruitment of shrimp and fish larvae, little or no fertilization or feeding, and low production costs. Yields were also low, typically 50 to 500 kg per hectare per year (Bailey 1986, Chamberlain 1991).

Over the last 60 years techniques have been developed to raise shrimp intensively in ponds (Weidner 1992). Developed first in Japan, and spreading later to Taiwan, China, Thailand and other countries, intensive shrimp culture controls the pond where shrimp are raised so as to optimize the environment. In this type of shrimp culture shrimp postlarvae produced in hatcheries are stocked into ponds where the water has been fertilized to create an algal bloom, aerated to maintain dissolved oxygen, and replaced regularly to prevent the buildup of metabolic wastes. The shrimp are fed formulated diets made from imported ingredients so as to produce rapid growth. Two harvests per year are normally produced in such ponds. Yields from this type of shrimp farming can vary from 5000 to more than 10,000 kg per hectare per year. In short the intensive shrimp ponds functions as a form of brackishwater feedlot for shrimp.

The aquaculture of shrimp is an important form of income for tropical coastal countries. For example Ecuador's shrimp exports, about 60% of which come from ponds, is the second largest foreign exchange earner after oil (Olsen 1989, Meltzoff, 1986). Shrimp exports provide the third largest source of export earnings for Bangladesh (Bashirullah,1989). For countries such as Bangladesh and the Philippines, shrimp culture has the potential of becoming the leading source of foreign exchange for these nations.

Shrimp Farming

Formerly all shrimp were captured in the wild. Traditional forms of aquaculture produced shrimp by impounding coastal marshes and mangrove swamps with levees and gates that allowed shrimp to enter with the tide. Subsequently they were netted on the falling tide or harvested by cast nets from the ponds. With the spread of refrigeration techniques it became possible to freeze shrimp and sell them on the world market. The reward for this was high enough to justify intensifying this form of shrimp farming through the use of hatchery produced shrimp post-larva, feed and aeration. Taiwan has gone the furthest in intensifying this form of shrimp farming. This intensive form of shrimp culture has the greatest impact on the environment (Chamberlain 1991).

Rapid expansion of shrimp farming took place in Latin America and Southeast Asia in the 80's. Estimated acreage of brackishwater coastal ponds in Southeast Asia are in excess of 400,000 hectares (Bailey 1992). However much of this acreage is utilized for the extensive cultivation of milkfish. I will now give a brief description of how shrimp are cultured intensively in Southeast Asia (specifically Indonesia).

Description of Shrimp Farming

Species cultured

The dominant species of shrimp cultured in Southeast Asia is Penaeus monodon. This is the species normally cultured in hatcheries and is normally the only species cultured intensively. Penaeus indicus and P. merguiensis are also cultured primarily in extensive shrimp culture systems.

Pond Construction - Site Selection

Pond location can be an existing extensive coastal fish pond or a newly excavated pond in a mangrove area. Normally the pond site will be located on an estuary or next to a coastline to provide a source of brackish or marine water. The pond usually dug to a depth of at least one meter. The pond levees are dug either by hand or by earth moving equipment. If the pond is excavated alongside an estuary or brackish water canal a screened gate allows water to enter and leave with the tide. Intensively managed ponds require the use of pumps to exchange water and to drain the pond during harvest (Boyd 1989).

Stocking with post-larvae

Ponds are stocked with hatchery produced postlarvae in the intensive style of shrimp culture. Extensively managed shrimp ponds continue to depend on wild caught postlarvae, either from tidal creeks, shorelines or by natural entry with the tides into the ponds. Due to the varying amount of postlarvae gathered in this way, stocking density can vary from less than 0.5 to more than 2 postlarvae per meter of pond surface. Intensive shrimp culture, on the other hand, requires hatchery produced postlarvae stocked at a density of up to 30 per meter squared.

Water management

Ponds are fertilized with urea and triple superphosphate. Intensive culture systems do not require much fertilization since the heavy feeding needed for the shrimp has sufficient nutrients present to maintain an algae bloom in the pond water. Both extensive and intensive ponds are treated with calcium carbonate (in the form of agricultural limestone) to help neutralize the pond bottom, which tends to become acidic anaerobic conditions (Boyd 1989).

Intensively managed ponds depend on diesel or electrically driven pumps to exchange water during the production cycle. Pumps are also necessary to drain the ponds for harvest. Aeration by means of paddlewheel aerators is essential to maintaining sufficient dissolved oxygen in the ponds. In very intensive systems, pond water is replaced by using fresh brackish or salt water. After draining the ponds for harvest they are allowed to dry completely and sometimes the bottom sediment is pumped out or removed manually (Chamberlain 1991).

Feeding

All intensive forms of shrimp culture depend on supplemental feeding to produce a dependable harvest. The natural fertility of brackishwater pond contributes significantly to shrimp growth in extensive systems but as the stocking rate increases its relative contribution to shrimp production decreases. Intensive systems stocked at 30 postlarvae per square meter are normally given artificial feed formulated with fish meal, tapioca flour, soybean meal, rice bran, or other local ingredients including a vitamin premix. In highly intensive systems this diet supplies all of the shrimps nutritional requirements. In extensive systems less supplemental feed is also used, usually inexpensive local ingredients derived from "trash" fish, mysid shrimp, snails, farm wastes etc.

Harvest

Harvesting systems depend on the design of the pond but in general the pond is drained as far as is possible. In deep ponds where pumps are available the pond is seined and the shrimp harvested by hand or by cast net. In shallow ponds cast nets or electrified seines may be the only methods used. When heavily stocked intensive ponds are drained, large amounts of turbid, fertile water are discharged into the receiving estuary or coastline.

Intensive systems of shrimp and fish farming can be highly profitable when the price of the product being cultured is high. However these systems are very vulnerable to disease outbreaks that can precipitously reduce production (Weidner 1992) and to price declines in the product being cultured. Extensive systems, while much less sensitive to disease outbreaks and price declines are also much less productive. Although only 10% of Indonesian shrimp farms culture shrimp intensively they produce the majority of the shrimp exported (Chamberlain 1991).

Ecological Impacts

Since most shrimp farming in Southeast Asia takes place on reclaimed mangrove forests I will briefly discuss the importance of this habitat.

Importance of mangrove wetlands

1. Serve as a nursery ground for marine coastal fisheries (Staples 1985, Zimmerman 1989).

2. Protect coastal shores from erosion and storm damage (Bashirullah 1989).

3. Provide construction material and fuel through charcoal production.

4. Mangrove ares are very productive. Leaf litter from mangrove trees provides raw material for nearshore ecosystems (Twilley 1989).

Effects on environment from aquaculture operations

The loss of mangrove habitat eliminates nursery grounds for larval shrimp and fish. Mangrove forests are critically important habitats for the reproduction and growth of shrimp postlarvae and juveniles (Turner 1986). Their replacement by shrimp ponds will adversely affect the recruitment of larval fish and shrimp (Zimmerman et al 1989).

Ecuador and Bangladesh are still dependent on collecting wild shrimp post-larvae to stock shrimp ponds (Olsen 1989). Depletion of local populations of shrimp postlarvae can occur due to this collecting (Bashirullah 1989, Turner 1986). Harvesting of postlarvae to stock shrimp ponds may have changed the dominant species of shrimp caught by fishermen in coastal Ecuador (Meltzoff 1986). In Bangladesh collectors of shrimp postlarvae also catch fish larvae and small invertebrates. This bycatch is allowed to die on the beach. Practices such as this may adversely affect populations of other fish and invertebrates in the Bay of Bengal.

Eutrophication of surrounding coastal areas from nutrients discharged in shrimp pond effluents can affect receiving waters. This is especially true for intensive shrimp culture systems for the high feeding, fertilization and water exchange rates require frequent discharge of pond effluents. Chemicals used for predator and pest control, for example tea seed cake and malachite green, and for pond soil sterilization, calcium carbide, may kill non-target organisms after discharge of pond effluents. Copper compounds used for algae control in shrimp ponds can be toxic to crustaceans and benthic fauna (Clifford 1992).

Intensive and semi-intensive shrimp culture involving discharging large amounts of pond water can affect estuary or other receiving waters. Since ponds are shallow, evaporation is greater than in neighboring mangrove or estuary. Effluents discharged from these ponds will be more saline and during periods of low flow can affect the salinity of receiving waters. Shrimp pond effluents are often high in organic matter, both suspended and dissolved (Boyd 1992). This high biological oxygen demand can cause oxygen depletion in receiving waters - especially since these estuaries already receive organic wastes from nearby urban and agricultural areas. If all the ponds are pumping out effluents during periods of low water, problems can arise due to this surplus organic matter and increased salinity (Twilley 1986).

If a intensively cultured shrimp pond is abandoned the bottom soil is usually saline making unavailable for agriculture or other uses. Therefore conversion of land to shrimp farming may for practical purposes be irreversible (Meltzoff 1986). Salt water intrusion into the water table of nearby agricultural land can occur when shrimp ponds discharge effluents into the irrigation systems supplying farm lands. This is a serious concern in Indonesia where the same canals supply both fresh and brackishwater, depending on the season (Chamberlain 1991).

Another consequence of using saline waters to raise shrimp is the need to maintain a particular salinity in the pond. Since the ideal salinity for P. monodon is 15 to 25 parts per thousand, freshwater is needed for pond dilution if full-strength seawater is used. In Taiwan land subsidence has occurred due to well water extraction to dilute coastal shrimp ponds (Avault 1993).

In addition to nutrients discharged from shrimp culture ponds sediments removed from pond bottoms are often discharged into receiving waters. (Boyd 1992). These sediments can increase turbidity in receiving waters.

As in other forms of agriculture, shrimp farming makes use of exotic species and varieties in areas where these species are not native. In oceanic islands such as Hawaii, Seychelles, Tahiti, etc. where shrimp farming has been introduced, the species cultured are all foreign to their environments. What effects this will have on the local ecosystem is unknown. Even if the presence of an exotic species of shrimp is innocuous, diseases and parasites can spread to local penaeid species from the exotic cultured shrimp. Cultured shrimp are vulnerable to a wide assortment of parasitic fungi and virulent bacteria and viruses (Brock et al 1992). If these pathogens spread to a local shrimp or invertebrate fishery it could have serious economic consequences (Hoffman 1984).

The use of antibiotics in shrimp feed has led to the occurrence of antibiotics in shrimp tissue (Weidner 1992). This may conceivably lead to the spread of antibiotic resistance inhumans. Since shrimp ponds are downstream from agricultural lands, pesticides may accumulate in shrimp tissue as well. Harmful pollutants present in estuaries - radioactive isotopes, heavy metals, etc. can also occur in shrimp tissue.

All these impacts described above occur on top of the impacts coastal areas already get from industrialization, urbanization, increased use of agricultural chemicals, recreational development, petroleum exploitation, etc. Coastal areas are especially affected by these impacts because they are downstream from sources of urban (sewage) and agricultural pollution (pesticides). In addition large urban centers are often on or near coasts (Lima, Jakarta, Manilla, Bangkok, etc.). These environmental stresses all reduce the capacity of the coastal environment to absorb the effects of mariculture (Bailey 1986).

Self-inflected impacts of shrimp farming

If shrimp ponds are built close together they share their water supply. If the wastes from one pond are discharged close to the water supply intake of another pond, that pond's effluent may enter the next pond, adversely affecting the shrimp growing within it. This recycling of pond water between ponds increases the incidence of disease and parasites. This recycling of water between heavily stocked ponds contributed to the collapse of the shrimp farming industry in Taiwan (Avault 1993). Dense algal growth followed by an algal population collapse can lead to die- off of shrimp and fish in the affected area. Hatcheries can also be affected if they pump from waters polluted by pond discharges (Chamberlain 1991).

If shrimp farming is to expand there must be a trade-off between reclaiming new mangrove areas for shrimp ponds or intensifying existing shrimp ponds, with concomitant increased pumping and nutrient discharge. The present policy of many countries is to protect mangrove environments and intensify production from existing ponds (Villalon 1986).

Social Impacts

Intensive shrimp farming in coastal areas of SE Asia has denied the use of these areas to local residents for traditional activities such as fishing, gathering construction materials, food collection, fuel gathering and hunting (Skladany 1991).

The benefits of shrimp farming development are for the most part confined to a limited number of entrepreneurs, government officials and foreign experts. Local residents suffer because of the loss of traditional livelihoods such as fishing and wood cutting to be replaced by low wage employment on shrimp farms. In addition if shrimp farms replace rice fields there is an absolute decrease in the number of jobs available in that area (Bailey 1988).

Shrimp produced by an intensive form of aquaculture are exported overseas, while the local need for high protein foods goes unmet. Declining fish catches due to loss of mangrove habitats adversely affect traditional fishermen in areas of intensive shrimp farming (Bailey 1988).

When coastal rice fields and other forms of agriculture are replaced by shrimp ponds producing shrimp for export, local food needs are denied (Bailey 1988). If local "trash fish" are harvested to provide protein for locally made shrimp feed, a supply of food fish may be denied to local residents as well (Newkirk 1991).

Another consideration in the growth of export-oriented shrimp farming has been the diversion of money and other resources from freshwater fish culture for local markets to shrimp farming for export markets. Although money is still invested in production for local markets shrimp culture activity has gained priority in government planning and international development assistance (Bailey 1992).

Intensive Cage Culture of Fish in Asia

Pond culture of freshwater fish has grown less quickly than commercial shrimp farming. It normally is less environmentally destructive since its more likely to be practiced by subsistence level farmers using local materials. Therefore fish farming is usually integrated into local farming systems (Costa-Pierce 1992). This type of aquaculture normally is less intensive than shrimp farming since it usually caters to a local market using local inputs of feed and fertilizer.

When fish culture is used to produce products for an export market, as the cage culture of salmon does in temperate waters, problems of local pollution can occur due to the accumulation of uneaten feed and feces beneath the cage. The cage culture of tropical food fish such as groupers and sea bass can cause problems similar to that caused by salmon culture when it is practiced in a similar way. For example in Singapore and Malaysia groupers (Epinephelus tauvinia), snappers (Lutjanus sp) and sea bass (Lates calcarifer) are raised in floating net pens. These cages are stocked quite heavily at about 40 fish per meter squared. The fish fingerlings are purchased from fishermen and are caught in the wild, often from distant waters. Since all species cultured in these cages are carnivorous they are fed with small "trash fish" caught specifically to feed these cultured species. All the fish raised in this manner are harvested and sold locally (Anon 1986).

The environmental impacts of intensive fish farming are similar to those caused by intensive shrimp farming. They include the following:

1. Deterioration of water quality in confined fjords or bays.

2. Deposition of uneaten feed and feces beneath cages.

3. Changes in benthic fauna beneath cages.

4. Introduction of exotic species for culture which later escape into the surrounding ecosystem.

5. Spread of diseases from domestic fish to native fish.

6. Possible spread of antibiotic resistant bacteria through the use of feed containing antibiotics (Getchell 1988)

7. Hybridization between native and domestic species of fish (Sletto 1992).

Like shrimp farming, the culture of salmon has expanded enormously in the last 10 years. Most of the salmon produced in Norway in 1992 was cultured in cages. The most serious environmental problems related to salmon farming result from the escape of domestic salmon from their cages. These domestic salmon can then breed with native salmon, reducing the ability of the hybrid offspring to find their native streams to spawn in.

A more serious harmful effect of domestic salmon on wild stocks is the transfer of bacteria causing furunculosis and parasites like Gyrodactylus salaris to wild salmon. This has caused population declines among these wild stocks, already seriously affected by acid rain. To prevent the genetic adulteration of native wild salmon by domestic salmon efforts are being made to develop sterile domestic salmon for culture (Sletto 1992).

Thus far the serious problems faced by salmon culture have not occurred in tropical fish culture (Beveridge 1984). Significant problems with this form of aquaculture include fouling of the nets and supporting structures with benthic organisms, vulnerability to algal blooms and sewage, pollution from passing ships and oil tankers, and unreliable food supplies.

Problems similar to those caused by salmon in Norway have not occured in Singapore or Malaysia yet. However cage culture of fish has not expanded to the extent it has in Northern Europe. However if cage culture systems were developed in the tropics using artificial feed to produce fish for export for the world market, then such problems could arise as well.

Mitigation strategies

The FAO met in January 1991 to discuss ways to reduce the impact aquaculture has on the coastal environment. They stressed the importance of the following:

1. Coastal aquaculture development and management planning.

2. Environmental impact assessment.

3. Criteria for site selection.

4. Carrying capacity of the associated ecosystem.

5. Use of mangroves, wetlands, and bioactive compounds.

6. Stock transfer and introductions.

7. Improvement in farm operation and management.

8. Monitoring of ecological changes. (Pruder 1992).

Formulating coastal aquaculture development and management plans

The allocation of production sites must be preceded by an adequate survey of the relevant area with a realistic appreciation of its potentials and limitations. Involve local residents in decisions concerning aquaculture development policies.

Use environmental impact assessment process

The environmental impact assessment is a process whereby the potential impacts of a proposed action can be evaluated. These impacts include effects on the biological, social, physical and economic environments. Once this has been done attempts can be made to mitigate the negative effects. For example environmental impacts on mangroves and the extent of their destruction can be documented. The effect of mangrove conversion to shrimp ponds had on local artisanal fishermen can then be acknowledged and its social effect mitigated.

Improve management of aquaculture operations

Wastes from shrimp farming can be reduced by reducing the feeding rate and stocking density in shrimp farms (Boyd 1992). A major cause of pollution from shrimp culture (and fish culture) is the attempt to culture too many shrimp in a pond, in a situation where the capacity of the pond ecosystem to decompose the feces and uneaten feed produced by the shrimp has been exceeded. Although shrimp can be cultivated successfully in super high densities, special techniques and treatment facilities are needed to do this successfully (Hirono 1992). Commercially viable systems for fish and shrimp culture will remain semi-intensive for small to medium sized farmers for the foreseeable future. Ways must be found to regulate the intensity of pond production so as not to exceed the capacity shrimp pond-coastal water systems have to assimilate and decompose effluent wastes.

Develop treatment systems for aquaculture effluents

Various techniques to reduce or recycle wastes are available to shrimp farmers. They include culturing shellfish like mussels or oysters in shrimp ponds that can utilize some of the surplus algae growing in that pond. Herbivorous fish like tilapia or milkfish can also be cultured in shrimp pond effluents (Lin 1992).

To reduce the biological oxygen demand and turbidity of effluents, sediment basins to settle suspended solids should be constructed to receive pond effluents before discharge (Boyd 1992).

Mangroves can also be used to treat shrimp pond effluents. Mangroves planted on levees and fringing areas of shrimp ponds will act as nutrient sinks for surplus nutrients discharged from shrimp ponds (Twilley 1987). They will also reduce erosion from bare soil exposed when ponds are excavated.

Determine carrying capacity of coastal environments

Assess the ability of a coastal ecosystem to sustain mariculture with minimal ecological change. To do this a program to monitor water quality in surrounding estuaries, rivers, groundwaters and ocean must be established (Ziemann 1992). Adverse ecological changes can be limited by ensuring that the scale of development does not exceed the capacity of the ecosystem to assimilate the changes resulting from aquaculture production (Zieman 1992).

Establish and enforce guidelines governing shrimp pond management, mangrove development and use of coastal wetlands

Prevent destructive expansion of unplanned shrimp farms by educating the industry and public on the harmful effects of mangrove reclamation. Integrate aquaculture development and regulation with other coastal zone planning activities (IMO 1992) Aquaculture development can be made complementary to other coastal uses (fisheries) through the planning process (Chua 1987). Pond construction and operation must take into consideration the watershed and environment in the surrounding area - both with respect to the effect proposed farming operations have on the environment and the effects the neighboring environment (agriculture, industry, urban development, fishing etc.) will have on coastal aquaculture.

Increased production of shrimp over the last 10 years, both cultured and wild-caught, has caused the price of shrimp to fall, especially in the mid-size ranges produced from shrimp ponds. More tropical countries are exporting shrimp, while the major markets for block-frozen shrimp, Japan, the US and Europe, are not likely to increase consumption significantly in the near future. Furthermore environmental side-effects of mariculture are increasing in all the producing countries. The profits obtained by those few benefiting from the mariculture industry will have to be weighed against the environmental impacts affecting the many local residents who depend on coastal resources for their livelihood. With proper planning and concern for both the environment and the economy, mariculture can continue to supply both food and foreign exchange for those nations capable of producing shrimp and fish.

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