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Aquaculture Impacts on the Environment
(Released December 1999 )

 
  by Craig Emerson  

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Whether as an economic windfall for developing countries, or as one of the most environmentally-destructive food industries, aquaculture has come under increasing scrutiny and criticism as the world tries to supply food for a population exceeding six billion. Aquaculture, the farming of aquatic organisms such as fish, molluscs, crustaceans and plants, is the fastest growing food production sector in the world1, but its sustainability is not assured. Pollution, destruction of sensitive coastal habitats, threats to aquatic biodiversity and significant socio-economic costs must be balanced against the substantial benefits. Aquaculture has great potential for food production and the alleviation of poverty for people living in coastal areas, many of who are among the poorest in the world. A balance between food security and the environmental costs of production must be attained.

Aquaculture Development & Techniques

For over 3,000 years, fish have been farmed in China, a country that continues to dominate the industry by producing 83% of the world's aquaculture output2. Other key producers include India (6%), Philippines (4%), Indonesia (3%), Republic of Korea (2%), and Bangladesh (1%), a list overwhelmingly concentrated in the developing world. Everything from sea cucumbers to sea horses is farmed, but the vast majority of production is carp, accounting for ~50% of aquaculture production measured as weight or value. The remaining top cultured species include kelp, oysters, shrimp and salmon. Salmon mariculture is often in the news, but the fish farming industry is concentrated inland, with over 15 million tonnes of fish produced in freshwater systems compared to 9.7 million tonnes produced at sea. The remaining 1.6 million tonnes is produced in brackishwater ponds. Seaweed farming accounts for another 7.7 million tonnes.

There are a variety of production systems around the world, including ponds, tanks, raceways, and cages or "netpens". There are hundreds of variations in technique, but there are only two significant differences: water processing and feeding regime. By economic necessity, most inland facilities use a flow-through system where water is diverted from surface water (lakes, rivers) or from natural underground reservoirs (aquifers). In many parts of the United States, aquaculture has been legally classified as a beneficial, nonconsumptive use of water, but in some states such as Idaho, the trout industry's raceways require huge quantities of freshwater which combine with drought to result in a drawdown of the aquifer. Recirculating systems only require periodic additions to top-up the water level, but the accompanying cost of filtration or aeration to maintain water quality restricts implementation. For cultured species held in natural water bodies, restrictions generally reflect site selection because water quality is heavily dependent on natural currents in and around the farm.

Although water resource issues are significant, there is a great deal of environmental concern focused on feeding techniques. The source of food for all aquaculture species can be divided into: 1) the use of artificial feed (aquafeed) in finfish and some shellfish operations, 2) provision of natural food (e.g. phytoplankton) in shellfish operations, and 3) a combination of natural and artificial feed. Whether inland or coastal, any operation that relies on artificial feed to grow fish faces the quandary of increasing production at the expense of increasing pollution from farm effluent.

Aquaculture Effluent: Pollution of Inland & Coastal Waters

In 1989, a sudden and catastrophic collapse of wild seatrout populations in areas close to salmon rearing cages in Ireland gave aquaculture critics a focus for protest. Although a link between fish farming and the decline of natural stocks cannot always be established, some environmental effects are clear. Unlike mollusc farming, many species of fish depend on a diet of artificial feed in pellet form. This feed is broadcast onto the surface of the water, and is consumed by the fish as it settles through the water column. Because not all the feed is eaten, a great deal of feed can reach the bottom where it is eaten by the benthos or decomposed by microorganisms. This alteration of the natural food web structure can significantly impact the local environment.

Many studies have implicated overfeeding in fish farms as the cause of changes in benthic community structure4 because a high food supply may favour some organisms over others. Moreover, sedentary animals may die in water depleted of oxygen resulting from microbial decomposition, while the mobile population may migrate to other areas. Antibiotics and other therapeutic chemicals added to feed (e.g. Ivermectin, Terramycin and Romet-30) can affect organisms for which they were not intended when the drugs are released as the uneaten pellets decompose5. Nonetheless, many drugs used in fish farms have been found to have minimal (if any) deleterious effects on the aquatic environment6. Feed additives, however, are not the only source of potentially toxic compounds in culture operations. A variety of chemicals are used to inhibit the growth of organisms which foul netting and other structures, reducing water flow through the cages.

Eutrophication

An increasingly significant effect of intensive fish culture is eutrophication of the water surrounding rearing pens or the rivers receiving aquaculture effluent. Fish excretion and fecal wastes combine with nutrients released from the breakdown of excess feed to raise nutrient levels well above normal, creating an ideal environment for algal blooms to form. To compound the problem, most feed is formulated to contain more nutrients than necessary for most applications. In Scotland, an estimated 50,000 tonnes of untreated and contaminated waste generated from cage salmon farming goes directly into the sea, equivalent to the sewage waste of a population of up to three quarters of Scotland's population7. Once the resulting algal blooms die, they settle to the bottom where their decomposition depletes the oxygen. Before they die, however, there is the possibility that algal toxins are produced.

Although any species of phytoplankton can benefit from an increased nutrient supply, certain species are noxious or even toxic to other marine organisms and to humans. The spines of some diatoms (e.g. Chaetoceros concavicornis) can irritate the gills of fish, causing decreased production or even death8. More importantly, blooms ("red tides") of certain species such as Chattonella marina often produce biological toxins that can kill other organisms. Neurotoxins produced by several algal species can be concentrated in filter-feeding bivalves such as mussels and oysters, creating a serious health risk to people consuming contaminated shellfish (e.g. paralytic shellfish poisoning9).

Fish is low in fat and considered a healthy alternative to other meats, but consumers cannot ignore the potential health risks of cultured species, just as they must not ignore the risks associated with terrestrial agriculture. In addition to shellfish contaminated with toxic algae, cultured seafood can pose additional concerns from disease transmission. Most fish pathogens are not hazardous to humans, but some fish pathogens such as Streptococcus bacteria can infect humans10. High levels of antibiotics and genetically-engineered components in fish feed (e.g. soya additives) can also pose risks. The challenge for regulatory agencies like the Food & Drug Administration in the United States is to ensure that these risks are "acceptable".

Pollution Control

Although aquaculture development has often occurred outside a regulatory framework, government oversight is increasingly common at both the seafood quality control level, and at baseline initiatives addressing the basic problem of pollution generated by culture operations. The impact of coastal aquaculture depends on a number of physical, chemical and biological factors, most notably the local hydrodynamics. In areas of high currents, waste accumulation is minimized by hydrodynamic dispersal. Excess nutrients aren't eliminated, but the lower level of waste is more easily assimilated into the local food web. Water movement also helps to replenish anoxic water with oxygen-rich water from surrounding areas. Accordingly, site selection is a primary factor in the mitigation of coastal pollution. There are a number of studies that have developed mathematical models to predict the hydrodynamics around culture operations to optimize selection18. Others are experimenting with new systems for growing flounder Paralichthys dentatus and mussels Mytilus edulis in offshore areas of New England11. Despite potential net entanglements with marine mammals and conflict with traditional trawling grounds, these operations can take advantage of waste dilution from offshore currents and deeper water.

Aquaculture effluent from inland operations can be treated much more effectively than coastal operations because the outflow can be controlled, and therefore treated, in much the same way as municipal sewage treatment. In addition, a firm in Japan has developed an odourless, environment friendly organic fish waste treatment system which uses a colony of micro-organisms active at high temperature to process up to 5 tonnes of fish waste daily12. Coastal operations can also take advantage of innovative techniques to reduce pollution. In China, polyculture of scallops, sea cucumbers and kelp reduces eutrophication and the use of toxic antifouling compounds. Nutrients from scallop excreta are used by kelp, which used to require the addition of tonnes of fertilizers. Antifouling compounds and herbicides can be reduced because sea cucumbers feed on organisms which foul nets and other structures. For shrimp and catfish culture, deeper ponds can be constructed to reduce weed growth to further limit herbicide use.

Culturing finfish with mussels, oysters and other filter feeders can minimize feed accumulation, as can the reformulation of feed and design of new feed delivery systems. Pellets are no longer packed with more nutrition than the target fish can possibly use, and feed pellets are designed to stay longer in the water column, rather than rapidly sink to the bottom where they become unavailable to the target species. Drugs added to feeds to combat diseases can be reduced by enclosing fish in what are essentially bags, rather than nets, and by vaccinating individual fish. Many techniques are being developed to minimize environmental impact but the most basic and cost-effective pollution control is implementation of an efficient aquaculture management regime. Unfortunately, many operations continue to have a lack of trained manpower, resulting in waste and misapplication of chemicals.

Using Natural Fish Stocks to Feed Farmed Fish

Ironically, fish culture is dependent on a diet of wild fish because fish meal and fish oils from natural stocks are the primary components of artificial compounded feed (aquafeed)13. It can be argued, therefore, that aquaculture cannot provide an alternative to fishing unless only herbivorous fish and shellfish are farmed. However, the source of the fish meal is pelagic fish such as menhaden and mackerel, species not normally consumed by humans. Additional fish meal comes from bycatch which would otherwise be discarded as waste. Nonetheless, it is not clear that the conversion of "trash fish" into human food via aquaculture is preferential to using fish meal in swine and poultry feed.

As farms intensify, there is a growing trend toward the increased use of aquafeed. Almost 31,000,000 megatonnes (MT) of the world's total wild fisheries production is used for animal feed each year, 15% of which is used in fish feed. Feed is specially formulated to ensure high conversion efficiencies, (amount of feed needed to produced one pound of animal), and in general, aquatic animals are far more efficient at feed conversion than terrestrial animals. Given these facts, the strategy of feeding fish to fish seems logical, however it should be noted that only a few percent of feed for swine and poultry is composed of fish meal, compared with 70% for finfish and shellfish13, and inefficient practices can lead to a great deal of waste. Growing a pound of salmon may require 3-5 pounds of wild fish, and between 1985 and 1995 the world's shrimp farmers used 36 million tons of wild fish to produced just 7.2 million tons of shrimp. In general, the quantity of input of natural fish stocks exceeds outputs in terms of farmed fishery products by a factor to 2 to 314.

A potential solution to the fish-eating-fish dilemma would be to shift culture operations to herbivorous species such as tilapia, catfish, carp, oysters and clams which rely little, if at all, on supplement feed. Unfortunately, the vast majority of world aquaculture production is already concentrated on these species, and it is much more lucrative to grow salmon and shrimp that rely heavily on fish meal. There have been gains made in substituting terrestrial animal byproduct meals, plant oilseed and grain legume meals, and cereal byproduct meals for fish meal but dependence on natural fish stocks for aquaculture feed will be slow to disappear.

Impacts on Natural Stocks

Clearly, feeding fish to fish leads to a net loss of protein in a protein-short world and impacts directly on natural stocks, but aquaculture may also have a myriad of indirect effects on the natural environment. Almost all marine or brackishwater culture is dependent upon natural fisheries for some aspect of operations. Although more and more hatcheries are being constructed to provide seed for shellfish and finfish culture, most farms still capture wild animals for broodstock or for a source of larvae. In some cases, collection of wild-caught shrimp larvae to stock ponds has destroyed thousands of other larval species in the process.

The full consequences of removing natural fish stocks from food webs are difficult to predict. When fish are removed to make fish meal, less food may be available for commercially valuable predatory fish and for other marine predators, such as seabirds and seals. This effect exacerbates large-scale problems caused by global warming and the El Nino phenomenon. The El Nino of 1997-1998 is considered to be the second strongest "warm event" in the topical and subtropical Pacific this century. The shift in water temperature caused a severe decline in biomass and total production of small pelagic fish leading to altered food webs and a shortage of fish meal and fish oil.

Genetic Conservation & Aquatic Biodiversity

Not all impacts on natural stocks are detrimental. Fish culture can actually mitigate the decline of fish stocks decimated by overfishing and environmental changes. In addition to decreasing the dependence on natural stocks, fish culture may help to re-stock populations by the release of cultured larvae or juveniles into the wild to bolster natural populations. Since 1890, Japan has engaged in stock enhancement in coastal waters through seeding from hatcheries in a bid to maintain fishing sustainability and genetic conservation of endangered stocks15.

It has been suggested, however, that the genetic diversity of natural stocks is hurt, more than helped, by aquaculture. In Norway, acid precipitation, hydroelectric development, and salmon parasites have all contributed to the extinction of over 40 salmon stocks and the endangerment of others, but stocks are also threatened by the spread of salmon escaping from fish farms. Cultured species are often bred or otherwise genetically engineered to exhibit abnormally high growth rates, usually at the expense of other characteristics unimportant in an aquaculture operation. Through selective breeding, aquaculturists have tripled the growth rates of native coho salmon, supporting a $5 million domestic industry16. If these genetically engineered salmon escape and breed with native salmon, the genetic traits optimal for culture may break up local adaptations critical to survival in nature. In Maine, USA, federal officials estimate that only 500 Atlantic salmon with a native genetic makeup remain in the wild17.

Genetic impacts can originate from the genetic manipulation of cultured organisms, but they may also be minimized. By heat or chemical shocking at the larval stage, triploid mussels and scallops can be produced which allocate more resources to meat production than reproductive tissue19. As a result, these cultured bivalves are reproductively sterile, and of little threat to local populations. Unfortunately, a study that introduced supposedly sterile Pacific oysters into Chesapeake Bay found that 20% of the population reverted back to their sexually fertile state17. Whether intentionally or unintentionally released, the potential loss of natural biodiversity through genetic hybridization could make aquaculture difficult to rationalize, particularly since accidental release of cultured populations also results in ecological competition.

Introduction of Alien Species

In the northwest United States, abnormally high spring tides destroyed net pens in Puget Sound, releasing 100,000 Atlantic salmon, two years after an escape of 300,000 salmon. This species cannot breed with local fish, and it was suggested that the only effect was a field day for anglers20. Such releases, however, may have significant ecological effects that are difficult to detect immediately. In the above case, government officials indicated that the competitive threat to the native Pacific salmon was minimal since the released fish would not survive long enough to breed and increase in abundance. Nonetheless, escaped fish will compete for food and space, at least temporarily. Release of blue tilapia in Florida has lead to the loss of food, native habitat, and spawning areas for native species in Everglades National Park.

Although ship ballast water is often the cause of introduced species, the importing of non-indigenous animals for culture can also introduce diseases and non-target organisms. The Japanese oyster drill and a predatory flatworm were introduced to North America with the Pacific oyster (Crassostrea gigas), thereby contributing to the decline of west coast native oyster stocks. French shellfish farmers have been warned not to import the American oyster (Crassostrea virginica) or the Pacific oyster from Canada because of possible parasitic disease transmission from Haplosporidium nelsoni, Haplosporidium costale, Mikocytos mackini and Perkinsus marinus21.

Whether the cultured species is native or not, culture operations do introduce a high concentration of potential prey which may significantly alter the local ecology. Birds, seals, crabs, and starfish can significantly predate farmed species. In the southern United States, cormorants, herons, kingfishers and pelicans consume millions of dollars worth of commercially raised catfish. In some areas, these birds have increased their numbers dramatically, far exceeding the normal carrying capacity of an area and negatively impacting natural roosting areas and island habitats. Covering fish pens with nets is extremely expensive and is effective only for avian predators. Installation of sound devices by salmon farmers often provide a temporary respite from seals, but it is feared that the sound may scare humpback whales from feeding in the area. Inland and coastal operators often resort to killing predators, but because the predators are often rare or endangered, killing them is not acceptable politically.

Habitat Destruction: Mangrove Forests

Nowhere are the negative impacts on the natural environment more apparent than with shrimp farming and the associated destruction of mangrove forests22. In Asia, over 400,000 hectares of mangroves have been converted into brackishwater aquaculture for the rearing of shrimp. Farmed shrimp boost a developing country's foreign exchange earnings, but the loss of sensitive habitat is difficult to reconcile.

Tropical mangroves are analogous to temperate salt marshes, a habitat critical to erosion prevention, coastal water quality, and the reproductive success of many marine organisms. Mangrove forests have also provided a sustainable and renewable resource of firewood, timber, pulp, and charcoal for local communities. To construct dyked ponds for shrimp farming, these habitats are razed and restoration is extremely difficult.

Unfortunately, shrimp ponds are often profitable only temporarily as they are subject to disease and to downward shifts in the shrimp market. Growing political pressure in western countries may restrict the shrimp market in response to consumers' avoidance of environmentally-unfriendly products. More significantly, Japan's economy is experiencing difficulty at present, and Japan is the world's largest market for shrimp; when the market falls, ponds are abandoned. A return to traditional fishing is not always possible because the lost mangroves no longer serve as nursery areas which are critical for the recruitment of many wild fish stocks. Unemployment prospects cannot always balance short-term gains. It is clear that socio-economic effects are as important as pollution and ecological damage when evaluating the sustainability of aquaculture.

Socio-Economic Effects

In developed countries, visual pollution created by thousands of buoys in coastal farms and the inconvenience to recreational boaters and others sharing the coastal zone, pale in significance to the socio-economic effects of aquaculture in the developing world. The quest for profit often has devastating consequences. In the Indian province of West Bengal, four fishermen were killed and over 20 injured in a dispute between fishermen and shrimp farmers as a result of access rights to Lake Chilika, one of the largest freshwater lakes in Asia23.

Many nations embrace aquaculture, not as a direct way to provide food for their poor, but as a source of export wealth that can potentially lead to longer-term social benefits. Many rural communities enjoy the employment opportunities possible with aquaculture, but conflicts often develop within these communities when traditional employment clashes with the aquaculture industry. Local fishing communities often do not hold title to coastal wetlands, and have at times been displaced by shrimp consortia that have acquired leases along tropical shorelines. Resource ownership is often complex or ambiguous in prime aquaculture locations, and pollution and social concerns are often secondary to economic ones. Once touted as employment for individual operators, aquaculture is starting to reflect terrestrial farming strategies, where small farms are absorbed into large industrial farms. An increase in culture efficiency is obtained, but employment can be reduced and the remaining small farms cannot compete economically.

The Future of Aquaculture

Aquaculture will continue to be one of the most viable methods to supply growing world population needs, but the challenge to maintain profitability and environmental compatibility is daunting. Growth of aquaculture was fueled initially by governments eager only for economic success, but many governments have started to implement strict regulatory guidelines addressing environmental and social issues to ensure sustainability. In the United States, aquaculture is under close scrutiny from the Environmental Protection Agency, Food and Drug Administration, the National Marine Fisheries Service, the United States Department of Agriculture and numerous state environmental agencies and local groups. Canada has also developed stringent guidelines to maintain the health of the environment, and Brazil, Malaysia, Sri Lanka and others have all made progress in the establishment of legal and regulatory frameworks which are starting to have a positive effect on aquaculture development.

Despite such progress, there are still major aquaculture producing countries that do not have appropriate legal frameworks and policies for aquaculture. All to often, governments fail to provide the needed economic, legal, and social support to ensure economic and environmental sustainability24. Where governments were initially integral to development, contraction of government involvement is now prevalent, resulting in increased privatization and corresponding social conflicts. While social issues are notoriously intractable, water quality problems are not. New technologies such as recirculating and offshore systems hold promise for lessening the impact of aquaculture on the surrounding environment, but many countries cannot take advantage of these expensive innovations. Technology alone cannot determine the approach for sustainability; aquaculture development must adapt to the needs and capacities of developing countries. Politically, food production will remain an overriding priority, and aquaculture will continue to grow. Models must be developed to clearly predict whether the socio-economic benefits of aquaculture are worth the environmental cost25.

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