Aquaculture is defined as the farming of aquatic organisms, including fish, molluscs, crustaceans and aquatic plants. The term farming implies some form of intervention in the rearing process to enhance production, such as regular stocking, feeding and protection from predators (Pullin, 1990).
Aquatic pollution is here defined as “the introduction by man, directly or indirectly, of substances or energy into the aquatic environment resulting in such deleterious effects as harm to living resources, hazards to human health, hindrance to aquatic activities including fishing, impairment of quality for use of the water and reduction of amenities” (adapted after GESAMP1, 1991).
The type and scale of any ecological change related to aquaculture development will depend on the method, the level of production and the physical, chemical, and biological characteristics of the area in question. Ecological change has been associated with the large scale production of bivalves and seaweeds and the release of dissolved and particulate waste from fish, shrimp, and bivalve culture. Disturbance to wildlife has been caused by destruction of wetland habitats, while uncontrolled introductions and transfers have changed or depleted the biodiversity of the ecosystem. Aquaculture developments can also have effects on human health (for example introducing or increasing the prevalence of schistosomiasis) and endanger indigenous fishes with new diseases. Aquaculture can make for more efficient use of scarce resources but conflicts may arise between different water users. It is consequently important to balance the positive and negative effects when establishing new aquaculture systems (Anon., Undated, Barg, 1992, GESAMP, 1991, Pullin, 1990).
This paper examines the various environmental effects of aquaculture, as well as the extent to which environmental circumstances can limit aquaculture, with emphasis on tropical and subtropical areas. The intention is to increase awareness of the environmental effects of aquaculture operations so that steps can be taken to minimize these negative effects.
Aquaculture is not yet well developed anywhere in developing countries, with the exception of China. The majority of inland aquaculture in the tropics is concerned with pond-based finfish species. Lake-based cages and pens probably account for about 10%, while tanks and raceways account for 5% of production (Beveridge and Phillips, 1990). Marine and freshwater resources are often underutilized in relation to aquaculture resource availability (Phillips et al, 1990).
1 GESAMP: Group of Experts on the Scientific Aspects of Marine Pollution.
Aquaculture systems can be broadly classified into three groups on the basis of feed/fertilizer input (Edwards 1990):
Extensive systems rely on natural feed produced without intentional pond inputs in the form of feed/fertilizers.
Semi-intensive systems depend on fertilization to produce natural feed in situ in the pond and/or on feed given to the fish to complement the natural feed which develops in the pond.
Intensive systems depend on nutritionally complete feeds, either in moist formulations or in dried pellet form, with fish deriving little or no nutrition from natural feed production in the pond.
The amount of environmental impact caused by an aquaculture system is closely related to the intensity of the system. The more intense the system, the larger the environmental effects.
Freshwater extensive aquaculture which is practised in lakes and reservoirs is the least destructive system; even this can generate some negative environmental effects. These include the eutrophication of water bodies, spoiling of the beauty of the environment, destruction of ecosystems, public health risks, and the displacement of stocks. However, the environmental impact of this kind of system is less than for more intensive systems which utilize larger amounts of inputs (Barg, 1992, Martinez-Espinosa and Barg, 1990, Pullin, 1989).
Most tropical inland aquaculture is semi-intensive, generally relying on the use of plant-based agricultural by-products as feeds with very variable feeding rates that are usually empirically derived (Beveridge and Phillips, 1990). Environmental effects include health risks to farm workers from water-borne diseases such as schistosomiasis and salinization/acidification of soils and aquifers. It is also possible that heavy metals from livestock feeds in integrated aquaculture can accumulate in fish and pond sediments. Semi-intensive systems often cause less environmental disturbance than larger and more intensive systems, particularly if they are integrated with agriculture (Barg, 1992, Pullin, 1989, Pullin, 1990).
Intensive aquaculture depends on the use of formulated feeds which supply all or almost all of the animals' nutritional needs. Aeration and automatic feeders are also often employed, and as a result production costs are high, limiting culture to high-value species (Beveridge and Phillips, 1990). However, intensive systems may be appropriate for rearing high-value species such as salmonids in temperate waters. Environmental effects include eutrophication due to aquaculture effluents high in BOD2 (biological oxygen demand) and suspended solids and the accumulation of anoxic sediments below cages (Barg, 1992, Pullin, 1989).
Aquaculture uses resources such as water, land, labour, materials for construction, and feedstuffs. There are also outputs such as the fish that is being farmed, uneaten food, faeces and other waste products as well as therapeutants and other chemicals entering the environment.
2 BOD: Biochemical oxygen demand
Demand for land and water resources has caused problems in some areas, leading to competition between aquaculture and other resource users. When it comes to water, there are a number of ways to reduce water demand -- including reduction of seepage loss, intensification of use, and reuse of water, for example treated sewage effluent (Beveridge and Phillips, 1990).
Not all types of land are suitable for aquaculture. The gradient of the land may be too steep or the soil may be too permeable. Water-based aquaculture does not, of course, utilize land. However, it does occupy areas of lakes and rivers, and this can result in competition for resources; conflicts can arise because aquaculture operations occupy lake surface which was previously used for fisheries (Beveridge and Phillips, 1990).
Ponds and other land-based aquaculture systems usually generate little visual impact, and can actually increase the attractiveness of the landscape. But cages and pens are often unattractive and disturb scenic beauty, affecting the tourism industry in an area (Beveridge and Phillips, 1990).
Growing demand for water from the aquaculture industry results in increased competition with other water users if the water resources are limited (Phillips et al, 1990). Both quality and accessibility play a role in the suitability of water supply for aquaculture development. The term water quality in its broadest sense includes all physical, chemical and biological aspects of water. However, fish farmers are concerned with those aspects of water quality which regulate the suitability of water for rearing fish. Most aquaculture supplies are utilized without pre-treatment, making the fish farmer wholly dependent on the present water quality (Boyd, 1979, Phillips et al, 1990).
The amount of water required by a given aquaculture system depends on several factors: the quality of the water supply, the tolerance of the cultured species and the type of the aquaculture system. Intensification of production results in increased water use to maintain water quality (Beveridge and Phillips, 1990, Phillips et al, 1990).
In theory, freshwater aquaculture can utilize water at almost any stage in the hydrological cycle, while in practice, surface fresh waters, which constitute only 0.3% of the earth's water resources, are most commonly used. Much of the water is taken directly from flowing waters although some may have been used already for irrigation. Groundwater resources are considerably more abundant but usually seem to be less used than surface waters on a global scale (Beveridge and Phillips, 1990).
The demand for water in aquaculture, especially in very arid regions, can lead to extreme abstraction (utilization) from limited underground aquifers. Excessive removal of groundwater for aquaculture has caused problems. For example in Taiwan, utilization of groundwater on a large scale has led to subsidence (depletion of the water source) and salinization. However, the supply of water for aquaculture can be largely met by using irrigation water before it is used on to the land (Anon., Undated, Beveridge and Phillips, 1990, Nyman, 1988).
Ground water supplies usually have more stable chemical characteristics than surface waters, although they can contain undesirable levels of dissolved salts as well as metal ions and high levels of toxic gases. In many regions there is an increasing concern about contamination of groundwater supplies by persistent pollutants which also can prove harmful to aquaculture (Barg, 1992, Phillips et al, 1990).
Marine waters tend to have fairly stable characteristics, but coastal water, particularly near large freshwater sources, can be unstable, because of the risk from pollution as well as fluctuations in salinity and temperature. Additional problems associated with the use of marine water include the presence of fouling organisms and blooms of toxic algae (Barg, 1992, GESAMP, 1991, Phillips et al, 1990).
The uncontrolled abstraction (drawdown) of water can adversely affect conditions in surface waters by (from Beveridge and Phillips, 1990, and Phillips et al, 1990):
Seasonal changes in water needs can also be important. The demand of fish for dissolved oxygen varies between species and increases with water temperature and is also inversely related to fish size. This causes water requirements to change throughout the production cycle (Phillips et al, 1990).
Evaporation, and to a lesser extent transpiration, lead to a significant loss of surface water from aquaculture facilities. Evaporation occurs at all water-air interfaces and depends on water and air temperatures, the relative humidity of the air, and wind speeds. Although evaporation occurs everywhere, it is likely to be an important part of the water budget only in ponds with high retention time. Evaporation losses from open water vary from a maximum of 0.8 cm/day in Europe to 2.5 cm/day or more in tropical regions with an intermediate value of around 0.5 cm/day in the sub-tropics (Beveridge and Phillips, 1990, Phillips et al, 1990).
Seepage, which is a transfer from surfacewater to groundwater, depends on soil permeability and pond area and can lead to significant loss in earthen ponds. Reported rates of seepage for fish ponds vary from 0.01–0.66 cm/day up to 2.5 cm/day (Beveridge and Phillips, 1990, Phillips et al, 1990). Seepage not only reduces the amount of available water, but can also affect water quality. Ponds located on acid soils with high seepage rates will need significantly greater additions of lime to maintain alkalinity and hardness levels suitable for fish production (Teichert-Coddington and Phelps, 1989)
Combined losses from seepage and evaporation are a very important component of water losses from ponds and may significantly influence resources in extensive pond culture. Beveridge and Phillips (1990) give an example of water losses during aquaculture operations.
If one assumes total losses from evaporation and seepage of 1–2 cm/day as typical of tropical conditions, then each ha of fish pond will “consume” 100–200 m3 water per day. The total water requirement for ponds has been estimated to vary between 35 000 and 60,000 m3/ha/yr in order to maintain a mean water depth of 1.5 m during the growing season (240 days) and counteract losses estimated at between 1 and 2 cm/day.
Pond systems in the tropics and sub-tropics have a comparatively low water demand since the most common systems in these areas are low-flow, extensive or semi-intensive systems, which are defined as methods in which water is added simply to compensate for evaporation and seepage losses (Beveridge and Phillips, 1990). The culture of salmonids and carp in intensive flow-through systems has the highest water requirement, since the water is being used to supply dissolved oxygen and remove metabolites from the culture system (Phillips et al, 1990).
Pond farming can be recommended as a water-efficient culture technique compared to intensive flow-through systems. The negative aspect of pond culture is that large land resources may be required for large-scale pond systems, because of the relatively low areal production. It has been suggested that deepening of ponds to increase storage of rain water will conserve seasonal water resources. The pond draining that usually takes place during the fish harvesting can be avoided to reduce loss of water, although this technique can result in long-term deterioration of water quality. Cage and pen culture do not involve water abstraction and thus offer good scope for enhanced use of water resources (Phillips et al, 1990).
Integration of aquaculture with agriculture makes more efficient water resource utilization possible. Irrigation water previously used in aquaculture will be more fertile and will improve or maintain yields with less fertilization. Fish culture in rice fields in tropical regions is one example of integrated techniques and shows the potential for using aquaculture to increase the value of agricultural crops (Phillips et al, 1990). The utilization of treated sewage water for aquaculture purposes is also a possible way to use existing water resources more efficiently, taking into consideration possible health risks.
The quality of water is one of the most important factors influencing successful freshwater fish culture; if the water quality is good, survival, growth, and reproduction can reach high values; if not fish production will be decreased or impossible (Fast, 1986).
The susceptibility of surface fresh waters to pollution is related to the catchment area size as well as the degree of human influence; the water quality of running waters is largely decided by the solubility and topography of the surrounding catchment area, the climate and the kind and size of pollution sources. Pollution of catchments far away from the pollution source by acid rain and radiation has also shown that fresh water pollution can develop far away from the source of pollution (Nyman, 1988, Phillips et al, 1990).
Static fresh water usually has more stable physical and chemical characteristics than flowing waters. This stability, which depends on the retention time, allows the development of highly fertile pond ecosystems which have been used for fish production. The same stability can also cause negative effects, e.g. a build up of wastes from intensive aquaculture (Phillips et al, 1990).
The requirements of different fish species regarding water quality are highly variable. Values for desirable ranges of some water quality parameters are presented in Table 1.
MOST DESIRED VALUES OF SOME WATER QUALITY PARAMETERS
IN TROPICAL AND SUBTROPICAL AQUACULTURE
(Boyd, 1979, Wetzel, 1983).
| Parameter | Value |
|---|---|
| Oxygen (mg/1) | >5 |
| pH | 7–8 |
| Carbon dioxide (mg/1) | 0–10 |
| Ammonium (mg/1) | <0.6 |
| Hydrogen sulphide | Absent |
Temperature influences oxygen solubility, photosynthetic rates, respiration and metabolism (Wetzel, 1983) It has a significant influence on the species of fish that can be cultured, growth rates, the quality of fish flesh, food conversion efficiency, and the economics of a fish culture operation (Boyd, 1979, Fast, 1986). Dissolved oxygen concentration is one of the most important water quality parameters for aquaculture (Fast, 1986). All cultured aquatic animals and plants need oxygen for survival, growth and reproduction; and all aquaculture systems must be designed to carry oxygen to the cultured organisms (Phillips et al, 1990). The toxicity of many substances, such as ammonia and hydrogen sulphide, is greatly influenced by pH (Boyd, 1979, Fast, 1986). Salinity is one of the most important factors that decide the species of fish that can be reared at a given location. Most fresh water animals are capable of living in waters of low salinity, although the limit for most fresh water organisms appears to be salinities of less than 10 ppt (Fast, 1986, Wetzel, 1983).
Pond turbidity is produced by organic matter like phytoplankton and by suspended inorganic matter like silt and clay. Inorganic turbidity can cause difficulties, since it can decrease the light penetration that is needed for photosynthesis and thus reduce oxygen generation and phytoplankton production. Inorganic turbidity can also decrease the benefits of fertilization since phosphorus can absorb or adsorb on the sediment particles. Appreciable fish mortality occurs at clay turbidities above 175 000 mg/l. Turbidities in natural waters seldom exceed 20 000 mg/l and even “muddy waters” usually have less than 2000 mg/l. Even though turbidity caused by suspended soil particles will rarely have direct effects on fish, it can negatively affect fish populations through the sedimentation of soil particles that can smother fish eggs and destroy communities of bottom organisms. Turbidity caused by plankton is not harmful to fish. Turbidity can also limit the production of undesirable macrophytes in ponds (Boyd, 1979, Fast, 1986).
Blue-green algae synthesise geosmin, a compound with an earthy flavour and smell. Geosmin is excreted into the water from the algae and is absorbed by fish, giving them an “off-flavour”. Some fish may have enough off-flavour to make them un-saleable. Off-flavour can be removed from fish by depuration -- a process in which the fish is held for a week or longer in clean water (Boyd, 1979).
Undesirable growths of the bloom-forming blue green algae Microcystis have produced sufficient toxins to cause human health problems and death of cattle that drink the polluted water. It is also possible for Anabaena to cause similar problems (Symoens, J.J. et al, 1981). Microcystis aerginosa is often very common in fertilized fish ponds, and there is concern about potential negative effects on fish and on humans who may eat fish grown in these ponds. It is reassuring that there have been no reports of tilapia mortalities caused by algal blooms in the tropics. It is important to be aware of the potential danger of toxic blue green algae to aquaculture, but it is encouraging that the Microcystis toxin is sensitive to heat. Thus proper cooking of fish should render any toxin accumulated by fish harmless to humans (Edwards, 1990).
Eutrophication and sedimentation are important factors promoting the growth of shoreline vegetation. This vegetation in turn traps sediment; thus a gradual process occurs of the water body by getting filled through accumulation of sediments and invasion of vegetation (Symoens et al, 1981). The amount of coverage by macrophytes required to significantly restrict pond fish production is not known, but it is suggested that more than 10–20% cover may sometimes be harmful (Boyd, 1979).
Macrophytes also provide the snails that act as intermediate hosts for schistosomiasis with a favourable habitat, thus increasing the risk of spreading this disease, as well as providing a refuge to certain species of disease-spreading mosquitoes (Anon., 1957, Nyman, 1988).
Some existing aquaculture operations have been seriously influenced by pollution caused by land-based and coastal developments (GESAMP, 1991). Water supply is also an important medium for receiving and dispersing excretory products and other waste materials which can prove harmful if allowed to accumulate. It can also be a source of pollutants such as toxins (Phillips et al, 1990).
Water treatment may be used to improve stocking densities and reduce water requirements. Natural water treatment already exists in many low-cost extensive and semi-intensive pond systems in which phytoplankton growth is encouraged as a contribution to the fish food. Phytoplankton also produces oxygen by photosynthesis and assimilates metabolites which may otherwise accumulate and damage fish stocks (Phillips et al, 1990). But the phytoplankton growth has to be controlled. Excessive growth of phytoplankton causes oxygen deficiency during the night due to respiration.
Toxic substances include fertilizers, acids, alkali and heavy metals from municipal, industrial and mining wastes, chlorinated hydrocarbons and organo-phosphate pesticides used in agricultural crop protection, ammonia from sewage and industrial wastes, and toxic carcinogenic or mutagenic organic wastes. They may stay dissolved in the water or settle to the bottom with settling solids. They can also be taken up and concentrated in the organisms, or be degraded or transformed depending upon the chemical structure of the surrounding environment. Human beings can be exposed to harmful amounts of toxins by drinking the polluted water or eating fish or other aquatic organisms which have accumulated toxicity through the food chain (Symoens et al, 1981).
In Africa large amounts of pesticides are being used each year in control programs against human and animal diseases, and the amounts are increasing every year, constituting risks to the fauna (Symoens et al, 1981).
Many pesticides, and insecticides in particular, are extremely toxic to fish. Acute toxicity values for many of the commonly-used insecticides range from 5–100 μg/l, and much lower concentrations may be toxic if the fish is exposed for longer periods. Even if adult fish are not killed at once, they have long-term effects in an environment contaminated with pesticides. For example the abundance of food organisms can diminish, fry and egg can suffer mortality, and growth rates of fish can decline. Pesticides sprayed on to fields may drift over large areas and reach ponds and streams not directly sprayed. Therefore, ponds in agricultural areas are often more or less contaminated to some degree with pesticides. Important factors in protecting fish ponds are: distance from pesticide-treated fields, tree and other vegetative cover, topographic barriers to drift or runoff from fields, and proper method of application. If a watershed receives heavy applications of pesticides, ponds in this area are not suitable for fish production (Boyd, 1979, Muirhead-Thomson, 1988, Symoens et al, 1981).
Inland aquaculture often relies on hatchery-produced seed and thus does not lead to overfishing of wild stocks. Overfishing has been reported in connection with tropical marine fish and shrimp culture, causing depletion of wild stocks (Beveridge and Phillips, 1990).
The amount of seed available is not always enough, causing ponds to remain unstocked, thus providing mosquitoes and snails with ideal habitats. This can increase the prevalence of schistosomiasis and malaria, as well as other mosquito-transmitted diseases such as filariasis.
Extensive aquaculture relies on natural food production with the production related to primary productivity. Semi-intensive tropical aquaculture utilizes food diets made mainly from locally available agricultural materials (Beveridge and Phillips, 1990).
Intensive aquaculture that uses formulated feeds is not very common in the tropics and subtropics.
Materials such as woods and bamboo are needed to construct holding facilities such as cages and pens, and demand for these materials can be very large (Beveridge and Phillips, 1990), with local deforestation and destruction of ecosystems such as mangroves as a result.
Uneaten food, excreta, and chemicals and therapeutants are the three main sources of aquaculture wastes. Mortalities and escaped fish can also be considered as wastes (Beveridge and Phillips, 1990).
In intensive systems where effluent waste concentrations are high, faecal and urinary products as well as waste food can be a problem. They can also be a problem wherever the flow rates and nutrient levels in the receiving waters are low (Beveridge and Phillips, 1990).
Very little is known about the types or quantities of chemicals and drugs used in the tropics. Chemicals envelop compounds that are deliberately utilized in aquaculture operations. Beveridge and Phillips (1990) list different deliberate uses of chemicals: e.g. in the hatchery (to induce spawning, control stickiness of eggs, determine sex), to improve productivity (lime, fertilizers), control pests (pesticides, insecticides, herbicides, molluscicides, piscicides), treat and control diseases (fungicides, parasiticide, disinfectants), control fouling, and pacify fish during handling (anaesthetics). Materials used in construction of tanks or raceways may enter into the aquatic environment. Antifoulants are seldom used in fresh water cage or pen culture in the tropics (Beveridge and Phillips, 1990).
Low-alkalinity water is usually unproductive -- it has low nutrient concentrations, little plant growth, large variations in pH, and low fish yields (Fast, 1986). Lime increases the pH of pond soil and water, increases alkalinity and hardness, reduces humic acid content in the water, and improves benthic and phytoplankton production. Quantities used in fish ponds can be large -- up to 2–3 t/ha during a growing season. Negative effects on the environment from the application of lime are minimal (Beveridge and Phillips, 1990).
Liming and corresponding increases in pH and alkalinity result in increased fish production through several mechanisms. The lime increases the sediment pH and reduces the capability of the sediments to bind plant nutrients. This releases the nutrients to the water where the phytoplankton can use them and also make added nutrients more available for plant growth. Higher sediment pH also creates better conditions for microbial growth and thus leads to a more productive detrital food chain. This results in an increased production of benthic fish-food organisms. Liming also increases the available carbon dioxide for photosynthesis, produces a more desirable water pH range, and buffers against drastic pH changes (Fast, 1986).
Most pond farmers in the tropics use fertilizers. The types and quantities of these fertilizers are determined by cost, availability, species and intensity of culture. Fertilisation plays a major role in determining water quality. Fertilizers and animal wastes can be directly toxic if added in large quantities, especially if ammonia concentrations and water pH values are high. These materials can also lead to extreme nutrient concentrations in the fish pond, excessive plant growth, and eventually oxygen depletion. Erosion carries silt, sand and other materials into ponds where they settle and lead to a filling in of the pond. This shortens the useful life of the pond, creates problems with macrophytes, reduces the productive volume, sometimes increases turbidity (and thus reduces phytoplankton growth), and reduces fish production (Beveridge and Phillips, 1990, Fast, 1986).
The fertilization of fish ponds with human excreta and/or livestock manure is a widespread practice in Asia. However, the promotion of organic fertilisation is restricted by sociological and public health considerations for human excreta reuse and by the need to impound livestock in feedlots to utilize their manure in fish ponds (Edwards, 1990).
Many different kinds of chemicals are used in aquaculture. They include therapeutics, vaccines, hormones, flesh pigments, anaesthetics, disinfectants and water treatment compounds. Chemicals present in materials used in the construction of aquaculture systems can also enter into the water (Barg, 1992, Beveridge et al, 1990).
Bioactive compounds are considered a part of overall disease control strategies. The success or failure of intensive aquaculture can in certain circumstances depend on the correct use of such bioactive compounds to control infectious diseases and parasites (Anon., 1991).
The indiscriminate use of bioactive compounds has also caused concern about their release into the aquatic environment. The little data available on the quantity of chemicals present in aquaculture effluents suggests that they are low, but the use of chemicals varies greatly with species, intensity of culture and location (Beveridge et al, 1990). Also, there is increasing evidence showing that bioactive compounds remain in animal tissues for longer periods than have been recognized so far. Elimination of antibiotics from the fish flesh can take anything from a few days to a few months (Beveridge et al, 1990, GESAMP, 1991).
While traditional, plant-based compounds have long been used in the treatment of fish diseases in the tropics, the most common therapeutants in use today are formalin, potassium permanganate, dipterix and malachite green. The number of therapeutants used in the tropics is probably more limited than in temperate countries, although there are some important differences in the types and amounts used, especially concerning antibiotics and antimicrobials (Beveridge and Phillips, 1990).
Depending on how they are administered and how they are later eliminated from the body, therapeutics can enter the environment in several ways. Treatment is by bath, injection or inclusion of chemicals in food. Many chemicals used to treat gill and skin problems are given in bath form and these compounds are usually allowed to soak away through the soil once used. In cage and pen culture, treatment is at present usually carried out in situ, resulting in a release of all the chemicals directly into the environment (Beveridge et al, 1990).
Pesticides, particularly organophosphate compounds, are used in some areas of the world to regulate pests such as shrimps in fish ponds as well as ectoparasitic infestations. There have been no studies on the effect of these compounds on the pond ecosystem. It is known, however, that most of these chemicals are toxic to aquatic life at lower concentrations than those used to treat fish (Barg, 1992, Beveridge and Phillips, 1990).
Not only does intensive aquaculture carry much larger risks of serious disease outbreaks than extensive and semi-intensive aquaculture, it can also function as reservoirs of infection threatening other farmed and wild populations. Intensive systems make greater demands on water treatment chemicals, and drugs for disease prophylaxis and treatment. The misuse of these chemicals and drugs could have serious effects: e.g. pollution of adjacent waters as well as development of resistant strains of human pathogens (Barg, 1992, GESAMP, 1991, Pullin, 1989).
A species of bacteria can develop resistance to an antibiotic and it can transmit this resistance to different bacterial species by genes contained on extra chromosomal pieces of DNA3 called plasmids. Rates of spread of antibiotic resistance have been very fast. It is said that bacteria will develop resistance to most, and possibly all, antibiotics with which they are confronted (Brown, 1989, GESAMP, 1991).
It may not matter whether antibiotic resistance builds directly in human pathogens in the environment since it is possible for antibiotic resistance to be transferred to normal bacteria within the human gut if a number of antibiotic resistant bacteria are consumed. Thus antibiotic resistance could be transferred to human pathogens within the human gut (Brown, 1989).
An increasing number of hormones and growth promoters are used to change the sex, productive viability and growth of cultured organisms. Studies have been carried out to describe their physiological effect on the target organism, and the result suggests that the hormones are completely eliminated from the animal's body after several weeks. No studies of their wider ecological impact on the environment have been carried out (Beveridge et al, 1990, GESAMP, 1991).
Throughout the 1970s and 1980s, antifoulants were used to poison fouling organisms on cage and pen nets. These substances were not commonly used in the tropics. Today these chemicals are much less frequently used since fears have grown about the accumulation of organotin-and copper-based compounds in farmed fish flesh and the effects of these chemicals on the environment. (Beveridge et al, 1990, Beveridge and Phillips, 1990).
Some construction materials release substances into the aquatic environment (e.g. heavy metals, plastic additives). Many of these compounds are toxic to aquatic life, although some protection is given by their low water solubility, slow rate of leaching and dilution (Anon., 1991). Mortalities in coastal aquaculture have resulted from toxicant leaching from construction materials, and the environmental effects of these toxicants remain largely undetermined (GESAMP, 1991). In tropical aquaculture this problem is largely nonexistent since natural materials (such as wood or bamboo) are commonly used to construct cages and pens.
3 DNA: Deoxyribonucleic acid, the carrier of genetic information in cells.
GESAMP (Group of Experts on the Scientific Aspects of Marine Pollution) has come up with the following code of practice for the use of inhibitory compounds in aquaculture:
Medically important inhibitory compounds should be banned from use in aquaculture. However, some medically important compounds may need to be used in exceptional circumstances for certain specified diseases.
The availability of inhibitory compounds should be restricted to qualified individuals, such as veterinaries.
Access to inhibitory compounds should be denied to all laymen and inexperienced personnel.
The storage of inhibitory compounds should be in the manner recommended by the manufacturers/suppliers.
The use of inhibitory compounds should be strictly in accordance with written instructions from manufacturers/suppliers.
The use of pharmaceutical compounds should be by rotation. Thus, the repeated use of single compounds should be avoided.
The use of suitable withdrawal periods, after the use of pharmaceutical compounds, is necessary before animals are removed from the aquacultural facility.
The deliberate or accidental release of inhibitory compounds into the aquatic environment must be avoided.
Unused inhibitory compounds must be disposed of safely.
A surveillance program must be adopted to ensure that the code of practice is carried out.
Even in developed countries there are problems in implementing guidelines like these, and the question arises how it would be possible to follow them in developing countries. One of the most important precautions is to limit the availability of inhibitory compounds to laymen, and restrict the availability to only qualified persons such as veterinaries.
Beveridge et al (1990) define waste as “consisting of the materials used in aquatic animal production which are not removed during harvesting and which can finally find their way into the environment at large”.
It is impossible to predict aquaculture effluent characteristics with any accuracy. There are many variables regulating effluent quality, such as the quantities of uneaten food, faeces and urinary products. These vary with species cultured and the type of food provided, and are controlled by many factors such as body size and season, method and intensity of culture, management, and manner of discharge, whether the effluent is treated or diluted prior to discharge. The effect of aquaculture effluents on the environment is determined both by the nature and amount of the effluents and where and how they are discharged (Barg, 1992, Beveridge et al, 1990, Beveridge and Phillips, 1990).
Cage and other water-based systems differ from land-based ponds, tanks and raceways in that no wastes are trapped within the system but enter the environment directly (Beveridge and Phillips, 1990, Nyman, 1988, Kautsky and Folke, 1991). The handling, processing and marketing of fish can also cause some environmental effects such as waste from the fish processing industry and the use of insecticides in fish processing (Anon., Undated).
In both semi-intensive and intensive aquaculture, food is provided to the fish. A part remains uneaten because quantities and qualities are often unsuitable. Water exchange rate influences the probability with which a food item is encountered by the cultured organisms and thus the part of food which is uneaten, while flow characteristics will partially decide whether uneaten food and faecal particles remain unbroken and the proportion of these particles that settle within the system. The feed losses from cages are considerably greater than those from ponds (Beveridge et al, 1990, Beveridge and Phillips, 1990, Nyman, 1988).
Unless removed, mortalities may be another source of wastes from the aquaculture operation. While dead fish are easily detected and removed from tanks and raceways, they cannot be easily removed from ponds and cages. A number of studies have pointed out that unrecovered mortalities can add significantly to nitrogen and phosphorus waste loadings in the effluent (Beveridge et al, 1990, Beveridge and Phillips, 1990).
Even in quite intensive pond systems the water exchange is usually low. Also the water quality of the effluent is generally reasonable, even in intensively managed systems. It is during harvesting when up to 95% of the water may be discarded, that problems may arise, with a markedly increased effluent phosphorus, ammonia, BOD and COD4 (chemical oxygen demand) and settleable solids (Beveridge and Phillips, 1990).
In tanks and raceways there is a more or less continuous flow-through of water, the amount of which is determined by the biomass, species and environmental conditions. It has been shown that suspended solids and BOD effluent loadings in concrete raceways, expressed in terms of percentage of the food fed, fall with increased water exchange (Beveridge and Phillips, 1990).
All aquatic animals have a rich flora of micro-organisms in the gut, and the guts of fish can act as breeding places for Enterobacteriaceae, Aeromonas, and faecal Streptococci which enter the culture system through incoming water or feed. But little is known about the importance of gut bacteria excreted from aquaculture organisms and about the effects the bacterial loading may have on the environment (Beveridge et al, 1990, Beveridge and Phillips, 1990).
The effects of pollution by intensive aquaculture in temperate climates are well known. Wastes such as fish faeces and uneaten food in effluents and in sediments from cages have high BODs and contain large amounts of particulate matter. These can cause water quality to deteriorate and anoxic sediments to build up (Barg, 1992, Kautsky and Folke, 1991, Pullin, 1989). These sediments are richer than natural sediments in nutrients such as phosphorus, nitrogen and carbon. The organic matter in the sediment is decomposed by bacteria and this can cause anaerobic conditions within a few millimetres of the sediment surface. Under completely anoxic conditions the highly toxic gas H2S can develop. Release of gases is enhanced by bioturbation which is caused by the high number of pollution-tolerant macro-invertebrates that can occur (Barg, 1992, Beveridge and Phillips, 1990).
4 COD: Chemical oxygen demand.
At the current level of coastal fish farming, nutrient enrichment and eutrophication of open coastal waters is unlikely, but could occur where exchange of water with more open coastal waters is limited such as in semi-enclosed coastal embayment (Barg, 1992, GESAMP, 1991).
The primary effect of aquaculture effluents on running waters is to increase ammoniacal nitrogen and phosphorus concentrations in the water immediately downstream of the discharge. In lakes cage culture can cause long-term elevations of carbon, nitrogen and phosphorus levels. Carbon is not usually a limiting factor for productivity in fresh water systems. In the case of nitrogen and phosphorus, of which one or the other usually limits productivity in fresh waters, increased levels will lead to eutrophication. Eutrophication can also cause increased production and changes in the macrophyte and natural fish communities structure. It has been shown that fresh water aquaculture operations can increase growth of the natural fish community. This is partly caused by the ingestion of uneaten pellets by wild fish (Beveridge and Phillips, 1990).
Most aquaculture uses a large amount of water, and consequently aquaculture effluents are characterized by their relatively large volume. Usually they have relatively low concentrations of wastes compared with effluent from other industries (Beveridge et al, 1990).
Ponds, which represent the most common aquaculture system in the tropics and subtropics, differ from most other systems in that water exchange is usually low, even in fairly intensive systems. Only when the ponds are drained to facilitate harvesting are the effluent discharges large with extremely high levels of solids, BOD and nutrients. It has been difficult to show a correlation between the effluent solids loading and the biomass harvested to the pond soil erosion that almost always occurs (Beveridge et al, 1990).
The key objective in reducing effluent waste concentration is the fast and gentle removal of solids, since much of the BOD and nutrients are in the solid component of the wastes (Beveridge et al, 1990).
One of the most important ways of minimizing the impact of cage and mussel culture on the environment is the siting of cages in fast-flushing waters which quickly dilute and disperse wastes (Beveridge et al, 1990). In warm waters, integration of extensive with intensive cage culture can minimize eutrophication from intensive cage farming. One example is cages of filter-feeding bighead carp that have been used to remove phytoplankton from potable water reservoirs in Singapore (Phillips et al, 1990). This is one example of ecological engineering, where ecosystem processes and functions are integrated into the culturing process, thus minimizing negative environmental effects (Kautsky and Folke, 1991)
It is impossible to prevent species originally enclosed in aquaculture systems from eventual escape, often in large numbers. However, they do not necessarily colonize natural waters. Farmed fish have often been selected for characteristics which make them suitable for farming (for example, rapid growth and placid behaviour) but less well adapted to the natural ecosystem. Thus, escaped fish could at first outcompete native stocks, but later decrease, leaving the waters with a depleted fish stock. The progeny resulting from inter-breeding could also be inadequately adapted to the ecosystem (Anon., 1991, Beveridge and Phillips, 1990, GESAMP, 1991).
Production of transgenic fish (fish which have genetic material which incorporates material received from other fish) has started lately, mainly to improve performance traits in economically important species. It is not certain what ecological effects transgenic fishes will have on natural fish stocks. The effects will depend firstly on their changed phenotypes and secondly on the frequency and scale of introduction into ecosystems. It is possible that ecologically noxious transgenic fishes can be produced. This is because many fish varieties disperse easily and can get established in aquatic ecosystems and are fit in natural settings. They can also interact considerably with other organisms, and play a role in the various ecosystem processes (Kapuscinski and Hallerman, 1991).
Adverse impacts of escape of fish on the environment can be summarized (from Beveridge and Phillips, 1990 and Pullin, 1989) as follows:
The effects of the escapees in aquaculture operations include (from Carss, 1990):
A number of fish, invertebrate and seaweed species have been transferred or introduced from one region to another for aquaculture purposes. Transfers take place within the present geographical range of a species and are intended to strengthen stressed populations, reinforce genetic characteristics, or re-establish a species that has failed locally. Introductions are movements beyond the present geographical range of a species and are intended to introduce entirely new taxa into the flora and fauna. These movements can pose risks to human health, the integrity of ecosystems, agriculture, aquaculture and related primary industries. Transfers and introductions can alter or weaken the biodiversity of the receiving ecosystem through interbreeding, predation, competition for food and space and habitat destruction. If the introduced fish species are not suitable for local gear types or tastes, this can cause problems in local fishing and related communities (Anon., Undated, Anon., 1991, Beveridge and Phillips, 1990, GESAMP, 1991).
There have been very few reports of species introduced to the tropics for aquaculture purposes negatively affecting the host environment, the common problem being disruption of the host community. Beveridge and Phillips (1990) give the following reason for this: “Many of the traits that make species ideally suitable for aquaculture (i.e. high fecundity and rapid early development, flexible phenotypes, wide environmental tolerances, catholic habitat preferences and feeding habits) are the same as those found in invasive species”.
The effects of pollution from intensive aquaculture are the same as the effects from many other kinds of sources, such as sewage and agricultural run-off. It can be expected that intensive fish farming in the tropics causes changes in the environment that are similar to changes that occur in temperate climates -- provided the types of waste are similar and that productivity in tropical environments is also limited by light phosphorus and nitrogen levels. Due to the higher temperatures in the tropics, the reaction to pollution will probably be more rapid. Also, the understanding of tropical aquatic food webs is limited. It should be emphasized, however, that intensive aquaculture methods are not common in the tropics (Anon., 1991, Beveridge and Phillips, 1990).
Increases in nutrient due to aquaculture discharge of nitrogen and phosphorus will lead to eutrophication. This is because either nitrogen or phosphorus usually limits productivity in fresh waters. A general increase in algal densities may thus result. In temperate waters, cyanobacteria and other species tolerant of high P:N ratios are likely to become dominant, and shifts in phytoplankton community structure will affect water quality as well as the autotrophic food webs (Beveridge and Phillips, 1990).
Normally, eutrophication expresses itself as an overabundance of algae, with the waters turning into a green soup, but in the tropics there is more often a bloom of floating macrophytic vegetation, such plants as Eichhornia (the water hyacinth), Pistia and Salvinia. Macrophytes are a particular problem in shallow ponds where light can penetrate to the bottom during the early growing season. Symoens et al (1981) lists the negative effects of excessive macrophyte growth.
It may:
Deep ponds (greater than 1.5 m deep) with moderate-to-high turbidity seldom have a problem with macrophytes, except along their shorelines (Fast, 1986).
Also, the fish community itself is influenced by the eutrophication; it can cause increased production and change the structure of the fish community (Beveridge and Phillips, 1990).
Eutrophication is not always a negative effect. It can increase the production of the water and consequently the production of wild fish, thus increasing the yield of local fisheries.
The deposition of organic fish farm and bivalve waste has been shown to enrich the benthic ecosystem in the proximity of the aquaculture operation. The changes which take place include: the formation of anoxic sediments which can, in extreme cases, release carbon dioxide, methane and hydrogen sulphide; increased oxygen consumption through the sediment and efflux of dissolved nutrients; and changes in the community structure of the benthic macrofauna. The diversity of the macrofauna community can be reduced with increases in opportunistic pollution-tolerant species. In extreme cases, macrofauna can be completely absent. It has been suggested that the release of hydrogen sulphide gas (which is highly toxic) together with the hydrogen sulphide dissolved in the water, can cause farmed fish to deteriorate in health (through increased stress, reduced growth, gill damage and even mortality) and production to fall. A high level of enrichment leading to what has been termed souring of sites has been reported from a number of fish farms in several countries (Anon., 1991, Barg, 1992, Beveridge and Phillips, 1990, GESAMP, 1991).
In addition to the oxygen requirement of cultured species, wastes and biodeposits released by a farm need a lot of biochemical oxygen. Organic waste deposits increase the consumption of oxygen by the bacteria in the sediment, leading to depletion of oxygen in the bottom water. A reduction in the concentration of dissolved oxygen in water passing through cage farms has also been reported. A large-scale depletion of oxygen in coastal waters is, however, improbable. While the small short-term reduction in the concentration of oxygen in water passing through cage farms is important to the farmer, it is probably not ecologically significant (Anon., 1991, Barg, 1992, GESAMP, 1991).
All forms of aquaculture activity can affect wildlife. The activities of humans close to important breeding colonies and feeding grounds can be disturbing to wildlife, while the aquaculture facility itself can attract predatory species. These species can consist of a wide range of obligate (species that only eat fish) and facultative (species eating fish as well as other animals) fish-eating vertebrates such as mammals, fish and birds. Thus the predation of wild fish can be increased when predators are drawn to the impoundment site (Anon., Undated, Anon., 1991, Barg, 1992, Beveridge and Phillips, 1990, GESAMP, 1991, Nyman, 1988).
Coastal wetlands are among the most productive ecosystems in the world and play an important part in supporting the ecological productivity of nearby coastal waters. Mangrove forests are increasingly being cleared for aquaculture, especially in Asia. This can have negative effects on other human uses of the same areas; it can also adversely affect those commercial species which breed and grow in mangrove swamps since they are major nursery areas for many commercially important fish and shrimp species (Anon., Undated, Barg, 1992, GESAMP, 1991).
In recent years, fears that antibiotic-resistant bacteria could affect human health have increased. The use of antibiotics in aquaculture can cause antibiotic resistance in bacteria.
It may not matter if the human pathogens themselves are resistant to antibiotics, since it is possible that antibiotic resistance can be transferred to normal bacteria within the human gut if a number of antibiotic resistant bacteria are ingested. Thus antibiotic resistance could be transferred to human pathogens within the human gut. This is possible because a species of bacteria can transmit its resistance to an antibiotic to a different bacterial species through genes contained on extra chromosomal pieces of DNA called plasmids. (See Section 4.2.1). Brown (1989) states that “rates of spread of antibiotic resistance have been alarming, and it is possible that bacteria will evolve resistance to most, if not all, antibiotics with which they are challenged”.
A few fish pathogens have been connected with outbreaks of human disease, although it should be stressed that the extent of the problem is largely unknown. Outbreaks of human diseases associated with the consumption of shellfish are typhoid fever, cholera, infectious hepatitis and other viral diseases. Phycotoxins, produced by several armoured dinoflaggelates and diatoms, are responsible for shellfish poisoning (Brown, 1989, GESAMP, 1991).
Depuration, a cleaning process where the organism is kept in clean water for various lengths of time, is commonly used to diminish the risk of microbiologically contaminated filter-feeding invertebrates being sold for human consumption. Problems can occur with the cleanliness of the water used in depuration systems. For effective depuration, it is essential to use systems which encourage the animals to eliminate the pathogens in the water, while ensuring that there is adequate disinfection to kill them (Brown, 1989, GESAMP, 1991).
The creation of new fresh water environments may increase species diversity; but it can at the same time increase the incidence of water-borne human diseases such as schistosomiasis, malaria and yellow fever (Beveridge and Phillips, 1990). The invasion of man-made lakes by Eichhornia crassipes, Salvinia spp, Ceratophyllum spp, and Potamogeton pectinatus provide ideal habitats for snails, intermediate hosts of schistosomiasis in African man-made lakes. The effects of impoundments are advantageous to man in the case of onchocerciasis because the breeding places for the larvae of Simulium damnosum, the vector of the disease, are drowned for several kilometres above the dam, thus limiting its distribution (Symoens et al, 1981).
Aquaculture operations can have both positive and negative effects on the environment. Natural habitats and their biota can be negatively affected; so can human health. But aquaculture is also a way to make more efficient use of existing resources. Integrated aquaculture provides a way to use agriculture waste to make marginal lands more productive by converting plant and animal waste into high quality fish protein and by enriching pond mud for use as fertilizer and for improving soil quality on crop land.
It is important to realise that the effects on the environment from an aquaculture operation depend on the intensity of the system. Intensive aquaculture may have significant effects on the receiving aquatic environment, while the effects of extensive pond aquaculture are very small. In the tropics and subtropics this type of aquaculture system is the most common.
Aquaculture development, though generally desirable socio-economically, has considerable implications for water resource use. Integration of aquaculture with other activities is likely to be the most effective means of development. This enables water use to be shared, or enhances its value sufficiently to allow investment in improved water supply or treatment.
In any aquaculture development, the potential environmental effects must be taken into consideration at the planning stage itself. It is important to choose aquaculture methods and sites for development in such a way that the negative environmental effects are minimized.
One of the most important aspects is the availability of water. But through integration with agricultural practices, existing resources can be used in an efficient way, increasing the supply of protein to the rural population without causing negative environmental effects.
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