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Aquaculture, Freshwater
Aquaculture, Freshwater C E Boyd, Auburn University, AL, USA ã 2009 Elsevier Inc. All rights reserved.
Introduction Aquaculture is the production of aquatic animals and plants under managed and partially controlled conditions. It may be done for aesthetic or recreational purposes, e.g., aquarium keeping, water gardens, and sport fish ponds, but most aquaculture is for production of aquatic plants and animals for human consumption. Aquaculture is conducted by rural farmers in developing nations to supply food for their families. It also is done worldwide to produce fish and other aquatic organisms for domestic and international markets. This discussion will focus on freshwater aquaculture and particularly on fish farming.
Freshwater Aquaculture and World Fisheries The capture fishery had steadily increased in response to the demand for fish, shrimp, and other aquatic organisms by the growing human population. The capture fisheries apparently reached the upper limit in the late 1980s, since there have been fluctuations in annual production but no upward trend (Figure 1). Aquaculture began at least 2000 years ago in China, but it only became a common practice during the past century. Aquaculture has been growing at a remarkable rate since the 1970s. It now provides about 38% of total fisheries production. The world population is continuing to increase, and there will be a greater demand for fisheries products. Expansion of aquaculture is the only means of avoiding a gap between supply and demand for fisheries products in the future. The capture fisheries are mainly from marine ecosystems. In 2004, the marine catch was 87 242 000 metric tons (t) compared with 9 221 000 t from inland areas. Marine aquaculture production was slightly more than inland aquaculture production – 32 196 000 t as compared to 25 594 000 t, respectively. The farm gate value of marine aquaculture species was US$35.8 billion while the value of inland aquaculture products was US$34.5 billion. Freshwater aquaculture is conducted in many countries, but the greatest production is from Asia. In 2004, freshwater aquaculture production in Asia exceeded 24 000 000 t, and China alone reported production of 18 365 000 t. The rest of the world contributed about 2 322 000 t (Table 1). Although experts feel that China inflates its aquaculture data by a factor of 2 or 3, this country is the world’s leader in freshwater aquaculture.
234
The most commonly farmed fish species are carps (Table 2), but catfish, trout, and several other species are important. Species of crustaceans, mollusks, and aquatic plants also are cultured in freshwater. Warm water species that grow best at temperatures above 20 C dominate production, but trout and a few other cold water species are commonly farmed. Freshwater has a salinity below 1000 mg l1. In some arid, inland areas, surface waters have a greater salinity. Most freshwater aquaculture species can be cultured at salinities up to 3000 mg l1, and some can tolerate higher salinity. Brackish water species sometimes are cultured at inland sites where saline waters are available.
Aquaculture Production Units Ponds
Ponds are the most common production facilities in freshwater aquaculture. There are three basic types: embankment ponds, watershed ponds, excavated ponds. An embankment pond is made by building earthen embankments around the area in which water from streams, lakes, or wells is impounded. A watershed pond is formed by building a dam across a watershed to retain direct rainfall and runoff. Sometimes, several ponds may be constructed one above the other on a single watershed. The overflow from higher ponds passes into lower ones. Watershed ponds are sometimes called terrace or rain-fed ponds. Embankment and watershed ponds range in size from a few square meters to several hectares. They typically have discharge gates to allow overflow following rains. Dam boards in gates are removed to drain ponds. Ponds also can be equipped with a combination overflow and drainage system constructed of pipe. Excavated ponds are constructed by digging a basin to hold water. These ponds usually are built in areas where the water table is high, and ground water seeps into them. They also receive rainfall and a limited amount of runoff. Excavated ponds usually cannot be drained, but they can be emptied by pumping out the water. Enclosures
Cage and net pen culture also is a common technique for freshwater fish production. In cage culture, fish are confined in cages constructed of wire or
Applied Aspects of Inland Aquatic Ecosystems _ Aquaculture, Freshwater 235
Production (mt ⫻ 106)
160 Aquaculture Capture
140 120 100 80 60 40 20 0 1954
1961
1968
1975
1982
1989
1996
2003
Year Figure 1 Statistics for capture and aquaculture fisheries production.
Table 1 Inland fisheries production in 2004 by continent Continent
Production (t)
Asia Europe North America South America Africa
24 271 973 467 679 456 380 252 305 145 490
Table 2 World production of major freshwater aquaculture species for 2004 Common name
Scientific name
Production (t)
Silver carp
Hypophthalmichthys molitrix Ctenopharyngodon idelius Cyprinus carpio Hypophthalmichthys nobilis Carassius carassius Osteichthyes
3 933 792
Grass carp Common carp Bighead carp Crusican carp Miscellaneous freshwater fishes Nile tilapia Roho labro Catla Mrigal carp White amur bream Channel catfish Black carp Rainbow trout Miscellaneous tilapias Amur catfish Snakehead
Oreochromis niloticus Labeo rohita Catla catla Cirrhinus mrigala Parabramis pekinensis I. punctatus Mylopharyngodon piceus Oncorhynchus mykiss Oreochromis spp Silurus asotus Channa argus
3 836 483 3 386 129 2 101 688 1 949 758 1 802 430 1 315 405 761 123 615 576 573 657 516 869 351 357 296 446 290 007 257 173 246 857 239 056
fiber netting suspended from a floating frame. Cages range in size from 1 to 2000 m3 or more. Cages are secured by anchors in streams, lakes, and reservoirs. Net pens are formed by securing netting to poles driven into the bottom of a water body. Typically, net pens are formed by placing netting across a small embayment, but they may be installed along the shore. Net pens enclose areas of a few square meters to a hectare or more. Ideal locations for both cages and net pens are protected from heavy waves but have good water movement. Water movement flushes waste from cages and net pens and provides a continuous supply of dissolved oxygen.
Flowing Water Systems
These systems include raceways, tanks, and other culture units through which water flows continuously (Figure 2). Flow-through systems usually are supplied by gravity flow from springs, streams, or other bodies of surface water. Typically, water flow equals 2 or 3 times the volume of culture units per hour. Inflowing water provides oxygen and flushes out wastes. Raceway units often are constructed in series on sloping terrain so that water falls from the tail of one unit into the head of the next to affect oxygenation. Flowthrough systems are especially popular for rainbow trout culture. Water Reuse Systems
An outdoor water reuse system consists of culture units from which water is passed through a sedimentation basin to a treatment pond and returned to the culture units (Figure 3). Indoor reuse systems usually are installed in greenhouses and rely on conventional wastewater treatment equipment (Figure 4). Effluents
236 Applied Aspects of Inland Aquatic Ecosystems _ Aquaculture, Freshwater
Figure 2 A raceway for trout culture.
Treatment pond
Sedimentation area
stocking in sport fish ponds are produced in hatcheries. Adult brood stocks are placed in hatchery ponds to spawn naturally. After hatching, fry are left in ponds until they become fingerlings. Fingerlings are harvested and stocked in sport fish ponds. Fingerlings grow to adult size and spawn naturally in sport fish ponds and restocking normally is not necessary for many years. Channel catfish, Ictalurus punctatus, a popular aquaculture species in the United States also is spawned in ponds. Spawning boxes or cans are placed in ponds at specific locations. Mating pairs enter these containers to spawn. Workers remove the egg masses and transfer them to baskets in a hatchery trough where a slowly revolving paddlewheel moves water over each egg mass to assure adequate aeration. After eggs hatch, fry are reared for a short while in tanks before being transferred to nursery ponds where they grow to suitable size for stocking in grow-out units. Channel catfish typically spawn in mid spring, and seed stocks are held in nursery ponds until the next spring before they are stocked. Eggs and sperm usually are stripped from trout brood stock. These gametes are mixed to effect fertilization and eggs are held in hatching jars or hatching trays with continuous water exchange until hatching occurs. Fry are reared in hatchery containers until they are large enough to move to culture units.
Culture units
Grow-Out Methods Ponds Figure 3 Illustration of aquaculture production systems with intensive culture units, sedimentation area, and treatment pond. Drawing is not to scale.
may overflow from outdoor systems following rainfall, and water must occasionally be discharged from indoor systems when new water is applied to lower salinity or filters are cleaned.
Hatcheries Small animals for stocking aquaculture grow-out units are called seed stock. Once seed stock were captured from natural waters and stocked into grow-out units. This method gradually gave way to seed stock production in hatcheries. The practice of capturing wild, adult animals for spawning in hatcheries has been largely replaced by use of farmed animals for brood stock. This allows improvement of brood stock through selective breeding. Hatchery techniques vary greatly with species. Culture of sunfish Lepomis spp. in sport fish ponds is popular in the United States. Fingerling sunfish for
Fertilizers may be used in ponds to increase the phytoplankton base of the food web (Figure 5) and allow greater production of the culture species. Low concentrations of nitrogen and phosphorus in pond waters commonly limit phytoplankton growth. These two nutrients may be supplied in organic fertilizers such as animal manures and agricultural wastes. Organic matter decomposes releasing inorganic nitrogen and phosphorus. Decomposition of organic fertilizer also removes dissolved oxygen from water, and applications of manure must be limited to avoid excessively low dissolved oxygen concentration. Some species such as tilapia also may feed directly on organic fertilizer particles. Chemical fertilizers such as urea, ammonia nitrate, triple superphosphate, and ammonium phosphate are 20–60 times more concentrated in nitrogen and phosphorus than are manures. Their advantages are lower application rates, predictable composition, availability, absence of possible harmful microorganisms and antibiotic residues sometimes found in manures, and a lower demand for dissolved oxygen.
Applied Aspects of Inland Aquatic Ecosystems _ Aquaculture, Freshwater 237
Greenhouse enclosure Degassing and oxygenation
Settling basin Intermittent effluent Make-up water
Coarse filter
Culture units Pump
UV disinfecter
pH adjuster
Biofilter
Heat exchanger
Figure 4 Illustration of an indoor, intensive aquaculture system with waste treatment processes. Drawing is not to scale.
Sunlight nutrients
Phytoplankton
Insects
Detritus Zooplankton
Tilapia Benthos
Figure 5 The food web in a tilapia pond.
Excessive fertilization produces too much phytoplankton. These photosynthetic microorganisms produce dissolved oxygen during daylight hours as illustrated: 6CO2 þ 6H2 O
! C H
N; P; and other inorganic nutrients
6
12 O6
þ 6O2
However, they also use dissolved oxygen in respiration: C6 H12 O6 þ 6O2
! 6CO
2
þ 6H2 O þ Energy
During daytime, photosynthesis normally exceeds respiration and dissolved oxygen concentration is high. At night, photosynthesis stops but respiration continues and dissolved oxygen concentration declines. Dissolved oxygen concentration should not fall below 3 or 4 mg l1 for most warm water culture species, and concentrations below 5 mg l1 are undesirable for cold
water species. Low dissolved oxygen concentration may kill culture animals, but more commonly, it stresses them. Stress results in greater susceptibility to diseases, poor appetite, and slower growth. Ponds constructed in areas with highly-leached, acidic soils have waters with low pH and low total alkalinity. Most aquaculture species grow best at pH 7–8.5, and total alkalinity should be above 50 mg l1 (as equivalent calcium carbonate). Acidic pond waters are neutralized by treatment with agricultural limestone or other liming materials. Feeds may be used to increase production above that possible in fertilized ponds. Aquaculture feeds are made from plant meals, fish meal and other animal meals, mineral supplements, vitamins, and other additives (Table 3). Aquaculture feeds typically are formed into small pellets. They contain 8–10% moisture and 25–40% crude protein. Depending upon the species for which they are used, feeds vary from 5% to 20% in fat content. Feeds are applied one or more times per day at typical rates of 2–5% of body weight of culture species per day. Culture animals do not completely convert feed to biomass. Usually, 1.5–2.5 kg of feed must be applied to obtain 1 kg of weight gain. The ratio of feed applied to net gain in biomass is called the feed conversion ratio (FCR). If 2000 kg of feed results in a gain of 1000 kg biomass, the FCR is 2.0. Feed is 90–92% dry matter while culture animals are about 25% dry matter. Thus, the amount of wastes resulting from feeding is much greater than what the FCR suggests. Nevertheless, the FCR is an important indicator of feed use efficiency because fish are sold on a live weight basis.
238 Applied Aspects of Inland Aquatic Ecosystems _ Aquaculture, Freshwater Table 3 Major ingredients of typical feeds for some common, freshwater aquaculture species Ingredient content (%)
Soybean meal Cottonseed meal Corn meal Wheat middlings Fish meal Shrimp head meal Squid meal Rendered products Oil
Trout
Tilapia
Channel catfish
15.0 — — 27.0 25.0 — — 15.0 16.0
38.3 — 48.8 4.0 6.0 — — — 1.5
34.5 12.0 20.4 20.0 2.0 — — 4.0 2.0
Feeding wastes include uneaten feed, feces, and dissolved nutrients such as carbon dioxide, ammonia, and phosphate. Uneaten feed and feces decompose consuming dissolved oxygen and releasing carbon dioxide, ammonia, phosphate, and other inorganic nutrients. As the feeding rate in a pond increases, the demand for oxygen to decompose waste increases and the water becomes enriched with nutrients. Dense phytoplankton blooms occur because of abundant nutrients, and dangerously low dissolved oxygen concentrations may occur at night. Ammonia, the major nitrogenous metabolite of aquatic animals, sometimes accumulates to harmful concentrations. However, low dissolved oxygen concentration typically becomes problematic before ammonia toxicity occurs. Feeding alone can increase the production of omnivorous and carnivorous species by 10–20 times above that possible in fertilized ponds. With plankton-feeding species such as tilapia, feeding alone may only cause a 2- or 3-fold increase in production over that possible in fertilized ponds. Two methods may be used to increase the production possible in ponds receiving feed. Water exchange may be used to flush out phytoplankton, nutrients, and toxic metabolites and to add water of higher dissolved oxygen concentration. The disadvantages of water exchange are that energy often is required for pumping, potential pollutants are flushed into natural waters, and effectiveness is relatively low. Mechanical aeration is a more effective than water exchange for increasing the dissolved oxygen concentration and aquatic animal production in ponds. Also, higher oxygen concentration increases the ability of microorganisms to decompose organic matter and oxidize ammonia. There are two basic types of mechanical aerators. One type releases air bubbles into the water, and oxygen within the bubbles diffuses into the water. The other type splashes water into the air. The splashing action increases the surface area of contact
between air and water to allow oxygen from the air to enter the water. Aeration induces water circulation in ponds to prevent thermal stratification. Induced circulation also moves oxygenated water across the pond bottom to avoid low dissolved oxygen concentration at the sediment–water interface. The most common way of harvesting fish from ponds is to gradually lower the water level to concentrate fish in a small area from which they are captured in nets or seines. The water level usually is lowered in two or more stages to facilitate capture of fish. Finally, the pond is completely drained, and any fish remaining on the bottom are removed by hand. Some species, e.g., channel catfish, are harvested from ponds using large seines that are stretched across the entire pond and pulled by a tractor. Mesh size of seines is selected to allow small fish to escape. Fingerling catfish are stocked into ponds to replace the harvested fish. Flowing Water Systems
Phytoplankton blooms do not develop in flowing water systems because of rapid water exchange. Inflowing water is the primary source of dissolved oxygen in unaerated systems, and the main loss of dissolved oxygen is fish respiration. In the culture of trout and other cold water species, dissolved oxygen should not fall below 5 mg l1, and 5 kg day1 of feed can be applied for each kilogram of available dissolved oxygen. Available dissolved oxygen in cold water systems is calculated by subtracting 5 mg l1 (5 g m3) from the concentration of dissolved oxygen in inflow. Suppose an inflow of 1 m3 min1 contains 10 mg l1 (10 g m3) of dissolved oxygen. Over a 24-h period, there will be 7.2 kg of available oxygen (1 m3 m1 1440 min day1 5 g oxygen m3 10–3), and a feed input of 36 kg day1 (7.2 kg oxygen 5 kg feed kg oxygen1) would be permissible. At a typical feeding rate of 3% body weight daily, 2400 kg fish could be held in a culture unit through which water initially containing 10 mg l1 dissolved oxygen flows at 1 m3 s1. Flow-through systems can be used to produce warm water species, but in warm water systems, only about 4 kg feed may be applied for each kilogram of available oxygen. High stocking density is permissible in flowthrough units, for the main factor limiting carrying capacity is availability of dissolved oxygen not space. Fish can be grown under extremely crowded conditions provided water quality is good. Mechanical aeration may be applied to flowthrough systems to increase dissolved oxygen availability, carrying capacity, and production. Aerators such as those used in ponds can be operated in flowing
Applied Aspects of Inland Aquatic Ecosystems _ Aquaculture, Freshwater 239
water culture, but devices that bring water and pure oxygen into contact are used more commonly. Solid wastes consisting of uneaten feeds and feces that accumulate in the bottoms of flow-through culture units can be removed by suction devices or through drains in the bottoms of units. However, ammonia is excreted by fish and accumulates in water of culture units. Thus, there are limits on the increase in carrying capacity through aeration, for ammonia may become a limiting factor at high fish densities.
Cages and Net Pens
Natural food organisms are scarce in cages and net pens, and manufactured feed is applied to enhance production. Soluble and suspended wastes are flushed from culture units by water movement. Larger waste particles settle beneath or near cages and on the bottom area enclosed by pens. Density of fish in cages may reach 80–100 kg m3 where water movement is adequate to replenish dissolved oxygen used in fish respiration and flush out ammonia and other wastes. Fish densities similar to those of cages probably would be permissible in pens at sites with rapid water exchange, but pens usually are operated at lower densities. Low dissolved oxygen concentration in water bodies containing cage and net pen facilities can cause fish mortality. A common cause of oxygen depletion in lakes and reservoirs is sudden thermal destratification resulting from heavy rains, strong winds, cool weather, or a combination of these factors. Fish culture often contributes to these events because nutrients from feeding wastes stimulate phytoplankton growth to favor strong thermal stratification, and wastes settling into the deeper water contribute to oxygen depletion in the lower stratum. The amount of cage and net pen culture in water bodies should be limited to avoid excessive eutrophication and periodic dissolved oxygen depletion.
culture facilities usually is no greater per total water area and volume than that achieved in static pond culture. Indoor, water reuse systems are equipped with water treatment equipment such as mechanical and biological filters, aerators, clarifiers, skimmers, pH adjusters, etc. This equipment provides treatment to allow water reuse. A very small proportion of freshwater aquaculture production is from water reuse systems. There is much interest in these systems for they require less space, use less water, and discharge less pollution per unit of production than other grow-out methods. Their contribution to production will likely increase dramatically in the future.
Aquatic Animal Health Fish and other culture species may have parasites and be infected by fungal, bacterial, and viral diseases. Many chemicals, including antibiotics, have been used to combat parasites and diseases. Some antibiotics are banned for use in aquaculture because of possible harmful effects of their residues on humans. Environmental advocacy groups are concerned over antibiotic use in aquaculture because of the possibility for development of antibiotic-resistant strains of bacteria and other disease organisms. A better way of controlling diseases is to implement an aquatic animal health management program. Specific pathogen-free brood stock can be developed and seed stock checked for diseases before stocking them in culture units. Culture units can be managed to avoid stressing animals and making them more susceptible to disease. After harvest, culture units can be dried out to eliminate disease organisms. When the previous crop was infected with disease, culture units may be treated with chlorine compounds or lime to kill possible pathogens that might survive dry-out.
Resource Use Issues Water Reuse Systems
In outdoor reuse systems, culture units are similar to those used in traditional flowing water aquaculture. Crowding of animals facilitates grading for size, feeding, harvesting, and other management operations. Water is passed through a pond or other water body for purification by natural physical, chemical, and biological processes, and mechanical aeration may be applied to enhance waste treatment capacity. The area and volume of the treatment system must be many times that of the culture units to allow adequate time for water purification. Production in such
Freshwater aquaculture is an important component of world fisheries production, and growth of this sector will be needed as the demand for fisheries products increases in response to human population growth. There are several problems that must be overcome if freshwater aquaculture is to continue to expand.
Land and Water Use
Expansion of pond aquaculture competes with traditional agriculture and other human endeavors for land and water. Thus, intensive production should be
240 Applied Aspects of Inland Aquatic Ecosystems _ Aquaculture, Freshwater
encouraged. Channel catfish farming in the United States is a good example of intensification. Typical production has increased from less than 2000 kg ha1 in the 1960s to over 5000 kg ha1 today. This increase has been achieved primarily by development of better feeds and application of mechanical aeration. Water use also has declined drastically. In the 1960s, water exchange was used when dissolved oxygen concentrations were low and ponds were drained annually to harvest fish. Water exchange is no longer used, and fish are harvested by seining. Ponds are typically drained at 5- to 10-year intervals.
Wetland Destruction
Aquaculture farms constructed in wetlands destroy habitat critical to maintaining balanced ecosystems. Disruption of wetlands can have serious ecological consequences, including deterioration of surface water quality because wetlands purify runoff before it enters streams, lakes, and other water bodies. Many nations have restricted development in wetlands, and others should be encouraged to do so.
Water Pollution
Effluents from aquaculture facilities contain elevated concentrations of nutrients, organic matter, and suspended solids and can pollute natural waters. Aquaculture associations, governments, international development agencies, and environmental advocacy groups promote better practices to lessen pollution by aquaculture. These practices include measures such as better feeds and feeding practices, mechanical aeration, reduction in water exchange, water reuse, harvest without draining ponds, and treatment of effluents in sedimentation basins. Some governments have developed effluent regulations for aquaculture. Regulations may require water quality monitoring and compliance with effluent water quality criteria such as limits on concentrations of nutrients, biochemical oxygen demand, dissolved oxygen, pH, and suspended solids. However, it is more common for regulations to mandate use of better production practices.
Marine Fish Meal and Oil Use
Aquaculture feeds usually contain fish meal and fish oil of marine origin. Aquaculture uses around 50% of current fish meal production and about 80% of fish oil production. The growth of aquaculture could be greatly restrained by future shortages of fish meal and fish oil. Research to develop feeds containing less fish meal and fish oil is needed, and greater use
of fish meal and oil made from fish processing wastes should be encouraged. An example of greater efficiency in fish meal use is afforded by the channel catfish industry in the United States. In the 1960s, channel catfish feed contained 12–14% fish meal. Research on feeds has allowed the fish meal content of catfish feeds to be reduced to 2–4%. Genetic Improvement and Escapes of Farm Animals
Genetic improvement of aquaculture species also provides promise in improving production. Strains of several species that grow faster, exhibit greater disease resistance, and have other more favorable characteristics for aquaculture have been produced through selective breeding. There has been little interest in genetically-modified aquaculture species. Nevertheless, environmentalists are concerned that escapees of farm fish will result in changes in the gene pools of natural populations. From the consumer perspective, there does not appear to be much concern over products resulting from selective breeding. There is widespread concern over the use of genetically-modified organisms, and the role of such organisms in the future of aquaculture is uncertain. The escape of nonnative species introduced for aquaculture also can lead to changes in ecosystems. For example, tilapia is a very aggressive species that can out-compete most native species when introduced into a region. Although concerns about escapes of animals from aquaculture farms are real, aquaculture is widespread, and most of the damage has probably already been done. This is especially true for tilapias that have been widely introduced in the tropics and subtropics. Nevertheless, practices to prevent escapes of aquaculture species should be implemented to avoid as much further damage as possible.
Responsible Aquaculture Consumers are becoming more environmentally aware and expressing a preference for fisheries products produced by environmentally- and sociallyresponsible methods. Some retailers and restaurant chains are sourcing aquaculture products from farmers willing to comply with specific environmental and social practices. There is a growing interest in aquaculture certification. A certified farm would agree to comply with environmental, social, and food safety standards. The farm would be inspected by a third party for verification of compliance. Environmental labeling of aquaculture products is expected to become a common practice in the next few years.
Applied Aspects of Inland Aquatic Ecosystems _ Aquaculture, Freshwater 241
Further Reading Beveridge MCM (1996) Cage Culture, 2nd edn. Oxford: Fishing News Books. Boyd CE and Tucker CS (1998) Pond Aquaculture Water Quality Management. Boston, MA: Kluwer Academic. Boyd CE, McNevin AA, Clay J, and Johnson HM (2005) Certification issues for some common aquaculture species. Reviews in Fisheries Science 13: 231–279. Boyd CE and Watten BJ (1989) Aeration systems in aquaculture. Reviews of Aquatic Science 1: 425–472. Federal Register (2004, August 23) Effluent limitation guidelines and new source performance standards for the concentrated aquatic animal production point source category: Final rule. Federal Register, Vol. 69(162), pp. 51892–51930. Washington, DC:Office of the Federal Register, National Archives and Records Administration. Goldburg R and Triplett T (1997) Murky Waters: Environmental Effects of Aquaculture in the United States. Washington, DC: Environmental Defense Fund.
Nash CE and Novotny AJ (eds.) (1995) Production of Aquatic Animals: Fishes, Amsterdam, the Netherlands: Elsevier. Sedgwick SD (1990) Trout Farming Handbook, 5th edn. Oxford, England: Fishing News Books. Soderberg RW (1994) Flowing Water Fish Culture, Boca Raton, FL: CRC Press. Timmons MB, Ebeling JE, Wheaton FW, Summerfelt FT, and Vinci BJ (2001) Recirculating Aquaculture Systems, 2nd edn. Ithaca, NY: Cayuga Aqua Ventures. Tucker CS (1996) The ecology of channel catfish ponds in northwest Mississippi. Reviews in Fisheries Science 4: 1–55. Yoo KH and Boyd CE (1994) Hydrology and Water Supply for Aquaculture. New York: Chapman and Hall.
Relevant Website www.fao.org – Food and Agriculture Organization (FAO).
Introduction Aquaculture is the production of aquatic animals and plants under managed and partially controlled conditions. It may be done for aesthetic or recreational purposes, e.g., aquarium keeping, water gardens, and sport fish ponds, but most aquaculture is for production of aquatic plants and animals for human consumption. Aquaculture is conducted by rural farmers in developing nations to supply food for their families. It also is done worldwide to produce fish and other aquatic organisms for domestic and international markets. This discussion will focus on freshwater aquaculture and particularly on fish farming.
Freshwater Aquaculture and World Fisheries The capture fishery had steadily increased in response to the demand for fish, shrimp, and other aquatic organisms by the growing human population. The capture fisheries apparently reached the upper limit in the late 1980s, since there have been fluctuations in annual production but no upward trend (Figure 1). Aquaculture began at least 2000 years ago in China, but it only became a common practice during the past century. Aquaculture has been growing at a remarkable rate since the 1970s. It now provides about 38% of total fisheries production. The world population is continuing to increase, and there will be a greater demand for fisheries products. Expansion of aquaculture is the only means of avoiding a gap between supply and demand for fisheries products in the future. The capture fisheries are mainly from marine ecosystems. In 2004, the marine catch was 87 242 000 metric tons (t) compared with 9 221 000 t from inland areas. Marine aquaculture production was slightly more than inland aquaculture production – 32 196 000 t as compared to 25 594 000 t, respectively. The farm gate value of marine aquaculture species was US$35.8 billion while the value of inland aquaculture products was US$34.5 billion. Freshwater aquaculture is conducted in many countries, but the greatest production is from Asia. In 2004, freshwater aquaculture production in Asia exceeded 24 000 000 t, and China alone reported production of 18 365 000 t. The rest of the world contributed about 2 322 000 t (Table 1). Although experts feel that China inflates its aquaculture data by a factor of 2 or 3, this country is the world’s leader in freshwater aquaculture.
234
The most commonly farmed fish species are carps (Table 2), but catfish, trout, and several other species are important. Species of crustaceans, mollusks, and aquatic plants also are cultured in freshwater. Warm water species that grow best at temperatures above 20 C dominate production, but trout and a few other cold water species are commonly farmed. Freshwater has a salinity below 1000 mg l1. In some arid, inland areas, surface waters have a greater salinity. Most freshwater aquaculture species can be cultured at salinities up to 3000 mg l1, and some can tolerate higher salinity. Brackish water species sometimes are cultured at inland sites where saline waters are available.
Aquaculture Production Units Ponds
Ponds are the most common production facilities in freshwater aquaculture. There are three basic types: embankment ponds, watershed ponds, excavated ponds. An embankment pond is made by building earthen embankments around the area in which water from streams, lakes, or wells is impounded. A watershed pond is formed by building a dam across a watershed to retain direct rainfall and runoff. Sometimes, several ponds may be constructed one above the other on a single watershed. The overflow from higher ponds passes into lower ones. Watershed ponds are sometimes called terrace or rain-fed ponds. Embankment and watershed ponds range in size from a few square meters to several hectares. They typically have discharge gates to allow overflow following rains. Dam boards in gates are removed to drain ponds. Ponds also can be equipped with a combination overflow and drainage system constructed of pipe. Excavated ponds are constructed by digging a basin to hold water. These ponds usually are built in areas where the water table is high, and ground water seeps into them. They also receive rainfall and a limited amount of runoff. Excavated ponds usually cannot be drained, but they can be emptied by pumping out the water. Enclosures
Cage and net pen culture also is a common technique for freshwater fish production. In cage culture, fish are confined in cages constructed of wire or
Applied Aspects of Inland Aquatic Ecosystems _ Aquaculture, Freshwater 235
Production (mt ⫻ 106)
160 Aquaculture Capture
140 120 100 80 60 40 20 0 1954
1961
1968
1975
1982
1989
1996
2003
Year Figure 1 Statistics for capture and aquaculture fisheries production.
Table 1 Inland fisheries production in 2004 by continent Continent
Production (t)
Asia Europe North America South America Africa
24 271 973 467 679 456 380 252 305 145 490
Table 2 World production of major freshwater aquaculture species for 2004 Common name
Scientific name
Production (t)
Silver carp
Hypophthalmichthys molitrix Ctenopharyngodon idelius Cyprinus carpio Hypophthalmichthys nobilis Carassius carassius Osteichthyes
3 933 792
Grass carp Common carp Bighead carp Crusican carp Miscellaneous freshwater fishes Nile tilapia Roho labro Catla Mrigal carp White amur bream Channel catfish Black carp Rainbow trout Miscellaneous tilapias Amur catfish Snakehead
Oreochromis niloticus Labeo rohita Catla catla Cirrhinus mrigala Parabramis pekinensis I. punctatus Mylopharyngodon piceus Oncorhynchus mykiss Oreochromis spp Silurus asotus Channa argus
3 836 483 3 386 129 2 101 688 1 949 758 1 802 430 1 315 405 761 123 615 576 573 657 516 869 351 357 296 446 290 007 257 173 246 857 239 056
fiber netting suspended from a floating frame. Cages range in size from 1 to 2000 m3 or more. Cages are secured by anchors in streams, lakes, and reservoirs. Net pens are formed by securing netting to poles driven into the bottom of a water body. Typically, net pens are formed by placing netting across a small embayment, but they may be installed along the shore. Net pens enclose areas of a few square meters to a hectare or more. Ideal locations for both cages and net pens are protected from heavy waves but have good water movement. Water movement flushes waste from cages and net pens and provides a continuous supply of dissolved oxygen.
Flowing Water Systems
These systems include raceways, tanks, and other culture units through which water flows continuously (Figure 2). Flow-through systems usually are supplied by gravity flow from springs, streams, or other bodies of surface water. Typically, water flow equals 2 or 3 times the volume of culture units per hour. Inflowing water provides oxygen and flushes out wastes. Raceway units often are constructed in series on sloping terrain so that water falls from the tail of one unit into the head of the next to affect oxygenation. Flowthrough systems are especially popular for rainbow trout culture. Water Reuse Systems
An outdoor water reuse system consists of culture units from which water is passed through a sedimentation basin to a treatment pond and returned to the culture units (Figure 3). Indoor reuse systems usually are installed in greenhouses and rely on conventional wastewater treatment equipment (Figure 4). Effluents
236 Applied Aspects of Inland Aquatic Ecosystems _ Aquaculture, Freshwater
Figure 2 A raceway for trout culture.
Treatment pond
Sedimentation area
stocking in sport fish ponds are produced in hatcheries. Adult brood stocks are placed in hatchery ponds to spawn naturally. After hatching, fry are left in ponds until they become fingerlings. Fingerlings are harvested and stocked in sport fish ponds. Fingerlings grow to adult size and spawn naturally in sport fish ponds and restocking normally is not necessary for many years. Channel catfish, Ictalurus punctatus, a popular aquaculture species in the United States also is spawned in ponds. Spawning boxes or cans are placed in ponds at specific locations. Mating pairs enter these containers to spawn. Workers remove the egg masses and transfer them to baskets in a hatchery trough where a slowly revolving paddlewheel moves water over each egg mass to assure adequate aeration. After eggs hatch, fry are reared for a short while in tanks before being transferred to nursery ponds where they grow to suitable size for stocking in grow-out units. Channel catfish typically spawn in mid spring, and seed stocks are held in nursery ponds until the next spring before they are stocked. Eggs and sperm usually are stripped from trout brood stock. These gametes are mixed to effect fertilization and eggs are held in hatching jars or hatching trays with continuous water exchange until hatching occurs. Fry are reared in hatchery containers until they are large enough to move to culture units.
Culture units
Grow-Out Methods Ponds Figure 3 Illustration of aquaculture production systems with intensive culture units, sedimentation area, and treatment pond. Drawing is not to scale.
may overflow from outdoor systems following rainfall, and water must occasionally be discharged from indoor systems when new water is applied to lower salinity or filters are cleaned.
Hatcheries Small animals for stocking aquaculture grow-out units are called seed stock. Once seed stock were captured from natural waters and stocked into grow-out units. This method gradually gave way to seed stock production in hatcheries. The practice of capturing wild, adult animals for spawning in hatcheries has been largely replaced by use of farmed animals for brood stock. This allows improvement of brood stock through selective breeding. Hatchery techniques vary greatly with species. Culture of sunfish Lepomis spp. in sport fish ponds is popular in the United States. Fingerling sunfish for
Fertilizers may be used in ponds to increase the phytoplankton base of the food web (Figure 5) and allow greater production of the culture species. Low concentrations of nitrogen and phosphorus in pond waters commonly limit phytoplankton growth. These two nutrients may be supplied in organic fertilizers such as animal manures and agricultural wastes. Organic matter decomposes releasing inorganic nitrogen and phosphorus. Decomposition of organic fertilizer also removes dissolved oxygen from water, and applications of manure must be limited to avoid excessively low dissolved oxygen concentration. Some species such as tilapia also may feed directly on organic fertilizer particles. Chemical fertilizers such as urea, ammonia nitrate, triple superphosphate, and ammonium phosphate are 20–60 times more concentrated in nitrogen and phosphorus than are manures. Their advantages are lower application rates, predictable composition, availability, absence of possible harmful microorganisms and antibiotic residues sometimes found in manures, and a lower demand for dissolved oxygen.
Applied Aspects of Inland Aquatic Ecosystems _ Aquaculture, Freshwater 237
Greenhouse enclosure Degassing and oxygenation
Settling basin Intermittent effluent Make-up water
Coarse filter
Culture units Pump
UV disinfecter
pH adjuster
Biofilter
Heat exchanger
Figure 4 Illustration of an indoor, intensive aquaculture system with waste treatment processes. Drawing is not to scale.
Sunlight nutrients
Phytoplankton
Insects
Detritus Zooplankton
Tilapia Benthos
Figure 5 The food web in a tilapia pond.
Excessive fertilization produces too much phytoplankton. These photosynthetic microorganisms produce dissolved oxygen during daylight hours as illustrated: 6CO2 þ 6H2 O
! C H
N; P; and other inorganic nutrients
6
12 O6
þ 6O2
However, they also use dissolved oxygen in respiration: C6 H12 O6 þ 6O2
! 6CO
2
þ 6H2 O þ Energy
During daytime, photosynthesis normally exceeds respiration and dissolved oxygen concentration is high. At night, photosynthesis stops but respiration continues and dissolved oxygen concentration declines. Dissolved oxygen concentration should not fall below 3 or 4 mg l1 for most warm water culture species, and concentrations below 5 mg l1 are undesirable for cold
water species. Low dissolved oxygen concentration may kill culture animals, but more commonly, it stresses them. Stress results in greater susceptibility to diseases, poor appetite, and slower growth. Ponds constructed in areas with highly-leached, acidic soils have waters with low pH and low total alkalinity. Most aquaculture species grow best at pH 7–8.5, and total alkalinity should be above 50 mg l1 (as equivalent calcium carbonate). Acidic pond waters are neutralized by treatment with agricultural limestone or other liming materials. Feeds may be used to increase production above that possible in fertilized ponds. Aquaculture feeds are made from plant meals, fish meal and other animal meals, mineral supplements, vitamins, and other additives (Table 3). Aquaculture feeds typically are formed into small pellets. They contain 8–10% moisture and 25–40% crude protein. Depending upon the species for which they are used, feeds vary from 5% to 20% in fat content. Feeds are applied one or more times per day at typical rates of 2–5% of body weight of culture species per day. Culture animals do not completely convert feed to biomass. Usually, 1.5–2.5 kg of feed must be applied to obtain 1 kg of weight gain. The ratio of feed applied to net gain in biomass is called the feed conversion ratio (FCR). If 2000 kg of feed results in a gain of 1000 kg biomass, the FCR is 2.0. Feed is 90–92% dry matter while culture animals are about 25% dry matter. Thus, the amount of wastes resulting from feeding is much greater than what the FCR suggests. Nevertheless, the FCR is an important indicator of feed use efficiency because fish are sold on a live weight basis.
238 Applied Aspects of Inland Aquatic Ecosystems _ Aquaculture, Freshwater Table 3 Major ingredients of typical feeds for some common, freshwater aquaculture species Ingredient content (%)
Soybean meal Cottonseed meal Corn meal Wheat middlings Fish meal Shrimp head meal Squid meal Rendered products Oil
Trout
Tilapia
Channel catfish
15.0 — — 27.0 25.0 — — 15.0 16.0
38.3 — 48.8 4.0 6.0 — — — 1.5
34.5 12.0 20.4 20.0 2.0 — — 4.0 2.0
Feeding wastes include uneaten feed, feces, and dissolved nutrients such as carbon dioxide, ammonia, and phosphate. Uneaten feed and feces decompose consuming dissolved oxygen and releasing carbon dioxide, ammonia, phosphate, and other inorganic nutrients. As the feeding rate in a pond increases, the demand for oxygen to decompose waste increases and the water becomes enriched with nutrients. Dense phytoplankton blooms occur because of abundant nutrients, and dangerously low dissolved oxygen concentrations may occur at night. Ammonia, the major nitrogenous metabolite of aquatic animals, sometimes accumulates to harmful concentrations. However, low dissolved oxygen concentration typically becomes problematic before ammonia toxicity occurs. Feeding alone can increase the production of omnivorous and carnivorous species by 10–20 times above that possible in fertilized ponds. With plankton-feeding species such as tilapia, feeding alone may only cause a 2- or 3-fold increase in production over that possible in fertilized ponds. Two methods may be used to increase the production possible in ponds receiving feed. Water exchange may be used to flush out phytoplankton, nutrients, and toxic metabolites and to add water of higher dissolved oxygen concentration. The disadvantages of water exchange are that energy often is required for pumping, potential pollutants are flushed into natural waters, and effectiveness is relatively low. Mechanical aeration is a more effective than water exchange for increasing the dissolved oxygen concentration and aquatic animal production in ponds. Also, higher oxygen concentration increases the ability of microorganisms to decompose organic matter and oxidize ammonia. There are two basic types of mechanical aerators. One type releases air bubbles into the water, and oxygen within the bubbles diffuses into the water. The other type splashes water into the air. The splashing action increases the surface area of contact
between air and water to allow oxygen from the air to enter the water. Aeration induces water circulation in ponds to prevent thermal stratification. Induced circulation also moves oxygenated water across the pond bottom to avoid low dissolved oxygen concentration at the sediment–water interface. The most common way of harvesting fish from ponds is to gradually lower the water level to concentrate fish in a small area from which they are captured in nets or seines. The water level usually is lowered in two or more stages to facilitate capture of fish. Finally, the pond is completely drained, and any fish remaining on the bottom are removed by hand. Some species, e.g., channel catfish, are harvested from ponds using large seines that are stretched across the entire pond and pulled by a tractor. Mesh size of seines is selected to allow small fish to escape. Fingerling catfish are stocked into ponds to replace the harvested fish. Flowing Water Systems
Phytoplankton blooms do not develop in flowing water systems because of rapid water exchange. Inflowing water is the primary source of dissolved oxygen in unaerated systems, and the main loss of dissolved oxygen is fish respiration. In the culture of trout and other cold water species, dissolved oxygen should not fall below 5 mg l1, and 5 kg day1 of feed can be applied for each kilogram of available dissolved oxygen. Available dissolved oxygen in cold water systems is calculated by subtracting 5 mg l1 (5 g m3) from the concentration of dissolved oxygen in inflow. Suppose an inflow of 1 m3 min1 contains 10 mg l1 (10 g m3) of dissolved oxygen. Over a 24-h period, there will be 7.2 kg of available oxygen (1 m3 m1 1440 min day1 5 g oxygen m3 10–3), and a feed input of 36 kg day1 (7.2 kg oxygen 5 kg feed kg oxygen1) would be permissible. At a typical feeding rate of 3% body weight daily, 2400 kg fish could be held in a culture unit through which water initially containing 10 mg l1 dissolved oxygen flows at 1 m3 s1. Flow-through systems can be used to produce warm water species, but in warm water systems, only about 4 kg feed may be applied for each kilogram of available oxygen. High stocking density is permissible in flowthrough units, for the main factor limiting carrying capacity is availability of dissolved oxygen not space. Fish can be grown under extremely crowded conditions provided water quality is good. Mechanical aeration may be applied to flowthrough systems to increase dissolved oxygen availability, carrying capacity, and production. Aerators such as those used in ponds can be operated in flowing
Applied Aspects of Inland Aquatic Ecosystems _ Aquaculture, Freshwater 239
water culture, but devices that bring water and pure oxygen into contact are used more commonly. Solid wastes consisting of uneaten feeds and feces that accumulate in the bottoms of flow-through culture units can be removed by suction devices or through drains in the bottoms of units. However, ammonia is excreted by fish and accumulates in water of culture units. Thus, there are limits on the increase in carrying capacity through aeration, for ammonia may become a limiting factor at high fish densities.
Cages and Net Pens
Natural food organisms are scarce in cages and net pens, and manufactured feed is applied to enhance production. Soluble and suspended wastes are flushed from culture units by water movement. Larger waste particles settle beneath or near cages and on the bottom area enclosed by pens. Density of fish in cages may reach 80–100 kg m3 where water movement is adequate to replenish dissolved oxygen used in fish respiration and flush out ammonia and other wastes. Fish densities similar to those of cages probably would be permissible in pens at sites with rapid water exchange, but pens usually are operated at lower densities. Low dissolved oxygen concentration in water bodies containing cage and net pen facilities can cause fish mortality. A common cause of oxygen depletion in lakes and reservoirs is sudden thermal destratification resulting from heavy rains, strong winds, cool weather, or a combination of these factors. Fish culture often contributes to these events because nutrients from feeding wastes stimulate phytoplankton growth to favor strong thermal stratification, and wastes settling into the deeper water contribute to oxygen depletion in the lower stratum. The amount of cage and net pen culture in water bodies should be limited to avoid excessive eutrophication and periodic dissolved oxygen depletion.
culture facilities usually is no greater per total water area and volume than that achieved in static pond culture. Indoor, water reuse systems are equipped with water treatment equipment such as mechanical and biological filters, aerators, clarifiers, skimmers, pH adjusters, etc. This equipment provides treatment to allow water reuse. A very small proportion of freshwater aquaculture production is from water reuse systems. There is much interest in these systems for they require less space, use less water, and discharge less pollution per unit of production than other grow-out methods. Their contribution to production will likely increase dramatically in the future.
Aquatic Animal Health Fish and other culture species may have parasites and be infected by fungal, bacterial, and viral diseases. Many chemicals, including antibiotics, have been used to combat parasites and diseases. Some antibiotics are banned for use in aquaculture because of possible harmful effects of their residues on humans. Environmental advocacy groups are concerned over antibiotic use in aquaculture because of the possibility for development of antibiotic-resistant strains of bacteria and other disease organisms. A better way of controlling diseases is to implement an aquatic animal health management program. Specific pathogen-free brood stock can be developed and seed stock checked for diseases before stocking them in culture units. Culture units can be managed to avoid stressing animals and making them more susceptible to disease. After harvest, culture units can be dried out to eliminate disease organisms. When the previous crop was infected with disease, culture units may be treated with chlorine compounds or lime to kill possible pathogens that might survive dry-out.
Resource Use Issues Water Reuse Systems
In outdoor reuse systems, culture units are similar to those used in traditional flowing water aquaculture. Crowding of animals facilitates grading for size, feeding, harvesting, and other management operations. Water is passed through a pond or other water body for purification by natural physical, chemical, and biological processes, and mechanical aeration may be applied to enhance waste treatment capacity. The area and volume of the treatment system must be many times that of the culture units to allow adequate time for water purification. Production in such
Freshwater aquaculture is an important component of world fisheries production, and growth of this sector will be needed as the demand for fisheries products increases in response to human population growth. There are several problems that must be overcome if freshwater aquaculture is to continue to expand.
Land and Water Use
Expansion of pond aquaculture competes with traditional agriculture and other human endeavors for land and water. Thus, intensive production should be
240 Applied Aspects of Inland Aquatic Ecosystems _ Aquaculture, Freshwater
encouraged. Channel catfish farming in the United States is a good example of intensification. Typical production has increased from less than 2000 kg ha1 in the 1960s to over 5000 kg ha1 today. This increase has been achieved primarily by development of better feeds and application of mechanical aeration. Water use also has declined drastically. In the 1960s, water exchange was used when dissolved oxygen concentrations were low and ponds were drained annually to harvest fish. Water exchange is no longer used, and fish are harvested by seining. Ponds are typically drained at 5- to 10-year intervals.
Wetland Destruction
Aquaculture farms constructed in wetlands destroy habitat critical to maintaining balanced ecosystems. Disruption of wetlands can have serious ecological consequences, including deterioration of surface water quality because wetlands purify runoff before it enters streams, lakes, and other water bodies. Many nations have restricted development in wetlands, and others should be encouraged to do so.
Water Pollution
Effluents from aquaculture facilities contain elevated concentrations of nutrients, organic matter, and suspended solids and can pollute natural waters. Aquaculture associations, governments, international development agencies, and environmental advocacy groups promote better practices to lessen pollution by aquaculture. These practices include measures such as better feeds and feeding practices, mechanical aeration, reduction in water exchange, water reuse, harvest without draining ponds, and treatment of effluents in sedimentation basins. Some governments have developed effluent regulations for aquaculture. Regulations may require water quality monitoring and compliance with effluent water quality criteria such as limits on concentrations of nutrients, biochemical oxygen demand, dissolved oxygen, pH, and suspended solids. However, it is more common for regulations to mandate use of better production practices.
Marine Fish Meal and Oil Use
Aquaculture feeds usually contain fish meal and fish oil of marine origin. Aquaculture uses around 50% of current fish meal production and about 80% of fish oil production. The growth of aquaculture could be greatly restrained by future shortages of fish meal and fish oil. Research to develop feeds containing less fish meal and fish oil is needed, and greater use
of fish meal and oil made from fish processing wastes should be encouraged. An example of greater efficiency in fish meal use is afforded by the channel catfish industry in the United States. In the 1960s, channel catfish feed contained 12–14% fish meal. Research on feeds has allowed the fish meal content of catfish feeds to be reduced to 2–4%. Genetic Improvement and Escapes of Farm Animals
Genetic improvement of aquaculture species also provides promise in improving production. Strains of several species that grow faster, exhibit greater disease resistance, and have other more favorable characteristics for aquaculture have been produced through selective breeding. There has been little interest in genetically-modified aquaculture species. Nevertheless, environmentalists are concerned that escapees of farm fish will result in changes in the gene pools of natural populations. From the consumer perspective, there does not appear to be much concern over products resulting from selective breeding. There is widespread concern over the use of genetically-modified organisms, and the role of such organisms in the future of aquaculture is uncertain. The escape of nonnative species introduced for aquaculture also can lead to changes in ecosystems. For example, tilapia is a very aggressive species that can out-compete most native species when introduced into a region. Although concerns about escapes of animals from aquaculture farms are real, aquaculture is widespread, and most of the damage has probably already been done. This is especially true for tilapias that have been widely introduced in the tropics and subtropics. Nevertheless, practices to prevent escapes of aquaculture species should be implemented to avoid as much further damage as possible.
Responsible Aquaculture Consumers are becoming more environmentally aware and expressing a preference for fisheries products produced by environmentally- and sociallyresponsible methods. Some retailers and restaurant chains are sourcing aquaculture products from farmers willing to comply with specific environmental and social practices. There is a growing interest in aquaculture certification. A certified farm would agree to comply with environmental, social, and food safety standards. The farm would be inspected by a third party for verification of compliance. Environmental labeling of aquaculture products is expected to become a common practice in the next few years.
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Further Reading Beveridge MCM (1996) Cage Culture, 2nd edn. Oxford: Fishing News Books. Boyd CE and Tucker CS (1998) Pond Aquaculture Water Quality Management. Boston, MA: Kluwer Academic. Boyd CE, McNevin AA, Clay J, and Johnson HM (2005) Certification issues for some common aquaculture species. Reviews in Fisheries Science 13: 231–279. Boyd CE and Watten BJ (1989) Aeration systems in aquaculture. Reviews of Aquatic Science 1: 425–472. Federal Register (2004, August 23) Effluent limitation guidelines and new source performance standards for the concentrated aquatic animal production point source category: Final rule. Federal Register, Vol. 69(162), pp. 51892–51930. Washington, DC:Office of the Federal Register, National Archives and Records Administration. Goldburg R and Triplett T (1997) Murky Waters: Environmental Effects of Aquaculture in the United States. Washington, DC: Environmental Defense Fund.
Nash CE and Novotny AJ (eds.) (1995) Production of Aquatic Animals: Fishes, Amsterdam, the Netherlands: Elsevier. Sedgwick SD (1990) Trout Farming Handbook, 5th edn. Oxford, England: Fishing News Books. Soderberg RW (1994) Flowing Water Fish Culture, Boca Raton, FL: CRC Press. Timmons MB, Ebeling JE, Wheaton FW, Summerfelt FT, and Vinci BJ (2001) Recirculating Aquaculture Systems, 2nd edn. Ithaca, NY: Cayuga Aqua Ventures. Tucker CS (1996) The ecology of channel catfish ponds in northwest Mississippi. Reviews in Fisheries Science 4: 1–55. Yoo KH and Boyd CE (1994) Hydrology and Water Supply for Aquaculture. New York: Chapman and Hall.
Relevant Website www.fao.org – Food and Agriculture Organization (FAO).