Definition and History
Aquaculture can be most simply defined as underwater agriculture. It is the rearing of aquatic plants and animals through some type of intervention by humans. Aquaculture is conducted in freshwater and saltwater. The term mariculture is often used in relation to marine aquaculture. Aquaculture involves plants and animals produced for human food, as ornamentals, for bait, and in recent years, as sources of nutritional supplements and pharmaceuticals. Some plants and animals are also grown as foods for other aquaculture species. For example, shrimp hatcheries typically grow one or more species of algae to feed brine shrimp (Artemia salina), which are fed upon by larval shrimp.
While there is some production of rooted aquatic plants,echinoderms (e.g., sea cucumbers), tunicates (e.g., sea squirts), amphibians (frogs), and reptiles (turtles and alligators), the majority of the world’s aquaculture production comes from seaweeds, molluscs (e.g., abalone, clams, oysters, scallops), crustaceans (e.g., shrimp, crawfish), and finfish.
Aquaculture is widely acknowledged to have been born in China, perhaps as long as 4,000 years ago (Avault 1996) when carp culture was developed. The first written document on aquaculture, a very short book by Fan Li, entitled Fish Breeding, appeared in 475 BC in China (Borgese 1977). Carp culture was also developed in Europe, possibly during the Middle Ages. Tilapia, a type of fish native to north Africa and the Middle East, is depicted on the tombs of the Pharaohs in ancient Egypt. Whether tilapia was being cultured or not is unclear. Oyster culture appears to have been practiced during the days of the Roman Empire, while shrimp were being produced in China by AD 730 (Borgese 1977). Native Hawaiians constructed fishponds and apparently developed a primitive form of aquaculture at least several hundred years ago. Thus, aquaculture in one form or another has a long history.
By the mid-nineteenth century, trout culture had become a well-developed art in both Europe and North America (Kirk 1987; Stickney 1996). In the United States, brook trout were being produced and at least a few commercial fish farms had been established.
Overfishing in the United States was first reported in the 1750s, but it was not until 1871, when Spencer F. Baird convinced Congress to create the US Fish and Fisheries Commission (Stickney 1996), that any attempt to rebuild wild fisheries was made. Baird enlisted the best fish culturists of the time and put them to work learning how to spawn an array of fishes and invertebrates with the goal of restocking both the inland and marine waters of the nation. In addition, the Commission shipped fish to foreign countries (e.g., establishing populations of rainbow trout in Europe and both rainbow trout and chinook salmon in New Zealand). Fish such as brown trout and common carp were introduced to the United States by the Commission.
While in most cases the stocking of eggs or newly hatched larvae by the Commission did little more than provide food for wild fishes, a significant amount of information and technology were developed, remnants of which can be seen in the most up-to-date of modern hatcheries. Techniques were developed for successfully producing fingerlings of many recreationally and commercially important species, including largemouth bass, channel catfish, walleye, striped bass, and Pacific salmon to name but a few.
The Commission was responsible for producing and distributing the eggs and larvae of hundreds of millions of fishes and invertebrates. Attempts were made to establish Atlantic salmon on the Pacific coast and Pacific salmon in the waters of New England. Both attempts failed, though not because of a lack of effort. Millions of eggs were shipped over a number of years to accommodate the program. Pacific salmon were ultimately established in the Great Lakes, but not until the 1980s.
Ultimately, attempts to enhance marine fisheries through stocking by the Commission and its successor organizations were terminated, though new enhancement programs, largely by the various individual states, have been initiated in recent years. Stocking programs for inland waters, by both state and federal agencies have continued.
Modern commercial aquaculture can arguably be attributed to have its origins in one or more of several countries and decades within the twentieth century. For instance, breakthroughs that made possible the development of commercial shrimp culture occurred in Japan during the 1930s, though the industry did not develop significantly until the 1980s after which considerably more information had been developed in Japan, Taiwan, the United States, and other nations; information that was critical to move the technology beyond the research and demonstration phase.
The late 1960s and the 1970s serve as a benchmark period during which commercial aquaculture developed rapidly and began making significant contributions to the world’s foodfish supplies. Fish farmers in the southern United States initially produced buffalo (Ictiobus sp.), but soon turned to channel catfish (Ictalurus punctatus). While the potential for rearing catfish to food size profitably was demonstrated in the 1950s (Swingle 1956, 1958), commercial activity did not expand from producing and selling catfish fingerlings to growout of foodfish until the mid- 1960s (Wellborn and Tucker 1985).
During the same two decades (1960s and 1970s), commercial craw-fish culture developed in Louisiana and rainbow trout production increased dramatically, particularly in the Thousand Springs area of Idaho (Stickney 1996).
Marine shrimp research with various species in the former genus Penaeus (which has recently been split into several genera) was being conducted in both the United States and Taiwan, which led to two different types of larval rearing systems, both of which have been employed extensively. Taiwan and Japan were leading shrimp producing nations in Asia until the 1980s when Thailand, China, Indonesia, the Philippines, India, and others developed active industries. In the Western Hemisphere, the shrimp industry began in the 1980s and was centered in Ecuador, which continues to dominate Latin American production, though many other nations are now involved. The 1970s saw a great deal of interest in the culture of freshwater shrimp, Macrobrachium rosenbergii, but for various reasons, there are very few farms currently producing that species.
Also, during the 1980s, the feasibility of producing salmon in net-pens placed in the marine environment was demonstrated (Stickney 1994). Much of the initial work was conducted on the west coast of the United States with coho (Oncorhynchus kisutch) and chinook (Oncorhynchus tshawytscha) salmon, and some commercial production occurred in both the United States and Canada. However, it soon became apparent that Atlantic salmon (Salmo salar) was more amenable to culture and the industry concentrated on that species, with Norway becoming the dominant salmon-growing nation. Chile became a major salmon-producing nation in the 1990s. Canada continues to produce significant quantities of salmon, and Maine has become the dominant salmon producing state in the United States, though there is still some production in the state of Washington. Large hatchery programs to produce Pacific salmon for release into the wild continue in the Pacific Northwest.
Many other aquatic species have been commercially produced within the past few decades. At present, there are well over 100 species under culture around the world, with more being developed each year. In Europe, sea bass (Dicentrarchus labrax) and sea bream (Sparus aurata) are being produced in the Mediterranean region, while Atlantic halibut (Hippoglossus hippoglossus) are cultured in Norway and Scotland along with the previously mentioned Atlantic salmon. Plaice (Pleuronectes platessa) and sole (Solea solea) are also among the fishes cultured in Europe.
Popular marine fishes in Japan are salmon (Oncorhynchus spp.), red sea bream (Pagrus major), and yellowtail (Seriola quinqueradiata). Milkfish (Chanos chanos) are produced primarily in the Philippines, Thailand, and Indonesia.
Tilapia (Oreochromis spp.) are being reared in both salt and freshwater throughout the tropical world and indoors in some temperate areas. Their fast growth and excellent flavor make them excellent aquaculture species.
Among the invertebrates being reared around the world are oysters (e.g., Crassostrea spp. and Ostrea spp.), including of course, pearl oysters, mussels (primarily Mytilus spp.), abalone (e.g., Haliotis spp.), and scallops (e.g., Pecten spp. and Patinopecten yessoensis).
Then there are the plants, dominated by red and brown seaweeds, but including green seaweeds (all of which are forms of algae), but also including higher plants such as water chestnuts, watercress, and such ornamental plants as water lilies, to name but a few. A wide variety of microscopic single-celled algae are produced as food for larval stages of aquacultured animals and, in the case of species such as Spirulina, as nutritional supplements for humans (Spirulina is also used as a food additive to produce color in fish). Extracts from aquatic plants are also being developed into pharmaceutical products.
Seaweed aquaculture comprises nearly one-quarter of total world production (New 1999). Seaweeds play a large role in Asian cuisine, particularly that of Japan. Extracts from seaweeds, including agar and carageenin, are components of products utilized by people around the world. Seaweed extracts are used in automobile tires, toothpaste, ice cream, pharmaceuticals, and a wide variety of other commonly employed products.
World aquaculture production for various groups of organisms in 1996 (data from FAO 1998)
World aquaculture production in 1996 by continent and the top 10 nations in terms of total aquaculture production (data from FAO 1998)
Percentage of world total
Republic of Korea
The world’s capture fisheries peaked and now hover around 80 mmt, while demand continues to increase. Aquaculture expansion has been identified as the only means of meeting that demand, and currently available data show that aquaculture production has rapidly increased in recent years. New (1999) developed a comprehensive overview of global aquaculture production that includes a comparison of data between 1987 and 1996. Production over the period covered increased from 13.4 mmt in 1987 to 34.1 mmt in 1996. That rate of expansion cannot be maintained indefinitely, however. Many of the best places to conduct aquaculture are currently being utilized, while various constraints mitigate against aquaculture development in regions where climate might be conducive to good growth of aquatic species. Water supply and quality are constraints in many areas. In the coastal zone, the high cost of land and competition with other users often mitigate against using it for aquaculture.
One approach is to establish fish farms in the open ocean. The costs and logistics associated with that idea present significant problems, but at least some success has been realized. Enhancement stocking, first undertaken by the US Fish and Fisheries Commission over a century ago, is currently being re-evaluated. Revitalization of the red drum (Sciaenops ocellatus) fishery in Texas through an aggressive stocking program coupled with strict fishing regulations (including a ban on commercial fishing) provide an indication that such programs have potential and may be cost effective. One approach would be to pay for the programs through license fees, which would apply to commercial and/or recreational fishermen depending upon the species employed.
Much of the increase in aquaculture production over the years, at least in developed nations, can be attributed to increased knowledge that has led to improved technology. One example is the channel catfish industry in the United States. When the industry was in its infancy, a profit could be made if the fish were sold at $1.10 per kg. Forty years later, a profit can still be made when catfish prices to the producer are in the range of $1.65–1.80 per kg. The reason that catfish farmers can still realize a profit given the small increase in price per unit weight over nearly a half century of production is related to increases in production per hectare, which is a function of improvements in management of water quality, improvements in the food provided, and to a lesser extent, selective breeding programs.
In the 1950s, when catfish farming was in its infancy, a production level of about 500 kg/ha was considered credible. By the late 1960s that had increased to 1,500 or even 3,000 kg/ha. Currently, some catfish farmers produce as much as 10,000 kg/ha in ponds. Without those increases in production, the catfish industry would have probably collapsed.
The vast majority of aquaculture species are grown in ponds, though a number of other types of culture systems are also in use. They include linear and circular raceways (the latter are also known as tanks), cages, and net-pens. Specialized culture systems for molluscs and seaweeds include bottom culture in bays, pole culture, net culture, basket culture, and raft culture (Stickney 1994).
Culture systems are often classified as extensive or intensive, though in reality the intensity of culture occurs along a continuum and relates to the complexity of the culture system. An example of a highly extensive system would be the culture of oysters in natural waters by spreading shell (cultch) material on the bottom and allowing larval oysters (spat) to settle and grow. Very little intervention by humans, other than preparation of the bottom and the spreading of shell is involved.
At the other extreme are high-intensity closed-recirculating water systems. Such systems employ raceways as culture chambers. Water is continuously exchanged in each raceway. Exiting water is treated, at a minimum to remove solids (usually by settling or mechanical filtration) and convert ammonia (NH3 or NH4+) and nitrite (NO2−) to less toxic nitrate (NO3−) in a unit called a biofilter. Additional components that maintain temperature in the desired range, remove foam, convert nitrate to nitrogen gas (N2), and remove other nutrients, may also be present. Most closed systems, receive supplemental aeration to maintain a high level of dissolved oxygen in the culture chambers. In truly closed systems, new water is added only to replace that lost to splashout, evaporation, or to flush solids from the system. Systems that have the same basic components but in which some percentage of the water is routinely or continuously changed by adding new water, are called semi-closed.
Between the extremes are the other systems mentioned above. Culture ponds, unlike most farm ponds, are most commonly rectangular in shape, are usually no more than 2 m deep, have a drain structure of some type incorporated into them and are, in most cases, located in an area where there is a reliable source of water. While some culture ponds depend on rainwater runoff to fill them, most ponds, at least in the developed world, employ well water or water from a stream, lake, or reservoir for filling and maintenance. Culture ponds come in many sizes, but most used for commercial production range from about 0.1–10 ha, with those between about 1 and 5 ha being the most commonly seen. Small ponds are used most often for breeding and rearing of early life stages, while the growout commonly occurs in larger ponds.
If two or more species are reared in the same culture unit-a process known as polyculture-that unit is most likely a pond. The Chinese have a very long history of carp polyculture, which involves different species with different feeding habits. Different species are stocked to consume phytoplankton, zooplankton, benthic invertebrates, and rooted aquatic plants. Benthic feeding carp may specialize on worms and insect larvae or molluscs. Today, the Chinese may also provide agricultural byproducts and prepared feeds. They, like aquaculturists in many other countries, maintain a high level of pond fertility to encourage the growth of natural food organisms. Cattle, swine, and poultry manure are common sources of fertilizer. Livestock are routinely seen being reared adjacent to ponds into which they deposit fresh manure. Rearing livestock adjacent to fishponds is common in many countries, particularly in the developing world.
The majority of raceway systems are of the flow-through variety. That is, water is continuously exchanged. Depending on stocking density and water quality, the exchange rate may range from several times an hour to several times a day.
Cages are relatively small structures with ridged walls, commonly made of welded wire or rubber-coated wire. Cages are used to contain fish in water bodies that are not enclosed (bays, rivers), and in those which are too large, too deep, or have some physical impediment to harvest (such as standing dead timber). Many reservoirs and lakes are good examples. Cages are not usually more than one to a few cubic meters in volume. Cages may be taken to the shoreside to facilitate harvesting.
Net-pens are large cages where nylon netting is employed for the sides and bottom instead of ridged material. The standard design most commonly used in protected marine waters where the net-pens are not exposed to high waves is typically 20 × 20 m2 on a side and can be 20 or more meters deep. Both cages and net-pens are provided with floats to keep them at the surface. Cages usually sit low in the water and are fitted with tops, while the sides of net-pens typically extend above the water surface sufficiently to prevent fish from jumping over the open top. Walkways around the perimeter of each net-pen allow access for feeding and harvesting.
Recently, there has been a move toward rearing fish in offshore locations. Net-pens designed for the open ocean are usually fully enclosed by netting because they may be subjected to storms. Some are designed so they can be totally submerged throughout the growing cycle or during stormy periods.
Oysters, mussels, clams, and scallops are less susceptible to predation by starfish and other organisms if they are grown off the bottom. Poles, lines, and baskets have been used in conjunction with off-bottom culture. In Spain, for example, ropes to which oysters or mussels are attached are suspended from large rafts.
Enhancement, as previously mentioned, is an option that deserves further consideration. While the culturists associated with the US Fish and Fisheries Commission in the latter half of the nineteenth century failed in their efforts to increase the numbers of fish in the sea, advances since that time greatly increase the potential for success.
Hatchery technology has developed to the point where the larvae of many species of interest can be reared from egg to appropriate stocking size economically. Enhancement efforts need to be conducted in an environmentally sound manner, however. Fish and invertebrates should not be stocked in such numbers that they overwhelm the available food supply; they should not cause declines in the populations of other valuable species in the community; and their genetic composition should reflect that of wild populations, to name but a few considerations.
Several steps should be taken in advance of actually establishing an aquaculture facility. First, the prospective culturist should determine what species will be produced and type of water system to be employed. A suitable location should also be identified. All three factors tend to be co-dependent. Sometimes, the prospective aquaculturist will own property selected as the site of the new facility. If that property is in a subtropical area and there is no suitable source of cold water, it would be undesirable to attempt to produce trout or salmon. Similarly, if the property is distant from the coast and there is no convenient source of saline groundwater, producing marine fish might not be wise. The exception to those examples might be employment of a closed water system within which temperature and salinity could be controlled. Economic analyses of each option under consideration should be conducted to help determine the best course of action.
Once the above matters have been resolved, it is necessary to produce a business plan. Such a plan will be required by lending institutions and venture capitalists. The plan should include a complete financial analysis showing all costs associated with constructing, equipping, and operating the facility over a period of several years, along with an estimate of profit or loss for each year projected. All estimated costs and income should be based on information that can be documented. The culturist should provide a listing of the permits that may be required for the facility as well as a timetable for obtaining those permits and the fees that are associated. Conceptual drawings, or even better, detailed engineering blueprints, should be included with the business plan.
The prospective aquaculturist should determine what, if any, local, state, or national governmental permits are required. In some instances, the permitting process can be long and arduous. It may also be costly, particularly if the applicant is required to battle opponents within the legal system and conduct an extensive environmental impact study prior to establishing a facility. In general, inland aquaculture sites and those located along the coast that employ recirculating technology or other water systems that do not impact the marine or estuarine environment are more easily permitted than those that are established in public waters.
On the surface, aquaculture is a biological discipline. In reality, like terrestrial farming, aquaculture involves a variety of sciences as well as engineering, business management, and such skills as plumbing, welding, carpentry, and electrical wiring. In the science arena, while expertise in biology is critical on any fish farm, an understanding of chemistry may be even more important. As the size and intensity of aquaculture operations increase, so does the need for additional and better-trained personnel. A subsistence culture operation in a developing country might be operated by an individual or a family, with perhaps a few additional laborers. A large pond operation that involves a hatchery, larval rearing, and production facilities, or an intensive raceway operation may require a large number of unskilled laborers as well as employees proficient in various specialties. Some of the technical aspects associated with operation and management of an aquaculture facility are discussed in the following subsections.
Maintenance of good water quality is critical in any aquaculture facility. There are virtually hundreds of thousands of chemicals that can be found dissolved in water. In addition, there are physical characteristics such as temperature and transparency that can affect performance of organisms in an aquaculture facility. It is neither possible nor necessary to monitor all aspects of water chemistry. In most culture systems, after an initial screening of the water supply for toxicants (e.g., herbicides, pesticides, high levels of trace metals), routine monitoring involves only a few variables. Water temperature and dissolved oxygen, and in marine systems, salinity are typically measured at least several times a week and records of them are maintained. Some culturists employ sensors that constantly monitor those parameters and download the information into computers. Water transparency monitoring is also conducted in pond systems and provides a simple means of determining the state of plankton blooms. This is particularly important in systems that employ fertilization to provide natural food.
In high intensity systems, ammonia may be routinely monitored. Nitrite levels might also be tracked, particularly during colonization of biofilters with bacteria, since the types of bacteria that convert nitrite to nitrate tend to lag behind those that convert ammonia to nitrite. Thus, if only ammonia is measured, the culturist may get the impression that the water is not toxic, when in fact, it may contain a high level of nitrite. Toxic nitrite levels have occurred in heavily stocked catfish ponds in the United States during the late summer, so routine monitoring during that critical period is often undertaken. While ponds are not equipped with biofilters, the bacterial conversion of ammonia to nitrite and nitrate does occur.
Water temperature is of critical importance because it controls metabolic rate in the species being cultured. Aquatic animals can often be categorized as being warmwater or coldwater species. Warmwater animals are those that have an optimum temperature for growth of 25 °C or higher, while coldwater species have an optimum below about 15 °C. There are some animals, such as walleye (Stizostedion vitreum vitreum) and northern pike (Esox lucius) that have an optimum between 15 °C and 25 °C. Such animals are sometimes referred to as midrange species.
Most species have a broad tolerance for environmental temperature. Many species of fish live over a range of temperatures from near 0 °C to somewhat above 30 °C. Channel catfish and largemouth bass (Micropterus salmoides) are but two examples. Many coldwater species, such as trout and salmon, cannot tolerate warm temperatures, while many tropical fishes, such as tilapia, die at temperatures where trout and salmon thrive. Tilapia grow most rapidly when the water temperature is about 30 °C, show significantly reduced growth at about 25 °C, and stop growing around 20 °C. Below the latter temperature tilapia lose their disease resistance and will die when temperatures fall much below 15 °C (Avault and Shell 1968).
Terrestrial animals live in an atmosphere that contains some 20% oxygen, while aquatic species live in a medium that contains only a few parts per million (ppm) of the same essential element. Depending on species, the minimum level of dissolved oxygen in the water that should be present to ensure that the animals have a sufficient supply varies from about 3 ppm (e.g., for catfish) to 5 ppm (e.g., for salmon and trout). From a practical standpoint, it is always desirable to have dissolved oxygen at saturation.
The amount of oxygen that water can hold (the saturation level) varies with temperature, salinity, and altitude. As each of those variables increases, the ability of water to hold oxygen decreases. Thus, warm, saline water at high altitude would hold less oxygen than cool, freshwater at sea level. In virtually all locations where aquatic animals are cultured, dissolved oxygen saturation level reaches or exceeds 5 ppm.
Dissolved oxygen follows a diurnal (daily) cycle. During daylight hours, phytoplankton and other types of vegetation in the aquatic environment produce oxygen as a result of photosynthesis. As a result, the oxygen level in water tends to increase during the daytime. Oxygen is also introduced into water through diffusion from the atmosphere, a process that is enhanced through wind turbulence or mechanical aeration. At night, both plants and animals respire, thereby consuming oxygen during a period when no photosynthesis is occurring (though diffusion is still operating). The result is a cycle in the availability of oxygen in the water. The cycle is also influenced by weather conditions and season of the year. During cloudy days, the rate of photosynthesis is reduced (because of reduced light level), while during clear weather the rate of photosynthesis is accelerated. Recall that temperature plays a role in the saturation level of dissolved oxygen in water; thus the seasonal variation. In most cases, problems associated with low dissolved oxygen occur during warm weather after a period of cloudy days. This often occurs during the late summer or early fall in temperate areas (the time just prior to harvest).
Aquaculturists routinely measure dissolved oxygen in pond systems early in the morning, particularly during the time of year when depletions are likely. Dissolved oxygen level typically peaks in the late afternoon before dusk and gradually falls during the night, and begins to increase after dawn. The lowest dissolved oxygen level will usually occur just prior to dawn, so the aquaculturist needs to check conditions at that time and determine which, if any, ponds may be approaching critically low oxygen levels.
When oxygen levels are low, the culture animals may alert the observant culturist. Fish may swim at the surface with their mouths open. They appear to be gulping for air. Shrimp may climb above the water level on rooted plants to expose their gills to the atmosphere. By the time such signs are noted, the animals may have already been severely stressed. It is much better to routinely measure dissolved oxygen during the night and take action to prevent severe depletions before they occur.
Under moderate stocking regimes only a few ponds on a given aquaculture facility will experience oxygen depletions on the same day and it is not necessary to have each pond provided with emergency aeration equipment. In many cases today, however, stocking densities are so high that oxygen depletions may occur with some frequency in nearly every pond. Such facilities may have aeration equipment permanently assigned to each pond. Paddlewheel aerators are commonly seen in culture ponds. They may be turned on only when the oxygen level falls below a specific level, or they may routinely be run during the night, or in some cases, continuously.
Mechanical aeration is not the only means of aerating ponds. New, oxygen-rich water can be added, though that can be expensive. Mechanical aeration is not inexpensive, however. It requires the equipment and the electricity or fuel required to operate that equipment.
Aquatic animals may have a wide range of tolerance for salinity (euryhaline) or may be restricted to a rather limited salinity range (stenohaline). Salinity is measured in parts per thousand (ppt). The salinity of freshwater is typically about 0.5 ppm, while full strength seawater is usually about 35 ppt. The optimum salinity for an aquatic species may be constant throughout the life cycle or it may change at various life stages. Salmon, for example, spawn and their eggs hatch in freshwater where the larvae live for a few weeks to 2 years before migrating to the ocean. Fish that spawn in freshwater but grow to adulthood in seawater are called anadromous. Those that spawn in the ocean and migrate to freshwater to grow to adulthood, such as eels, are catadromous.
While most aquatic organisms survive and grow well at a constant optimal salinity, it may be necessary to change the salinity when rearing certain species. It is important to know when those salinity changes should be made and the extent of the changes.
Ammonia is produced by many aquatic species as a metabolic waste product. Ammonia enters the water through the gills. As previously indicated it can be present in the water in two forms: ionized (NH4+) or unionized (NH3). It is the unionized form is highly toxic to aquatic animals. The ratio between the two forms is determined by factors such as water temperature and pH. As either or both temperature and pH increases, the amount of unionized relative to ionized ammonia increases. Under most conditions, total ammonia concentration should not exceed about 1 mg/l. That will ensure that the level of unionized ammonia is not excessive.
Water pH can undergo fairly dramatic changes diurnally, particularly when there is a strong algae bloom present. As plants and animals respire at night in the absence of sufficient light to promote photosynthesis, the increased level of carbon dioxide in the water causes the pH to drop unless the system is well buffered. During the daytime, carbon dioxide is removed from the water by photosynthetic organisms and pH may rise. Carbonate (CO32−) and bicarbonate (HCO3−) ions make up the foundation of the buffer system. They can absorb hydrogen ions (H+) or in the case of bicarbonate, act as a source of H+. If the supply of bicarbonate becomes exhausted, pH can change dramatically. Poorly buffered systems can be as acidic as pH 6 in the early morning and as basic as pH 10 in the afternoon if the buffer system is weak. In recirculating systems, pH will tend to drop over time as organic acids become concentrated in the water. Most aquatic species cannot tolerate such swings in pH. Calcium carbonate in the form of limestone or crushed oyster shell can be employed in the water system to help maintain pH by providing a source of carbonate ions.
Routine measurement of pH and alkalinity (a measure of carbonate and bicarbonate ions) every few days is common on aquaculture facilities. Hardness (a measurement of the calcium plus magnesium level in the water) may also be monitored. Some species, such as the euryhaline red drum that can be found in estuaries as well as in the coastal ocean of the southeastern Atlantic and Gulf of Mexico coasts of the United States, can survive and grow well in hard freshwater, but performance is not as good in soft freshwater.
Genetics and Reproduction
Most species being successfully produced by aquaculturists can be maintained throughout their life cycles. Closing the life cycle has been relatively simple for some species, but very difficult for others. Aquatic animals with large eggs, such as salmon, trout, catfish, and tilapia-each of which has eggs 3 mm or larger in diameter-readily spawn in captivity. Their eggs can be incubated relatively easily and the newly hatched fish will consume prepared feeds as first food after yolk sac absorption.
Many aquatic animals of aquaculture interest have very small eggs and larvae so small as to be nearly invisible to the naked eye. The larvae tend to be very fragile and often have extremely limited swimming ability. If exposed to water currents in a hatchery raceway, they may become impinged on screens and killed. For many species, providing a complete feed that will be accepted has proved difficult so it is often necessary to culture living zooplankton as first food.
The most simple approach is to allow the animals to spawn naturally. Providing the proper conditions for spawning is not always possible, and in some cases, culturists still do not know how to duplicate what the animals experience in nature. Hormone injections are sometimes used to induce spawning. In the case of marine shrimp, removal of an eyestalk from females can trigger ovulation.
Fertilized eggs may be allowed to incubate naturally or they may be taken to a hatchery for artificial incubation. Various types of specialized hatching facilities have been developed. Not only is the survival rate generally higher in a hatchery than in a pond, but the culturist can get a good estimate of the number of offspring that have been produced since they are easy to observe in the hatchery as compared with an outdoor pond situation.
Only a few aquatic animals have been truly domesticated through many generations of selective breeding. Most are still only a step or two removed from their wild counterparts. In some cases, for example, in conjunction with hatcheries that produce fish for stocking into nature, a conscious effort is made to maintain the genetic integrity of the wild stock. For animals bred to be reared in captivity only, selection of brood stock for rapid growth, improved body configuration, disease resistance, and other desirable qualities may be employed.
The use of genetic engineering in conjunction with aquaculture species is in its infancy, but there has been some research activity in that controversial arena. Researchers have successfully incorporated selected genes from one species into another, but there do not appear to be any commercially produced transgenic aquatic animals in the market.
Nutrition and Feeding
Feed represents the largest variable cost for most types of aquaculture. The exception is mollusc culture where wild phytoplankton is fed upon in most instances (cultured algae is used in mollusc hatcheries and fertilization of ponds is widely used in China and other countries to produce natural food). Many fishes and crustaceans are provided with commercial feeds. When prepared feeds are employed, they often represent nearly half of the variable costs associated with operating the aquaculture facility.
Most fishes and crustaceans will either accept prepared feeds upon first feeding or can be weaned from natural foods within a few weeks after first feeding. Prepared feeds may be a mixture of ground and mixed feedstuffs that are fed directly, or the ingredients may be formed into pellets. Feed pellets can be made in a variety of sizes and shapes; they can be dry, moist, or semi-moist; they may be hard or soft, and they may either sink or float depending upon the species being fed, local ingredient availability, and the type of equipment used in feed manufacture.
For some species, the nutritional requirements have been determined in some detail. Examples are salmon, trout, catfish, tilapia, carp, and to a lesser extent shrimp, striped bass, red drum, and a number of others (Wilson 1991; D’Abramo et al. 1997). Depending upon the species, requirement information may be available on dietary energy, protein and amino acids, lipids and fatty acids, carbohydrates, vitamins, and minerals.
Feeds may contain animal protein from a few percent (channel cat- fish) to one-third or more of the diet (some salmonids), or they may be made up entirely of plant feedstuffs with or without added vitamins and minerals. Most of the animal protein in aquaculture feeds comes from fish meal, though poultry byproduct meal, and meat and bone meal are among the other options. Locally available ingredients are typically employed when possible. Fish feeds in Asia, for example, often contain relatively high percentages of rice bran, which has limited nutritional value but is readily available. Soybean meal is a major ingredient in many fish feeds in the United States. Diets may also contain corn meal, cottonseed meal, and/or peanut meal depending on availability and cost. A wide variety of other ingredients are also used.
Aquatic animals on prepared feeds are usually fed once or twice daily during the growout period. The total amount of feed provided on a daily basis is usually not more than 3–4% of the biomass of the animals being fed. Young animals are usually fed at a higher rate (as much as 50% of body weight initially, and declining as the animals become more proficient at finding feed and their relative growth rate slows). Young animals may be fed more frequently. Some carnivorous species may be offered small amounts of feed as often as every few minutes to keep them satiated and reduce cannibalism.
Feeding may be done by hand, though increasingly, automated feeding systems are being employed. Such systems have been developed for virtually every type of water system. Some feed automatically when activated by timers, while others may be activated when animals go to the feeder. The latter, called demand feeders, are used in conjunction with some finfish rearing operations and feature a rod or plate that is suspended in the water which when bumped by the fish causes a few feed pellets to be released.
Aquatic animals are susceptible to a wide variety of diseases. Organisms responsible for disease in aquatic species include fungi, bacteria, nematodes, cestodes, trematodes, as well as parasitic protozoans, copepods, and isopods. Some can cause death while others may stress the affected animal to the point that it becomes more susceptible to additional diseases. Not all aquatic animal diseases are caused by other organisms. Some, such as gas bubble disease, are caused by water quality problems (in this case gas supersaturation). Nutritional deficiency diseases also exist. For example, vitamin C deprivation can lead to decalcification of bone, which can lead to spontaneous fracturing of the vertebral column.
While a variety of disease treatments have been developed and are in widespread use around the world, only a very small number of drugs have been approved for use on aquatic organisms in the United States. The US Food and Drug Administration must approve each drug utilized on foodfish. Because the industry is relatively small, the market for drugs often does not justify the financial investment required by the pharmaceutical companies to obtain clearance.
Controversies in Aquaculture
In recent years, aquaculture has come under increasing criticism from environmentalists. Particularly heavy criticism has been leveled against aquaculture being conducted in pubic waters. The conduct of aquaculture on private land has received less opposition, except in cases where effluents from such facilities enter public receiving waters.
Waste feed and feces causing dead zones under net-pens
Nutrients released from decaying feed and feces leading to eutrophication
Antibiotics in feed entering the food chain leading to the development of resistant bacterial strains
Net-pens interfering with recreational and commercial fishing
Net-pens interfering with navigation
Fish in net-pens transmitting diseases to wild fish
Escapees from net-pens interbreeding with wild fish, or in the case of nonnative Atlantic salmon successfully colonizing and displacing wild salmon
Net-pen aquaculture operations being noisy and producing noxious odors.
Many of the claims have at least some basis in reality or hold the potential of becoming problems, though each can be effectively dealt with if the net-pen facility is properly constructed and placed in the proper location (Parametrix 1990).
If located in areas where tidal exchange and currents are insufficient to carry away waste products, an anoxic zone can occur. In Japan, uncontrolled development of net-pen facilities in several bays led to so-called self-pollution that resulted in heavy mortalities. Strict regulations on the number of net-pens per unit area are now in force in Japan and the problem has been resolved.
Salmon farmers, as other aquaculturists, are dedicated to maintaining the highest possible water quality and other environmental conditions for the animals under their care. Degradation of the culture environment causes stress and can lead to disease. If disease does occur, an approved antibiotic may be employed, but only for 10 days at recommended rates, so large amounts of such pharmaceuticals are not used.
Atlantic salmon have been known to escape but they cannot interbreed with Pacific salmon (the situation is different in Maine where Atlantic salmon are native). Many attempts were made to establish Atlantic salmon in the Pacific Northwest by the US Fish and Fisheries Commission over a century ago. All of those attempts failed. Atlantic salmon that have escaped into the waters of the Pacific Northwest in North America do not seem to compete well with native Pacific salmon.
Proper siting will help avoid problems with nutrient concentration and interference with other users. Excessive noise is not a valid criticism of net-pen operations and there would only be noxious odors in the event of a fish kill that was not immediately cleaned up.
The Texas shrimp farming industry has come under criticism for releasing sediment and nutrient-laden water into adjacent public waters. In one part of the state the suspended solids load from shrimp pond effluents has been blamed for heavy siltation of navigable waters, while nutrients in the effluent are blamed for harmful algae blooms. There has also been concern expressed that diseases in cultured shrimp could spread to wild populations. The industry is dealing with the situation by constructing wetlands through which water exiting the production ponds passes. Solids can settle and nutrients can be taken up by the wetland vegetation. The water may then be recirculated back to the ponds rather than released to the environment. Significant improvements in water quality have been achieved and production costs reduced when constructed wetlands are employed.
The other major criticism of the shrimp industry in Texas and other states involves the use of exotics. Most of the shrimp raised on farms in the United States are exotics from Asia or Latin America. Research on native species was all but halted in the 1970s, but many feel that attempts should be initiated to solve some of the problems so the industry can move away from exotic or nonindigenous species.
The development of extensive pond culture for shrimp has led to destruction of mangrove areas in many tropical countries. Some nations have recognized the importance of mangroves as habitat for marine organisms and wildlife and as buffers that can dampen the effects of storms on upland regions. As a result, mangrove habitat destruction has been outlawed or significantly reduced due to governmental regulation. In addition, mangroves grow in acid soils, which are not as conducive to supporting aquaculture operations as other soil types.
Many countries have brought in nonindigenous species and introduced them into culture. Besides Atlantic salmon and marine shrimp as previously discussed, examples include tilapia in North, Central, and South America as well as throughout much of tropical Asia; Latin American species of marine shrimp in the United States; various species of carp in the United States and Latin America; and walking catfish (Clarias spp.) in Europe. For species which have only recently been introduced into aquaculture as exotics, there is concern about escapement and establishment of wild populations that could compete with and perhaps displace native species. The rapid spread of common carp throughout North America beginning in the nineteenth century and the dispersal and establishment of tilapia in parts of Asia (where tilapia are now considered native though they have been present in Asia only since the 1930s) and Latin America are good examples.
While transgenic fish are not apparently being commercially produced for sale, researchers have been able to incorporate growth hormone producing genes from one species into another and obtained an increased growth rate. Transgenic fish with so-called antifreeze genes that might, for example, allow tilapia to survive at much lower temperatures than normal have been discussed. This type of activity is controversial and there are many stories in the media that have convinced at least a portion of the public that transgenic fish are threats to the environment as well as to the health of consumers. A great deal of debate surrounding this activity will occur before transgenic fishes become commonplace in the market.
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