Culture Systems

At one extreme, aquaculture can be conducted with a small amount of intervention from humans and the employment of little technology. At the other extreme, aquaculture involves total environmental control and the use of computers, molecular genetics, and complex modern technology. Many aquaculturists operate between the extremes. The range of culture approaches can be described as running from extensive to intensive, or even hyperintensive, with extensive systems being relatively simple and intensive systems being complex to very complex. In general, as the level of culture intensity increases, stocking density and, as a consequence, production per unit area of culture system or volume of water increase.

The most extensive types of aquaculture involve minimal human intervention to promote increases in natural productivity. One good example is the scattering of cultch material to provide substrates for oyster spat settlement as previously described. A second is the stocking of water bodies used in conjunction with recreational fishing.

The stocking of ponds, lakes, and reservoirs to increase the production of desirable fish that depend on natural productivity for their food supply and are ultimately captured by recreational anglers is practiced primarily in developed nations. In developing nations, community ponds are sometimes stocked to provide food for people living at the subsistence level. Some would consider the practice of stocking fish for recreation or subsistence as lying outside of the realm of aquaculture, but since it involves human intervention and often employs fish produced in hatcheries, such programs are certainly associated with, if not an actual part of, aquaculture. Since the stocked animals are expected to live off available natural food supplies in most instances (agricultural waste products are sometimes thrown into ponds by subsistence farmers to enhance productivity), production levels are often less than 100 kg/ha/yr.

Most of the aquaculture practiced around the world is conducted in ponds (Fig. 1). Ponds range in size, but production units are generally 0.1 to 10 ha in area. The intensity of aquaculture in ponds can range from a few kilograms per hectare to thousands of kilograms per hectare of annual production. Production levels in the U.S. channel catfish industry were only a few hundred kilograms per hectare in the early 1960s and rose to about 3000 kilograms per hectare by the end of that decade. A decade

Figure 1. Catfish culture pond in Texas with paddlewheel aerator used to increase dissolved oxygen level, as necessary.

later, some farmers were able to produce 8000 to 10,000 kg/ha/yr in open ponds. Technological advances in feed quality, control of diseases, and water quality management were among the factors that allowed for the increased production rates.

The first step up from the recreational stocking or subsistence level of culture is to fertilize the water to increase natural productivity. Fertilization encourages algae blooms, which in turn stimulate the production of plankton and benthic organisms, all of which can provide a source of nutrients to the target culture species. Moving up in intensity from fertilization is the provision of supplemental feeds, which are those that provide some additional nutrition but cannot be depended upon to supply all the required nutrients. Provision of complete feeds, those that do provide all of the nutrients required by the culture species, translates to another increase in intensity. Associated with one or more of the stages described might be the application of techniques that lead to the maintenance of good water quality. Examples are continuous water exchange, mechanical aeration, and the use of various chemicals to adjust such factors as pH, alkalinity, and hardness.

With the application of increased technology and control over the culture system, intensity continues to increase. Utilization of specific pathogen-free animals, provision of nutritionally complete feeds, careful monitoring and control of water quality, and the use of animals bred for good performance can lead to impressive production levels.

Where water is plentiful and inexpensive, raceway culture is an attractive option and one that allows for production levels well in excess of what is possible in ponds. Trout are frequently reared in linear raceways from hatching to market size. Linear raceways are longer than they are wide and are usually no deeper than 1 to 2 in. (Fig. 2). High-density raceways used in production facilities are commonly constructed of poured concrete. Small raceways of the type used in hatcheries and research facilities may be constructed of fiberglass or other resilient materials. Water is introduced at one end and flows by gravity through the raceway to exit the other end. Circular raceways, called tanks (Fig. 3), are also used by aquaculturists. Tanks are usually no more than 2 in. deep and may be from

Figure 2. Concrete raceways at a trout farm in Idaho.

Figure 3. Circular tanks for the commercial production of tilapia in geothermal water in Idaho.

less than 1 m to as much as 10 m in diameter. Concrete tanks can be found, but most are constructed of fiberglass, metal, or wood that is sealed and covered with epoxy or some other waterproof material. Plastic liners are commonly used in metal or wood tanks to prevent leakage, and in the case of metal, to avoid exposing the aquaculture animals to trace element toxicity.

Linear raceways are commonly used by trout and salmon culturists both for commercial production and for hatchery programs conducted by government agencies to produce fish for stocking. Large numbers of state and federal salmon hatcheries in Washington and Oregon, along with governmental and private hatcheries in Alaska, collect and fertilize eggs, hatch them, and rear the young fish to the smolt stage at which time they become physiologically adapted to enter seawater. It is following smoltifica-tion that the fish are transported to release sites. Migrating adults will have imprinted on the hatchery water as juveniles and can be counted upon to return to their hatcheries of origin with few exceptions.

Commercial salmon culturists can rear their fish to market size in freshwater raceways, although most salmon are grown from smolt to market size or adulthood in the marine environment, either as free roaming fish or in con-

finement. Releasing smolts into the open marine environment is called ocean ranching and is a technique that takes advantage of the homing instinct of salmon. When the fish that had been released as smolts return to spawn, sufficient numbers of adults are collected for use as broodfish to continue the cycle. The remainder may be harvested by the aquaculturist or by commercial fishers after which the fish are processed and marketed.

Salmon, steelhead trout, and a variety of marine fish are currently being reared in net-pens (Fig. 4). The typical salmon net-pen is several meters (sometimes as much as 20 m) on each side and may be 10 m or more deep (1). Smaller units, called cages, are sometimes used by freshwater culturists. Cages tend to have volumes of no more than a few cubic meters (Fig. 5).

Net-pen technology was developed in the 1960s but has only been widely employed commercially for salmon production since the 1980s, when the Norwegian salmon farming industry was developed. The Japanese began producing large numbers of sea bream and yellowtail in netpens during the 1960s. Other nations have used the technology as well. Most of the net-pens currently in production are located in protected waters since they are easily damaged or destroyed by storms.

Competition by various user groups for space in protected coastal waters in much of the world has led to strict controls and, in some cases, prohibitions against the establishment of inshore net-pen facilities. As a result, there is growing interest in developing the technology to move offshore. Various designs for offshore net-pens have been developed and a few have been tested (Fig. 6). A number of different designs, including systems that are semisub-mersible or totally submersible, have been able to withstand storm waves of at least 6 m, but the costs of those systems are very high compared with inshore net-pens, so commercial viability may be difficult to achieve unless the species being reared is of very high value. Tuna, for example, are being cultured to a limited extent and can bring very high prices, particularly if they are of high quality, for the Japanese sushi trade. Some success in spawning and larval rearing of tuna has been achieved, but in most instances juvenile fish are captured and placed in net-pens for growout to market size. Depending on the size at which the fish are stocked, the net-pen growout period may require only a few months.

The highest level of intensity that can be found in aquaculture is associated with reuse systems, often called recirculating systems. In these systems, the bulk of the wa

Figure 6. A salmon net-pen designed for use in areas where wave heights during storms may reach as high as 6 m.

Figure 4. A marine net-pen facility in Norwegian fjord.

Figure 5. Marine cages in Malaysia.

ter passing through the chambers in which the finfish or shellfish are held is continuously treated and reused. Once filled initially, reuse systems can theoretically be operated for long periods of time without much water replacement. It is necessary to add some water to such systems to make up for that lost to evaporation and splashout and in conjunction with solids removal.

Recirculating systems can be used for all phases of culture, from broodstock maintenance through spawning, hatching, larval rearing, to growout. High energy requirements because of the requirement to run pumps and aerators 24 hours a day, and sometimes greatly increased if water must be heated or chilled, have rendered many recirculating systems uneconomical. Exceptions include systems used for the production of high-priced products (such as ornamental species), those where water and/or heat is basically free (such as systems that employ power plant cooling water), or when only part of the rearing cycle employs recirculating technology (eg, fingerling production in a reuse system followed by pond growout).

Many of the recirculating systems in use today are operated in a mode between entirely closed and completely open (open systems typically have sufficient flow rates to exchange the water in the culture chambers every hour or less). In most systems there is a significant percentage of replacement water added either continuously or intermittently on a daily basis. Such partial recirculating systems may exchange from a few percent to several hundred percent of system volume each day.

The heart of a recirculating water system is the biofilter, a device that contains a medium on which bacteria that help purify the water become established (Fig. 7). Fish and aquatic invertebrates produce ammonia as a primary metabolite. If not removed or converted to a less toxic chemical, ammonia can quickly reach lethal levels. Two genera of bacteria are responsible for ammonia removal in biofilters. The first, Nitrosomonas, converts ammonia (NH3) to nitrite (NO2 ). The second, Nitrobacter, converts nitrite to nitrate (no3 ). Nitrite is highly toxic to aquatic animals, although nitrate can be allowed to accumulate to relatively high levels. If both genera of bacteria are active, the conversion from ammonia through nitrite to nitrate is so rapid that nitrite levels remain within the safe range.

In addition to the biofilter and culture chambers, recirculating systems typically also employ one or more settling chambers or mechanical filters to remove solids such as unconsumed feed, feces, and mats of bacteria that slough from surfaces within the system. Each recirculating system requires a mechanical means of moving water from component to component. That usually means mechanical pumping, though air-lifts can also be used. Most systems incorporate one pump to lift water and rely on gravity to provide flow through the various system components and back to the pump (Fig. 8). That approach reduces the need to balance flow rates through two or more pumps and reduces operating costs.

Control of circulating bacteria and oxidation of organic matter can be obtained through ozonation of the water. Ozone (03) is highly toxic to aquatic organisms. Ozone must be allowed to dissipate prior to exposing the water to the aquaculture animals. With time, and with the assis-

Figure 7. A circular tank filled with plastic balls to which bacteria attach is one type of biofilter that can be used by aquacul-turists; in this case, the biofilter helps maintain water quality at a baitstand in Texas.


Culture chamber

Settling chamber or mechanical filter

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