INTRODUCTION

AQUACULTURE (farming of fish under controlled conditions) is a growth industry striving to satisfy a growing market for food fish. It currently is one of the fastest growing sectors of agriculture in the United States. Farm-reared freshfish is increasing in popularity and profitability.

Growing public demand for a healthy tasty and affordable food is stimulating the "boom" in this industry. The decline in wild fish populations as a result of overharvest and water pollution has promoted the culture of farm-fresh that are grown in contaminant-free waters in indoor tank systems.

RAS DEFINED

Recirculation aquaculture systems (RAS) represent a new and unique way to farm fish. Instead of the traditional method of growing fish outdoors in open ponds and raceways, this system rears fish at high densities, in indoor tanks with a "controlled" environment. Recirculating systems filter and clean the water for recycling back through fish culture tanks.

New water is added to the tanks only to make up for splash out and evaporation and for that used to flush out waste materials. In contrast, many raceway systems used to grow trout are termed "open" or "flow through" systems because all the water makes only one pass through the tank and then is discarded.

Fish grown in RAS must be supplied with all the conditions necessary to remain healthy and grow. They need a continuous supply of clean water at a temperature and dissolved oxygen content that is optimum for growth. A filtering (biofilter) system is necessary to purify the water and remove or detoxify harmful waste products and uneaten feed. The fish must be fed a nutritionally-complete feed on a daily basis to encourage fast growth and high survival.

EXPERIMENTAL SYSTEM

RAS have been in existence, in one form or another, since the mid 1950’s. However, only in the past few years has their potential to grow fish on a commercial-scale been realized. New water quality technology, testing and monitoring instrumentation, and computer-enhanced system design programs, much of it developed for the waste water treatment industry, have been incorporated and have revolutionized our ability to grow fish in tank culture.

Nevertheless, despite its apparent potential, RAS should be considered a high-risk, experimental form of agriculture at this time. They can be used to culture high-densities (over a million pounds) of fish annually, but their ability to do so economically remains to be demonstrated, conclusively and repeatedly.

Location Flexibility:

RAS are particularly useful in areas where land and water are expensive and not readily available. They require relatively small amounts of land and water. They are most suitable in northern areas where a cold or cool climate can slow fish growth in outdoor systems and prevent year-round production. RAS provide growers who are geographically disadvantaged because of a relatively short growing season (less than 200 days) or extremely dry (desert) conditions, a competitive, profitable, year-round fish production system.

They can be located close to large markets (urban areas) and thereby reduce hauling distances and transportation costs. RAS can use municipal water supplies (dechlorination is necessary) and discharge waste into sanitary sewer systems. Nearly all species of food fish and sport fish that are commonly reared in ponds including catfish, tout, and striped bass can readily be grown in high densities when confined in tank systems.

Water Supply:

A good supply of water, adequate in both quantity and quality, is essential to a successful fish farming enterprise, RAS or otherwise. Ground water obtained from deep wells or springs is the best source of water for fish culture. It generally is free of pollutants and has relatively high hardness levels, which are beneficial under some circumstances. Municipal water supplies also can be used after chlorine, floride, and other chemicals are removed.

Other sources of water, particularly surface waters from streams, rivers, ponds, and lakes, are not recommended for fish culture. Surface waters may contain fish diseases, parasites, pesticides, and other pollutants that can kill or slow the growth of fish. Testing the quantity and quality of the available water supply is one of the first steps for a prospective fish farmer to take to insure an adequate supply of high quality water.

Because RAS recycle most of their water, they consume considerable less than other types of culture and are especially well adapted to areas with limited water supplies. The required quantity of water needed to grow fish varies with the species of fish selected, size of the culture system, and investment size. As a general rule, a minimum water volume of 1-5 gallons is needed for every pound of fish reared and minimum water flows of 10-25 gallons per minute (more for trout) are needed to grow 50,000 pounds of warmwater fish per year.

Open vs. Closed Systems:

Tank culture systems are referred to as recirculating (closed) systems because they recycle or reuse water. No system is ever completely "closed," because some water must be added periodically to replace evaporative loses and that used to flush out waste materials. Some water change is necessary since no filter is 100% effective. Nevertheless, RAS can operate efficiently by occasionally adding only a relatively small amount of water on a daily or weekly schedule.

Open (flow-through) systems refer to those that simply allow water a single pass through the system before it’s discarded. Flow through systems can also be used in indoor tank culture of fish if an abundant and continuous supply of high quality water is available. Trout farmers typically use open systems rather than recirculating ones, because trout require large volumes of high quality, cold water. Open systems "leak" most of the water quickly, whereas closed systems "leak" water slowly. RAS are more suitable for warmwater fish such as channel catfish, striped bass, and tilapia that can tolerate lower water quality conditions and higher temperatures than trout. They also are now being used to rear marine species such as redfish, oysters, clams, and softshell blue crabs.

Fish Culture Tanks:

Fish can be grown in tanks of nearly every shape and size. Fish tanks typically are rectangular, circular, or oval in shape. Circular or oval tanks with central drains are somewhat easier to clean and circulate water through than rectangular ones. Rectangular tanks are usually built with or set upon inclined floors to facilitate cleaning and circulation.

Rearing tanks range in size from 500 to 500,000 gallons capacity. The size of the tank depends on a variety of factors including: stocking rates, species selected, water supply, water quality, and economic considerations. The tank must be designed to correspond with the capacity of other components of the system, particularly size of the biofilter and sump so that all parts of the system are synchronized.

Tanks can be constructed of plastic, concrete, metal, wood, glass, rubber and plastic sheeting, or any other materials that will hold water, not corrode, and are not toxic to fish. Smooth surfaces on the inside of the tanks are recommended to prevent skin abrasions and infections to the fish, and to permit cleaning and sterilization.

Light weight, durable, plastic tanks can be conveniently moved and readily cleaned when necessary, but they require special support to prevent stretching when filled with water. Stainless steel also is a good tank material, but can be expensive. Marine-grade plywood tanks are inexpensive, but leak if not properly sealed and are not as durable as tanks of other materials. Concrete tanks may be the most economical to build, but they are relatively permanent and immovable structures once constructed. Non-toxic plastic or rubber liners can but used over frames made of wood, metal, concrete, or other materials.

Biofiltration:

The biological filter (biofilter) is the heart of the RAS. As the name implies, it is a living filter composed of a media (corrugated plastic sheets or beads or sand grains) upon which a film of bacteria grows. The bacteria provide the waste treatment by removing pollutants. The two primary water pollutants that need to be removed are (1) fish waste (toxic ammonia compounds) excreted into the water and (2) uneaten fish feed particles. The biofilter is the site where beneficial bacteria remove (detoxify) fish excretory products, primarily ammonia.

Ammonia and Nitrate Toxicity:

Ammonia and nitrite are toxic to fish. Ammonia in water occurs in two forms: ionized ammonium (NH4+) and unionized (free) ammonia (NH3). The latter, NH3, is highly toxic to fish in small concentrations and should be kept at levels below 0.05 mg/l. The total amount of NH3 and NH4 remain in proportion to one another for a given temperature and pH, and a decrease in one form will be compensated by conversion of the other. The amount of unionized ammonia in the water is directly proportional to the temperature and pH. As the temperature of pH increases, the amount of NH3 relative to NH4 also increases.

In addition to ammonia, nitrite (NO2) poisoning of fish also is an imminent danger in RAS. Nitrite levels should be kept below 0.5 mg/l. Brown blood disease (methemoglobinemia) occurs in cultured salmon and channel catfish when hemoglobin is oxidized by nitrite to form methemoglobin (a respiratory pigment of the blood that cannot transport oxygen). The disease can occur at nitrite concentrations of 0.5 mg/l or greater. As the name implies, the blood has a characteristic chocolate brown color. Adding salt (NaCl) at a rate of 1 pound per 120 gallons of water (a chloride to nitrite ratio of 16:1) will suppress this disease in soft water; a ratio of 3:1 is effective in hard water.

Nitrification:

Ammonia is a poisonous waste product excreted by fish. Since fish cannot tolerate this poison, detoxifying ammonia is fundamental to good water quality, healthy fish, and high production.

Detoxification of ammonia occurs on the biofilter through the process of nitrification. Nitrification refers to the bacterial conversion of ammonia nitrogen (NH3) to less toxic NO2, and finally to non-toxic NO3. The process requires a suitable surface on which the bacteria an grow (biofilter media), pumping an continuous flow of tank water through the biofilter, and maintaining normal water temperatures and good water quality.

Two groups of aerobic (oxygen requiring), nitrifying bacteria are needed for this job. Nitrosomonas bacteria convert NH3 to NO2 (they oxidize toxic ammonia excreted by fish to less toxic nitrite), the Nitrobacter bacteria convert NO2 to NO3 (they oxidize toxic nitrite to largely nontoxic nitrate).

Nitrification is an aerobic process and requires oxygen.

For every 1 milligram of ammonia converted about 5 milligrams of oxygen is consumed, and additional 5 milligrams of oxygen is required to satisfy the oxygen demand of the bacteria involved with this conversion. Therefore, tanks with large numbers of fish and heavy ammonia loads will require plenty of oxygen before and after the biofiltration process.

Nitrification is an acidifying process, but is most efficient when the pH is maintained between 7 and 8 and the water temperature is about 27-28 C. Acid water (less than pH 6.5) inhibits nitrification and should be avoided. Soft, acidic waters may require the addition of carbonates (calcium carbonate, sodium bicarbonate) to buffer the water. The addition of a salt as a therapeutic in striped bass as freshwater bacteria temporarily adjust to alteration in salinity.

Biofilter Design and Materials:

A biofilter, in its simplest form, is a wheel, barrel, or box that is filled with a media that provides a large surface area on which nitrifying bacteria can grow. The biofilter container can be constructed of a variety of materials, including plastic, wood, glass, metal, concrete, or any other nontoxic substance. In small-scale systems, some growers have used plastic garbage cans or septic tanks. The size of the biofilter directly determines the carrying capacity of fish in the system. Larger biofilters have a great ammonia assimilation capacity and can support greater fish production.

A biofilter must provide sufficient surface area for the colonization (attachment) of nitrifying bacteria. It needs to provide a large surface area to support bacterial populations at densities adequate to reduce the load of waste products (ammonia) excreted by the fish population in the tank. It is essential that the water flowing through the biofilter come into direct contact with the bacteria film growing on the surface media for a time period sufficient to allow the bacteria to convert toxic NH3 and NO2 to less toxic NO3. Careful calculation of the flow-through rates (turnover or contact time) and size (volume and depth of the biofilter) is fundamental.

The biofilter media can be corrugated plastic, styrofoam or glass beads, lava rock, sand, gravel, or similar material that supplies large surface area. The quality and quantity of surface area of the media provided for nitrifying bacteria are important determinants of the efficiency of the biofilter. The ideal biofilter media has (1) high surface area for dense bacterial growth, (2) sufficient pore spaces for water movement, (3) clog resistance, (4) easy cleaning and maintenance characteristics. We use and recommend plastic because it’s lightweight, flexible, and easy to clean, but it can be expensive.

Biofilter Sizing:

The biofilter in any RAS design must be sized to correspond with the other system components. Important factors that must be considered in designing a biofilter are: media surface area (square feet of surface for bacteria attachment), ammonia leading (ounces of ammonia that need to be converted per day per square foot of media area), and hydraulic loading (gallons of water per day per square foot media surface).

Types of Biofilters:

Biofilters can be configured in many ways. The two general categories are (1) submerged bed filters and (2) emerged bed filters. Submerged bed filters can have fixed (immobile) media in which the water flow can be upward, downward or horizontally through the media.

The fluidized bed reactor (FBR) is a commonly used submerged bed filter. The FBR consists of fine particles (sand, dense plastic, glass beads, minerals, etc.) in a container through which upwelling water flows thereby "fluidizing" or suspending the media in the water column. FBRs offer large surface area per unit volume and theoretically greater nitrification. However, as with other submerged bed filters, all of the oxygen needed for conversion of ammonia to nitrate must be dissolved. Submerged bed filters often need supplemental aeration both before and after the water passes through the filter. If the inlet dissolved oxygen is low, the efficiency of ammonia conversion is reduced.

Emerged bed filters are of two basic types: (a) trickling filter (TF) sometimes called packed columns, and (b) rotating biological contactors (RBC). These filters have the advantage of not requiring the addition of oxygen prior to water entering the filter. In fact, these filters frequently supply al the oxygen used to support fish respiration. For this reason these types of filters are often employed in RAS.

The trickling filter is designed to have water slowly cascade down through the media column, which is suspended above the water. Water enters the column (which is filled with biofilter media) from an overhead spray pipe and trickles down through the biofilter media where nitrification occurs. The waterfall action of this filter adds oxygen to (aerates) the water.

Trickling filters can become clogged and lose efficiency. When a filter accumulates too much organic material it can experience "ponding" and "short coming." In severe instances the filter can cease functioning altogether. We use trickling filters in the Virginia Tech Aquaculture Center on some of our smaller fish culture tanks that contain only a limited number of fish.

The rotating biological contractor (RBC) has a water-wheel configuration consisting of plastic media attached to a central axle which spins slowly, moving the media through the water in the RBC containment vessel. Advantages of the RBC are that it is self-aerating and self-cleaning. Once established, it tends to be very stable and can operate for years without failure.

The Virginia Tech system uses the RBC-type biofilter. The RBC has three circular stages (6’ diam x 1’ thick, 1,950 ft² per stage, 5,850 ft² total) mounted to a central shaft. The shaft is rotated at 2 rpm by a ¼ hp gearmotor. The biofilter media and tank (525gallon) are divided into three partitions or stages so that filtered water passes in sequence form one to another. Staging the RBC allows greater nitrification efficiency for a given amount of media or RBC size.

Recirculation Rates (turnover times):

The recirculation rate (turnover time) is the amount of water exchanged per unit of time. This can easily be determined by dividing the volume of water in the tank by the capacity of the pump (in gallons per minute). For example, the turnover rate in a 2,500 gallon tank system circulated with a water pump rated at 45 gallons per minute (63,360 gallons per day) would be 25.3 tank volumes per day (a rate of slightly more than one volume per hour).

Increasing the number of turnovers per day would provide increased biofiltration, greater nitrification (bacterial contact), and reduced ammonia levels. Most fish production recirculation systems are designed to provide at least one complete turnover per hour (24 cycles per day).

Other filters:

Other types of filtration (mechanical and chemical) are available and can sometimes be used to supplement the efficiency of biofilters in removing ammonia in fish production systems. Most of these measures are useful only to temporarily control ammonia and nitrite in small systems.

In chemical filtration, water is pumped through a chemical media of activated carbon, zeolite, or other substances. These chemicals have microscopic pores that trap ammonia ions and remove them from the water. The familiar activated charcoal filter, popular in aquaria, can be incorporated as an auxiliary filter to support biofiltration in fish production systems, but this form of filtration requires periodic replacement with large quantities of relatively costly activated charcoal.

Zeolite filters are frequently used to remove NH4 (and indirectly NH3) at an estimated rate of 1 mg NH4 per 1 g of zeolite. The use of zeolite requires regular and constant pumping of water through the filter and regular replacement of large quantities of expensive zeolite. Zeolite can be recharged with a salt solution (10%) and reused, but salt water disposal then becomes an environmental problem, particularly in inland waters.

Sump:

A sump (clarifier tank) is used to prevent the excessive accumulation of fish excretory products and waste feed. Waste products increase the biological oxygen demand (BOD), decrease the dissolved oxygen content, lower the carrying capacity (density of fish) that can be reared, and may result in off-flavor in fish products. Accumulation and decomposition of waste material results in the production of toxic compounds such as ammonia (NH4, NH3, NO2) and hydrogen sulfide (h2S) that can be hazardous to fish health.

The clarifier tank is designed as a settling basin (large volume tank with a slow flow rate to increase sedimentation). Its purpose is to concentrate and remove suspended solids (fish feces, uneaten feed particles) before they clog the biofilter or consume valuable oxygen supplies. The clarifier should be a separate tank, isolated from the fish tank and the biofilter, so that it can be cleaned periodically (daily) as needed. To increase the efficiency of the clarifier, various filters (plastic filters, sand filters, metal screens) can be inserted into the sump tank.

In the Virginia Tech RAS, water flow in the sump tank is directed upward through a multi-tube clarifier (corrugated plastic filter) which removes large, settleable particles from the water column by gravity (particles with densities great tan water will sink). The waste material sinks, collects, and is concentrated in the bottom of the sump where it can be drained out. The sump tank should have a v-shaped bottom to concentrate waste particles and facilitate cleaning. Another technique for the removal of waste particles for suspension in the water column includes the use of a hydroclone, a cone shape tank that uses centrifugal force to increase gravitational force and enhance waste particle settling.

The size of the clairifier tank needs to by fully integrated with the sizes of the fish tank and biofilter and also with the turnover rate of the system (pump size). Volume of the sump and flow rates through the sump must be adjusted to maximize sedimentation of suspended particles. Sump column in the Virginia Tech system represents about 15% of the total and flow rates average about 90 gallons per minute.

Oxygen Management:

Successful fish production depends on good oxygen management. The addition of oxygen in a pure form or as atmospheric air (aeration) is essential to (1) the survival (respiration) of fish held in high densities, (2) the survival of aerobic, nitrifying bacteria on the biofilter and, (3) for the decomposition (oxidation) of organic waste products. Supplying sufficient oxygen to sustain healthy fish and bacterial populations and to meet the biochemical oxygen demand (BOD) for fish waste and unconsumed food is critical. Maintain oxygen levels, near saturation or even at slightly super-saturation at all times. Low oxygen levels will reduce growth, feed conversion rates, and overall fish production.

The amount of oxygen needed in RAS depends on a number of factors. Oxygen demand is directly correlated with the density of fish in the tanks, feeding rates, water temperatures, flow rates, and nitrification. It is also a function of physical conditions such as water temperature and water volumes. Increasing dissolved oxygen concentrations through oxygen injection, aeration, and increasing water flow rates (turnover times) are ways to increase the density (carrying capacity) of fish that can be held in tanks of fixed size.

Atmospheric oxygen can be added to the tanks by surface agitation with aerators or by large blowers. Surface aerators may not be cost effective or efficient in evenly distributing oxygen throughout large commercial-scale systems. Blowers can be effectively used to supply oxygen and also to mechanically rotate RBS.

Pure Oxygen:

Pure oxygen injection systems are increasingly being used in aquaculture. They are particularly useful in maintaining oxygen-saturated conditions in recirculating systems with high densities of fish. Pure oxygen can be delivered and stored in a tank as liquid oxygen or it can e produced on-site by a oxygen generator. Bottled oxygen gas also is sometimes kept as an emergency backup system for RAS, but this alternative usually is too expensive and bulky to be practical.

Liquid oxygen technology is relatively simple, efficient, and cost-effective; especially if purchased in bulk quantities and if the site is located near a reliable supplier. A liquid oxygen system consists of a storage tank for the liquid gas, vaporizers to turn liquid oxygen to gas, and supply lines to the fish tanks. It conveniently requires no external power supply and is therefore free of power failures and the consequent fish kills. Most growers rent or purchase a liquid oxygen storage tank of a size sufficient to provide two to four week supply of oxygen. The size of the tank corresponds with the fish production capacity of the system.

Oxygen generators (pressure swing adsorption systems, PSAs) are particularly advantageous at remote sites where liquid oxygen deliveries would be costly and expensive. Generators produce oxygen by using electric energy. They are expensive to purchase and operate, and they are subject to power failures. Selecting the proper size of the oxygen generator and carefully calculating the cost of electric power needed are important considerations.

Ozone Sterilization:

Ozone (O3) is a naturally occurring gas (upper atmosphere) that consists of three atoms of oxygen. It is a powerful oxidizing agent that can be used to break down compounds. Ozone must be used with caution since it is directly toxic to aquatic life and may form harmful biproducts (hypochlorite, hypobromite). Careful redox potential measurements and special injection equipment apparatus are needed to determine and control ozone applications.

Carbon Dioxide:

In addition to toxic ammonia, carbon dioxide tends to concentrate in intensive fish production systems. As carbon dioxide increases, the pH of the water decreases, and fish respiration is affected. Carbon dioxide levels should be maintained at levels less than 30 mg/l for good fish growth. Some carbon dioxide is beneficial since it reduces pH and mitigates ammonia toxicity. Carbon dioxide removal can be accomplished with any device (RBC, packed column) that increases air-water contact.

Emergency Systems:

Fish farmers using RAS usually have made a substantial economic commitment in the system, feed, and fingerling fish. The standing crops of fish in the tanks at any particular time represent costly inventory that become increasingly valuable as they approach marketable size.

Fish kills caused by suffocation resulting from power failures (even short-term ones) are not uncommon. The oxygen demand in tanks with high densities of large fish is high. During emergency power failures, the grower generally has a response time of 10 minutes or less to avoid a total kill. Sublethal oxygen concentrations can be equally harmful because they cause slow growth, increase the susceptibility of fish to disease and parasites, and decrease production and profitability.

Fish farming in RAS is risky business where "an ounce of prevention" is a worthy and important consideration. Fish kills from oxygen deficits as a result of power failures constantly threaten growers with financial ruin. Protection by designing alarm systems, or using backups and alternatives for emergencies is essential.

An alternative bottled oxygen gas or liquid oxygen system is suggested for growers who rely primarily on oxygen generators. The Virginia Tech RAS uses liquid oxygen, but also has a secondary, standby surface agitator system in each tank and a backup electric generator for emergencies.

An emergency water supply reservoir consisting of a large volume of high quality water (oxygenated, dechlorinated) and the proper water temperature stored in a standby tank provides a good margin for safety. Sudden declines in water quality or outbreaks of disease can be effectively controlled in certain instances by rapidly flushing growing tanks with freshwater, by quickly altering water temperatures.

PRODUCTION CONSIDERATIONS

Fish Species Selection:

RAS can be used to rear nearly any species of fish, freshwater or marine or other aquatic animal. Hybrid striped bass, channel catfish, and tilapia are the most common freshwater species grown in these intensive culture systems. Red drum (redfish) and soft shell blue crabs are the main saltwater species being reared RAS. Because RAS requires high capital investment, it is best designed to grow high-value species such as striped bass for food or stocking in private recreational angling waters or aquarium fish.

Certain fish species are easier to grow and more available than others. Two species dominate the U.S. aquaculture industry today: channel catfish and rainbow trout. Their popularity is a result of the desirable traits that they express. For example, their culture requirements are well known, the demand high, and the economics viable. Small (fingerling) fish are available for grow out nearly year-round, they tolerate crowding and stress, and disease resistant, and grow rapidly to a marketable size.

Recirculating systems generally are not suitable for rearing coldwater species such as salmon and trout. Trout and salmon require clean, cold water and generally do not thrive in warm, recycled waters. However, a wide variety of fish, particularly catfish and hybrid striped bass, and other aquatic species, including shrimp and mussels are good candidates for farming in recirculation systems.

Warm Temperature:

Water temperature strongly influences feeding and growth rates of cultured fish. The water temperature preferences of most cultured fish are well known and can be maintained year-round in RAS. In general, cultured fish species can be classified as either cold water species that prefer temperatures of 50-65 F, (trout), cool water species prefer temperatures of 65-80 F, (yellow perch), and warm water species (channel catfish) that prefer temperatures 80-90 F. Water temperature also influences the water quality processes occurring in recirculation systems. For example, the optimum water temperature for bacterial nitrification activity is 85 F, which may or may not be optimal for the fish species being cultured.

Energy conservation is one of the major advantages of recirculation aquaculture systems. Once the tank water is heated to the optimal temperature for fish growth, only a small amount of heat energy is required to maintain the temperature. However, heat loss from the building and from the water via evaporation, splash-out, or waste water must be minimized to assure economic success.

A unique thermal property of water is its high specific heat – it heats and cools slowly. Therefore, once the optimal water temperature for fish growth is reached, only a small amount of energy is necessary to maintain the best thermal condition. Heat losses through convection and conduction are minimal in a well insulated building.

Heat can be provided by heating the air in the building, or directly heating the water, and by using heat exchangers. Water temperature in the Virginia Tech system is supplied by heating the air in the building with four propane space heaters. The room temperature is generally kept 2-4 degrees above that of the water to reduce condensation in the building. The Virginia Tech system is housed in a specially constructed 110- x 50- x 16-ft high farm utility building. The interior and exterior walls and ceiling are covered with 19 gage corrugated aluminum siding and insulated with R-19 and R-30 fiberglass blanketing. At times, room temperatures and working conditions can become hot and humid, especially when growing warm water fish in the summer. High temperatures and humidity can be controlled through ventilation with an electric fan.

Directly heating the water is another alternative, but this strategy requires expensive insulated tanks with lids that maintain the heat, but inhibit feeding and observation of the fish. Alternatives such as solar heating and heat exchangers are being considered.

Market Opportunities:

A wide variety of freshwater fish can be farmed in RAS for a diverse number of purposes. Many species are commonly grown as food fish for marketing as live or processed products to wholesalers or retailers and directly to supermarkets and restaurants. The major food fish species reared in the U.S. are channel catfish and rainbow trout. Other species with good potential for farming as food are hybrid striped bass, red drum, tilapia, yellow perch, walleye, and shrimp.

Fish can be marketed alive, thereby reducing processing concerns, or they can be sold fresh on ice, or frozen. Value-added products such as smoke, marinated, boiled, and pickled fish may expand the demand and induce a greater price per pound.

In addition for food fish, hatcheries can be established to sell fish eggs to other growers or to market fingerling (2-6 inches) sportfish for stocking in private ponds and lakes for angling. Bait minnows, crayfish, and some aquatic insects can be marketed to bait dealers for sale to anglers. Goldfish, tropical fish, turtles, and other aquatic animals and water plants are cultured and marketed as aquarium pets and for stocking in ornamental ponds.