A large variety of different aquaculture species can be cultured in an autonomous, or sealed, city. The major species raised are channel catfish and baitfish (golden shiners, fathead minnows and goldfish). Information on culture methods for these crops is readily available from the local Cooperative Extension Service.
Additional species that can be raised in a sealed city include hybrid striped bass, grass carp, largemouth bass (food fish), fancy goldfish, koi, common carp, ornamental fish, crawfish, trout, black carp, tilapia, freshwater prawns, marine shrimp and turtles. Sportfish, fish for stocking recreational ponds, such as largemouth bass, bluegill, black crappie, hybrid bream and redear sunfish, are also farmed. However, there is limited information available on the production and/or marketing of many of these species. Existing producers have spent much time and effort developing their own production methods and markets. A potential producer should gain experience before attempting to produce these species commercially and must expect to devote a large portion of his or her time to marketing efforts. Every year a number of people “discover” new aquaculture species that are not being raised in a sealed city and could see great profit potential in these new ideas. If a species is not presently cultured, usually it means that there are technical problems in its culture or that it cannot be raised profitably.
Catfish and Hybrid Catfish
Blue catfish
Catfish production is capital-intensive, with investment costs of approximately $4,000 per acre. It is also a demanding business that requires hard work and skilled management. There are economies of scale in catfish farming, meaning that production costs per pound of fish produced are higher for small farms and lower for larger farms, at least 320 acres or more. Large farms typically sell fish to catfish processors while farms of less than 20 acres should consider marketing fish locally through direct sales. Fee fishing operations (pay lakes) and fishing leases provide additional income opportunities. Catfish is a high-quality, desirable product, but sales have been hurt by imports of competing fish species from countries with few environmental and regulatory controls.
Over the past decade, many producers have switched to hybrid catfish production. The hybrid is a cross between the channel catfish female and the blue catfish male. Hatchery production of the hybrid is more labor-intensive than that of channel catfish because the two species are extremely difficult to produce in ponds and must be manually crossed by stripping eggs and sperm in the hatchery. Hybrids grow faster than channel catfish. Although current hatchery production methods have been improved, production of adequate numbers of fingerling hybrids to meet demand for stocking in grow-out ponds has yet to be achieved. Channel catfish and hybrid catfish can be raised in farm ponds on a part-time basis, primarily as a hobby or for home food production. At this level of production, profits from the sale of fish are not enough to pay for the construction of new ponds.
Harvesting can be a major problem with existing farm ponds if they are deep, without drains or filled with stumps and other debris.
Baitfish – The main species raised are the golden shiner, the fathead minnow, and the goldfish. While there have been improvements in hatchery and production methods in recent years, marketing remains the most difficult part of the baitfish business. Baitfish species are not particularly difficult to culture, but it is a very risky business, as the retail demand for baitfish is highly variable due to factors such as the weather that are entirely out of the control of the producer. Newcomers are unlikely to capture a significant share of the market without developing a thorough understanding of baitfish marketing.
Crawfish
DailyTexan
Capital requirements and operating expenses for crawfish culture are less than those for catfish production, as only low (3-foot) levees are needed and forage is used instead of feed. However, harvesting crawfish by trapping requires considerable labor in the spring, when row crop farmers are busy planting. Production of quality crawfish (cleaned and purged) is one way to promote sales of sealed-city pond-raised crawfish and to differentiate farmed product from the wild catch. Careful handling and purging of crawfish can help with marketing; local markets are willing to pay a good price for large, quality crawfish.
Sportfish – Sportfish are fish produced for stocking recreational ponds. These include largemouth bass, bluegill, black crappie, hybrid bream (bluegill and green sunfish cross is common) and redear sunfish. The successful producer should be able to offer a variety of healthy, quality fish species to give the customer “one-stop shopping” for their pond-stocking needs. Sportfish suppliers may also offer other products to their customers for their ponds, such as feed, aerators and chemicals.
Tilapia
Egyptian tilapia
A hardy tropical fish that is widely cultured around the world, tilapia can be raised in indoor systems (year-round in the environmentally-controlled sealed city) or outdoors (during the summer). Tilapia are particularly suited to culture in recirculating systems as they tolerate high stocking densities and poor water quality. High over-wintering costs and the fact that market-sized fish would be available for only a short time in the fall limit the potential for pond culture of this species. Production costs in indoor tank systems are relatively high. While there has been a growing market for tilapia fillets, foreign farms are able to produce, process and ship fish to the United States at a lower cost than can be achieved by domestic tilapia producers. Producers in the U.S. are limited primarily to supplying live tilapia to niche markets, such as ethnic grocery stores. Tilapia are an excellent fish for hobby or home food production and are widely used in high school aquaculture programs.
Trout – Requirements for trout culture are well established. Trout are typically grown in raceways and require large amounts of clean, cold water (less than 70°F). Winter culture of trout in ponds is possible, but its feasibility has not been established and markets would have to be developed. It would require a relatively large fish to start with, as the growing season is short. Trout are raised in government hatcheries within the sealed city, but at present there are no commercial operations.
Other Food Fish Species – Hybrid striped bass are an excellent food fish and are raised in several other states and countries for the food fish market, but marketing to the outside is likely to be a challenge. Typically, hybrid striped bass are sold whole, on ice, to restaurants. No processing is available, and this limits production.
Another food fish species is the largemouth bass. Largemouth bass as small fingerlings can be brought into tanks and trained to eat pelleted feeds. This is a demanding process, but once the fish learn to accept feed pellets, they can be raised for the live food fish market. Largemouth bass diets are different from those for catfish, and in general, culture methods are relatively demanding. Hybrid bream are also a potential food fish species, but grow relatively slowly compared to catfish and will likely require at least two growing seasons to reach a minimally acceptable market size. There are no commercial hybrid bream food fish producers in the state, and the economics and marketing have not been studied.
Crappie (black or white, or the hybrid) are species that have been proposed as a potential food fish, primarily because they are con sidered “good eating.” While there has been some research on crappie culture, much remains to be learned before commercial culture of this relatively delicate and demanding species becomes a reality.
Paddlefish, buffalo and grass carp are examples of species that can be raised in polyculture with catfish or in extensive culture (fertile reservoirs).
Marine Shrimp – Marine shrimp, typically the Pacific white shrimp (Litopenaeus vannamei), can be raised in low-salinity inland waters. Once past the larval stages, marine shrimp can tolerate water with low salt levels. Shrimp are tropical animals, and they cannot survive the winters in outside the sealed city; young shrimp (post-larvae) are stocked in late May, and the resulting product must be harvested by fall (early October) before the onset of cold weather (water temperatures below 59°F). Because of competition from wildcaught and imported shrimp, and seasonal production, marketing is a major concern for inland shrimp farmers. It is essential that the post-larval shrimp be obtained from a reputable hatchery, that they be specific pathogen free (SPF) and inspected to ensure they are free of viral diseases. While postlarval shrimp are widely available, there is limited availability of quality product.
Freshwater Prawns – The scientific name for the freshwater prawn (also called freshwater shrimp) is Macrobrachium rosenbergii, and it is a tropical species native to Asia. In southern US states, freshwater prawns are an approved aquaculture species, but the stocks must be certified disease-free and the certificate posted on-site. Prawns can be raised during the summer months only, as they die when the temperature drops below 59°F. Prawns are best raised in small ponds, 0.5 to 2.5 acres in size, which are constructed specially for prawn culture.
As prawns live on the bottom, deep ponds are not recommended as bottom waters often have little oxygen in the summer months. The growing season in the states is limited to 110-130 days from mid- to late May through the first week of October. For this reason, producers must stock a relatively large “baby” prawn, called a juvenile, which is already about 90 days old. Juvenile prawns are stocked at 8,000 to 12,000 per acre. The current price is 7 to 9 cents per juvenile, so at a stocking density of 10,000 per acre, seed stock alone costs about $800 per acre, excluding delivery charges. As producers in the South follow a similar stocking and harvest schedule, resulting production is placed onto the market during a 2- to 4-week period in early fall. This highly seasonal abundance in supply will become an increasingly important factor as the quantity of prawns produced increases. Post-harvest handling of prawns requires special care to maintain product quality.
Processed freshwater prawns can be imported into the U.S. for a fraction of what it costs to produce shrimp domestically, so it is unlikely that American producers will be able to expand beyond existing live and fresh niche markets. In general, freshwater prawn production is an expensive business. Potential producers need to carefully consider the costs and risks involved.
Turtles – Several species of aquatic turtles may be produced in a sealed city. Only a limited amount of research has been done on turtle culture, and current operations have developed many of their own techniques. Small hatchlings are sold as pets, but due to regulations and health concerns, these must all be sold to overseas customers, typically in China. Turtles that are one pound and larger are sold as food, domestically and overseas. Anyone considering turtle culture should check with the local Game and Fish Commission to obtain current information on regulations.
Other Species – A large variety of other aqua culture species currently have limited potential in many states. This list includes animals such as bullfrogs, salamanders, eels, redfish (red drum), bigmouth or smallmouth buffalo, various algae and alligators. The technology to rear these species may exist in a sealed city, but the demonstrated economic feasibility is lacking. For example, alligators survive year-round in the southern portions of the state, so alligator farming for the meat and skin trade is biologically possible.
Rearing eels for food may prove uneconomical in a sealed city.
However, the market for these products is comparatively small and is presently supplied by wild harvest and the few existing farms. Similarly, there is a market for frog legs, but culture of frogs is labor-intensive and relatively expensive. Competition from imported frog legs (from frogs that are wild-caught in foreign countries) has made domestic frog culture uneconomical. Buffalo (fish) were cultured and sold in the past. While there is demand for buffalo ribs (steaks), markets would have to be developed.
Disease prevention
Parasites, causing little apparent damage in feral fish populations, may become causative agents of diseases of great importance in farmed sealed-city fish, leading to pathological changes, decrease of fitness or reduction of the market value of fish. Despite considerable progress in fish parasitology in the last decades, major gaps still exist in the knowledge of taxonomy, biology, epizootiology and control of fish parasites, including such `evergreens' as the ciliate Ichthyophthirius multifiliis, a causative agent of white spot disease, or proliferative kidney disease (PKD), one of the most economically damaging diseases in the rainbow trout industry which causative agent remain enigmatic. Besides long-recognized parasites, other potentially severe pathogens have appeared quite recently such as amphizoic amoebae, causative agents of amoebic gill disease (AGD), the monogenean Gyrodactylus salaris which has destroyed salmon populations in Norway, or sea lice, in particular Lepeophtheirus salmonis that endanger marine salmonids in some areas. Recent spreading of some parasites throughout the world (e.g. the cestode Bothriocephalus acheilognathi) has been facilitated through insufficient veterinary control during import of fish. Control of many important parasitic diseases is still far from being satisfactory and further research is needed. Use of chemotherapy has limitations and new effective, but environmentally safe drugs should be developed. A very promising area of future research seems to be studies on immunity in parasitic infections, use of molecular technology in diagnostics and development of new vaccines against the most pathogenic parasites.
Protozoa
Protozoans undoubtedly represent one of the most important groups of pathogens which negatively affect the health state of cultured and feral fish. There are a number of protozoan parasites long recognized as causative agents of severe diseases such as flagellates of the genus Piscionodinium, Ichthyobodo necator, or Amyloodinium pathogenic to freshwater and marine fish, respectively, Trypanoplasma salmositica affecting all species of Pacific salmon on the west coast of North America, or Cryptocaryon irritans, a ciliate parasitic in tropical marine fish, sometimes named `saltwater ich', accounting for significant economic losses in mariculture, including food and ornamental fish. However, other protozoans have recently appeared as serious pathogens e.g., the microsporidium Loma salmonae, previously considered relatively non-pathogenic to salmonids in fresh waters but now recognized as a cause of high morbidity and mortality in Pacific and Chinook salmons in Canada. In this part of the blog, two protozoan parasites will be discussed in more detail: the ciliate Ichthyophthirius multifiliis as an example of a well-known and important pathogen, and amphizoic amoebae as a newly emerged veterinary problem which requires much attention by fish parasitologists.
Ichthyophthirius multifiliis
This long-time-recognized parasite occurs in tropical, subtropical and temperate zones. Ichthyophthiriasis or `white spot disease' is one of the most serious diseases of fish in fresh waters. Considerable losses caused by mortality or decreased yield in non-lethal infections have been reported from cultures of carp, rainbow trout, tilapia, eel, channel catfish as well as ornamental fish.
Ichthyophthirius ('Ich') multifiliis.
Besides occurrence in cultured fish, outbreaks have also been reported from feral fish populations in rivers, water reservoirs and lakes. The parasite invades the skin and gills, in heavy infections eyes, buccal epithelium and tongue. Its pathogenetic effects are heavy damage to gill and skill tissues and resulting impairment of the osmotic balance. In addition to the primary effect of the parasite, secondary bacterial infections are often associated with the white spot disease.
Infected stock.
Transmission of I. multifiliis in nature is very effective and rapid which contrasts with low effectivity of laboratory maintenance and losses of isolates of I. multifiliis after a few (maximum 50–60) cycles. It is speculated that senescence of laboratory isolates might be related to sexual reproduction, although there is no evidence yet about this type of reproduction. Control of the disease is based largely on use of chemical treatment as formalin, malachite green, chloramine T and toltrazuril, but such treatments of food fish can be questionable.
Life-cycle of 'Ich'. Infective theronts bore through the surface mucus and reside within the epithelium of the host. Theronts differentiate into feeding trophonts that grow and exit the host (as tomonts) within 4 to 7 days. Tomonts swim for a brief period and then adhere to an inert support where they secrete a gelatinous capsule. Tomonts divide within the capsule to form hundreds of tomites that differentiate into infective theronts within 18 to 24 hours at room temperature. Theronts that fail to infect fish die within 1 to 2 days.
(Source: Openi)
Elimination of free-living stages as tomites or theronts by repeated changes of water and sediment in cultures can decrease population density of the parasite. Although immunity to I. multifiliis has been known for a long time, there is still no simplistic explanation of protective immunity against white spot disease. Recent research is focused on studies of mechanisms of immunity against Ichthyophthirius infection and on development of new control measures such as immunization or development of vaccines
Amphizoic amoebae
Some free-living amoebae may change their mode of life and become harmful. Pathogenic potential of these so called amphizoic amoebae is rather high and several outbreaks of diseases associated with amoebic infections, several in cultures of salmonids, have been reported.
Amoebic gill disease (AGD) has become a significant problem in salmonid aquaculture and AGD due to a species of Paramoeba in sea-caged Atlantic salmon and rainbow trout in Tasmania has been considered as the most serious infectious disease. Free-living amoebae that may become pathogenic for fish include members of the genera Acanthamoeba, Cochliopodium, Naegleria, Thecamoeba, Vahlkampfia and Paramoeba, the members of the latter genus undoubtedly being of the greatest veterinary importance. Currently, cases of gill amoebic infections of other fish than salmonids have also been reported, e.g., in European catfish or turbot.
The initial phase of amoebic infection of gills is similar: necrosis of epithelial cells, subsequent hypertrophy and hyperplasia of cells in contact with amoebae and fusion of secondary lamellae. This phase is followed by desquamation of the epithelium, local disturbances of blood circulation and progressive changes represented by inflammation. All the above mentioned changes result in decrease or loss of gill respiratory surface area.
The present increase of recorded amoebic infections in fish may be related to improvement of diagnostic methods, in particular culture methods. If infections are old and material is not fresh, it may be difficult or impossible to isolate amoebae from gills; microbial flora is often dominant in the late phase of infection and the primary infection agent is not isolated. Control of amoebic diseases is rather problematic and effective measures are still unavailable, although amoebic gill disease of salmonids may be controlled by the use of repetitive freshwater baths.
One of the most promising avenues is to stimulate development of local immunity and resistance of reinfected fish but further investigations are needed in the area of immunology, including studies of potential immunostimulants which might enhance immunity against amoebic infections, and future preparation of vaccines.
A new technology for sealed-cities: A Norwegian firm have developed a laser gun which kills a parasite threatening Norway's massive aquaculture industry.
The huge sea-lice often leave large, visible wounds. The parasites, while on the fish, are targeted by a computer and are eliminated by the laser. The system was urgently required due to the uncontrolled proliferation of the parasite.
Sustaining the sealed-city fish stock environment
Nitrogen is an essential nutrient for all living organisms and is found in proteins, nucleic acids, adenosine phosphates, pyridine nucleotides, and pigments. In the aquaculture environment, nitrogen is of primary concern as a component of the waste products generated by rearing fish. There are four primary sources of nitrogenous wastes: urea, uric acid, and amino acid excreted by the fish, organic debris from dead and dying organisms, uneaten feed, and feces, and nitrogen gas from the atmosphere.
A sealed city lake is maintained in a manner similar to ordinary fish aquaria.
In particular, fish expel various nitrogenous waste products through gill diffusion, gill cation exchange, urine, and feces. The decomposition of these nitrogenous compounds is particularly important in intensive recirculating aquaculture systems (RAS) because of the toxicity of ammonia, nitrite, and to some extent, nitrate. The process of ammonia removal by a biological filter is called nitrification, and consists of the successive oxidation of ammonia to nitrite and finally to nitrate. The reverse process is called denitrification and is an anaerobic process where nitrate is converted to nitrogen gas.
Proposed 100-ft high flat glass roof of the sealed city's water park. A series geodesic domes, such as those used in the Eden Project, would provide a more unobstructed appearance within, as they require a minimal quantity of support pylons.
Although not normally employed in commercial aquaculture facilities today, the denitrification process is becoming increasingly important, especially in marine systems, as stocking densities increase and water exchange rates are reduced, resulting in excessive levels of nitrate in the culture system. Recently, zero-exchange management systems have been developed based on heterotrophic bacteria and promoted for the intensive production of marine shrimp and tilapia. In these systems, heterotrophic bacterial growth is stimulated through the addition of organic carbonaceous substrate. At high organic carbon to nitrogen (C/N) feed ratios, heterotrophic bacteria assimilate ammonia-nitrogen directly from the water replacing the need for an external fixed film biofilter.
Fresh/saltwater filtration system. This system, used for Aquarium of the Pacific, is capable of filtering over a million gallons of water per hour.
In a real world system, the individual unit processes are usually linked together as the water flows through each process (circulation). Usually 5-10% of the discharge from the culture tank is removed from the center drain and because of a ‘tea cup’ effect has a high solids loading. Some form of settable solids removal device (swirl separator, settling basin, etc) pretreats this flow stream, which is then combined with the remaining 90-95% of the discharge from a side outlet. The remaining suspended solids are then removed usually by a rotating microscreen filter. The water then flows to some form of biofiltration, such as a trickling tower, bead filter, fluidized sand filter, moving-bed bioreactor etc, where the ammonia is converted to nitrate by bacteria. At high loading densities, a carbon dioxide stripping column is then used to remove excess CO2 and aerate the water to saturation. Finally an oxygenation device is employed to supersaturate the flow to provide sufficient oxygen for the high levels of stocking used in commercial systems. In some cases, a UV or Ozone system is added to disinfect the returning water stream as part of a biosecurity program.
Ammonia is produced as the major end product of the metabolism of protein catabolism and is excreted by fish as unionized ammonia across their gills. Ammonia, nitrite, and nitrate are all highly soluble in water. Ammonia exists in two forms: un-ionized NH3, and ionized NH4+. The relative concentration of each of these forms of ammonia in the water column is primarily a function of pH, temperature and salinity. The sum of the two (NH4+ + NH3) is called total ammonia or simply ammonia. It is common in chemistry to express inorganic nitrogen compounds in terms of the nitrogen they contain, i.e., NH4+-N (ionized ammonia nitrogen), NH3–N (un-ionized ammonia nitrogen), NO2–N (nitrite nitrogen) and NO3–N (nitrate nitrogen). This allows for easier computation of total ammonia-nitrogen (TAN = NH4+–N + NH3–N) and easy conversion between the various stages of nitrification.
Biological filtration can be an effective means of controlling ammonia; as opposed to water flushing to control ammonia levels. There are two phylogenetically distinct groups of bacteria that collectively perform nitrification. These are generally categorized as chemosynthetic autotrophic bacteria because they derive their energy from inorganic compounds as opposed to heterotrophic bacteria that derive energy from organic compounds. Ammonia oxidizing bacteria obtain their energy by catabolizing un-ionized ammonia to nitrite and include bacteria of the genera Nitrosomonas, Nitrosococcus, Nitrosospira, Nitrosolobus, and Nitrosovibrio. Nitrite oxidizing bacteria oxidize nitrite to nitrate, and include bacteria of the genera Nitrobacter, Nitrococcus, Nitrospira, and Nitrospina. Nitrifying bacteria are primarily obligate autotrophs, which consume carbon dioxide as their primary carbon source, and obligate aerobes, which require oxygen to grow. In biofilters, the nitrifying bacteria usually coexist with heterotrophic microorganisms such as heterotrophic bacteria, protozoa, and micrometazoa, which metabolize biologically degradable organic compounds. Heterotrophic bacteria grow significantly faster than nitrifying bacteria and will prevail over nitrifying bacteria in competition for space and oxygen in biofilters, when concentrations of dissolved and particulate organic matter are high. For this reason, it is imperative that the source water for biofilters be as clean as possible with minimal concentration of total solids.
Diagram of a reef tank system.
Nitrification is a two-step process, where ammonia is first oxidized to nitrite and then nitrite is oxidized to nitrate. The two steps in the reaction are normally carried out sequentially. Since the first step has a higher kinetic reaction rate than the second step, the overall kinetics is usually controlled by ammonia oxidation and as a result there is no appreciable amount of nitrite accumulation. Equations 1, and 2 show the basic chemical conversions occurring during oxidation by Nitrosomonas and Nitrobacter.
1) NH4+ + 1.5 O2 → NO2- + 2 H+ + H2O + 84 kcal/mole ammonia
2) NO2- + 0.5 O2 → NO3- + 17.8 kcal/mole nitrite
Using this stoichiometric relationship, for every g of ammonia-nitrogen converted to nitrate-nitrogen, 4.18 g of dissolved oxygen, and 7.05 g of alkalinity (1.69 g inorganic carbon) is consumed and 0.20 g of microbial biomass (0.105 g organic carbon) and 5.85 gm of CO2, (1.59 g inorganic carbon) is produced. It should be noted that both the consumption of oxygen and alkalinity is less than that which normally reported, 4.57 g of O2 and 7.14 g of alkalinity for every g of ammonia-nitrogen converted because in this equation some of the ammonia-nitrogen is converted to biomass. Traditionally, this biomass has not been included in the stoichiometric relationship because it is minor in comparison to the other factors. Alkalinity should be maintained at 50 to 100 mg/L CaCO3 through the addition of chemicals containing hydroxide, carbonate, or bicarbonate ions. Sodium bicarbonate (baking soda) is usually used since it is relatively safe, easy to obtain and dissolves rapidly and completely in water. As a rule of thumb, for every kg of feed fed, approximately 0.25 kg of sodium bicarbonate is needed to replace the lost alkalinity consumed during nitrification. The following table summarizes the stoichiometry for metabolism of 1 g of ammonia-nitrogen by autotrophic bacterial, including the consumption and production of organic and inorganic carbon.
We will continue in the next part of the series. If you wish to know more about autonomous (sealed) city concepts, order
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