Sustaining the Autonomous City Aquaculture System

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 Beyond Smart Cities from Amazon.