(Recycling Aquaculture System)


Intensive RAS aquaculture process

An alternative to outdoor open ocean cage aquaculture, is through the use of a recirculation aquaculture system (RAS). A RAS is a series of culture tanks and filters where water is continuously recycled and monitored to keep optimal conditions year round. To prevent the deterioration of water quality, the water is treated mechanically through the removal of particulate matter and biologically through the conversion of harmful accumulated chemicals into nontoxic ones.

water purif WPOther treatments such as UV sterilization, ozonation, and oxygen injection are also used to maintain optimal water quality. Through this system, many of the environmental drawbacks of aquaculture are minimized including escaped fish, water usage, and the introduction of pollutants. The practices also increased feed-use efficiency growth by providing optimum water quality (Timmons et al., 2002; Piedrahita, 2003).

One of the drawbacks to recirculation aquaculture systems is water exchange. However, the rate of water exchange can be reduced through aquaponics, such as the incorporation of hydroponically grown plants (Corpron and Armstrong, 1983) and denitrification (Klas et al., 2006). Both methods reduce the amount of nitrate in the water, and can potentially eliminate the need for water exchanges, closing the aquaculture system from the environment. The amount of interaction between the aquaculture system and the environment can be measured through the cumulative feed burden (CFB kg/M3), which measures the amount of feed that goes into the RAS relative to the amount of water and waste discharged.

ras22Because of its high capital and operating costs, RAS has generally been restricted to practices such as broodstock maturation, larval rearing, fingerling production, research animal production, SPF (specific pathogen free) animal production, and caviar and ornamental fish production. ras8Although the use of RAS for other species is considered by many aquaculturalists to be impractical, there has been some limited successful implementation of this with high value product such as barramundi, sturgeon and live tilapia in the US.


Major species

Top ten freshwater, brackish water and marine cultured fish in 2010

Freshwater culture




Brackishwater culture


Grass carp


Atlantic salmon


Greasy grouper


Silver carp


Large yellow croaker


Flathead grey mullet


Catla (Indian carp)


Salmonids nei


Marine fishes nei


Common carp


Greasy grouper


Nile tilapia


Bighead carp


Sea trout


Cyprinids nei


Crucian carp


Japanese amberjack




Nile tilapia


Gilthead seabream


Marble goby


Pangas catfishes nei


Japanese seabass


Tilapias nei


Roho labeo


European seabass


European seabass


Freshwater fishes nei


Silver seabream


Mozambique tilapia


† not elsewhere identified in FAO statistics


Sustainable improvements in technological aspects of aquaculture will not be achieved unless they are accompanied by appropriate policies that address the social and economic environment within which the aquaculture system is placed.ras27 The development of such systems must lie within the context of environmentally sound regulatory frameworks (e.g. systems providing for monitoring and enforcement, and good  governance)

In the 21st Century, water resources will be at a premium, with water shortages expected after 2015. With such a pressure on this vital resource for aquaculture, business-as-usual scenarios will no longer be possible. Competition for this resource will increase with drinking water shortage expected to affect large populations by 2025. This important constraint will have a major bearing on how aquaculture can and will develop in the new millennium, and appropriate technologies and farming systems will be required to address this issue.

Within the context of this paper, the essential elements of aquaculture incorporate: the care of aquatic stocks; requires confinement or site allocation; isolation to varying degrees of the farmed stock from the external environment; allows for various levels of internal control of the system; and requires some form of ownership or contractual arrangement to that effect.

ras11Aquaculture systems must be considered in relation to natural resource systems and human development circumstances within which reside.

  This requires consideration of sustainability criteria, particularly socio-economics and the wider interaction between aquaculture and other processes and activities. These interactions have to be considered– both as aquaculture’s impact on other water and natural resource users, and the impact of these on aquaculture.

Types of aquaculture systems

Systems and species

Aquaculture systems range from very extensive, through semi-intensive and highly intensive to hyper-intensive. When using this terminology the specific characterization of each system must be defined, as there are no clear distinctions and levels of intensification represent a continuum.

Farming systems are also diverse for example including:

  • Water-based systems (cages and pens, inshore/offshore).
  • Land-based systems (rainfed ponds, irrigated or flow-through systems, tanks and raceways).
  • Recycling systems (high control enclosed systems, more open pond based recirculation).
  • Integrated farming systems (e.g. livestock-fish, agriculture and fish dual use aquaculture and irrigation ponds).



Various aquatic organisms are grown in different ways including:

  • Fish (ponds, polishing ponds, integrated pond systems).
  • Seaweeds and macrophytes (floating/suspended culture, onshore pond/tank culture).
  • Molluscs (bottom, pole, rack, raft, long-line systems also culture based fisheries)
  • Crustaceans (pond, tank, raceway, culture based fisheries).
  • Other minor invertebrates, such as echinoderms, coelenterates, seahorses, etc (tanks, ponds, culture based fisheries)

The phases of aquaculture include broodstock holding, hatchery production of seed, nursing systems, grow-out systems, and quarantining.

Together, this mix of intensity, culture systems, species, farming systems and different phase of culture create an extreme diverse collection of aquaculture systems and technologies.

mussels on rope
Management interventions, infrastructure and support technologies

The management interventions, infrastructure and supporting technologies utilized in aquaculture include a wide range of activities, such as seed supply and stocking, handling, feeding, controlling, monitoring, sorting, treating, harvesting, processing and use of prophylactic measures.

Recirculation systems

The uses of recirculation vary widely, from broodstock management, hatchery and nursery rearing, grow-out and quarantine holding. It is likely that use of recirculation systems in intensified commercial aquaculture will increase in future. There are many possible solutions, adaptable to specific local situations.production

The PAS system for catfish is one example. It combines an extensive set of channels within the pond, for water treatment, with a highly intensive growth enclosure. The very slow circulation with low energy requirement provides good control of pond environmental processes whilst conserving water.

The recirculation of water is not necessarily highly intensive. Shrimp farmers in Thailand are successfully using closed pond systems for removing the requirement for water exchange making efficient use of brackishwater and helping to reduce risks of introduction of shrimp pathogens to the farming system.

Active suspension ponds, which reduce the requirement for water exchange, have been demonstrated for tilapia in Israel and the USA and in shrimp culture in Belize Hyper-intensive recirculation systems have many advantages.


  These include minimum water demand, limited space demand, reduced water discharges, controlled conditions to optimise productivity, tight control of feeding to maximise feed conversion efficiency, fairly site-independent, exclusion of predators and climatic events, and necessarily little use of chemicals. But such systems often involve high capital costs, are more complex, and failures can result in serious crop loss. Such systems place greater demands on management control, feed design, health management, and demand professionalism in their use.

A well-designed recirculation system must be readily managed and competitive in terms of cost-efficiency, as such current applications are principally targeted at high value intensive aquaculture.ras24

Hyper-intensive recirculation is currently particularly suited to Europe due to environmental pressures and the market for high value aquacultured species. As economic and resource conditions change in the future, alternative applications of recirculation are likely.

Technology issues in recirculation approaches:

There are a number of technology issues in recirculation technologies that include:

  • Limited knowledge about component interactions (biofilters, mechanical filters, energy flows).
  • Interaction of pathogens and benign microbes in biofilters is very poorly understood.
  • Biofilms, biomats etc need more study.
  • Scale up problems are common: thorough testing is still necessary.
  • Modified processes may be required when using new feeds.
  • Accumulation of bi-products in the systems are poorly understood.
  • There is a need for predictive modeling to assess multifactor interactions in recirculation system design and testing.farming3

The design of feeds for recycling systems will: need to weigh conversion efficiency versus water treatment efficiency. Currently, feeds can be designed to facilitate the separation of faeces from the water and for reduction of nutrient leaching.


Recirculation systems would be preferred for culture of exotics species and GMOs, since escape to the wild can be more effectively controlled.grouper fish5Intensification can cause stress by disrupting fish social structures – but this varies with species – some do better at high stock densities, and we need to know more about such behavioural characteristics. Fish may require pre-adaptation to the recirculation environment. Recirculation techniques can also be highly species-specific. Species that are currently difficult to culture can be selected to perform better in recirculation systems. As expected, strains that have been cultured and adapted to recirculation systems seem to perform best.

Welfare concerns as well as the desire for improved productivity will compel us to design systems to suit the needs of the cultured animal.MEDIA FILTER

Water is not always the limiting factor that makes recirculation an attractive option– in some cases it may be energy conservation such as heated hatchery and/or grow-out systems.

New approaches

An important future environment for aquaculture expansion is the sea, particularly offshore waters. Currently coastal waters, bays and inlets etc. are utilized but the cost of open water development is currently prohibitive in most instances.

As we enter the new millennium, it is noticeable that the rate of increase in global aquaculture production is slowing. If this is due to production limitations, it suggests we are not using current technologies well, or alternatively those future increments will be more expensive to achieve. We therefore need fundamental innovations in aquaculture technology and it would also be useful to determine the potential performance of the available species, to help us optimise culture conditions.captive-bluefin-tuna-inside-a

The slowing of growth of aquaculture production is largely due to the effect of major current producers, as a result of saturation, problems with disease and environmental limitations. We should also take account of huge longer-term potential in South America and Africa, for which suitable technologies might already be available but have yet to be effectively transferred in a manner suitable to the prevailing local conditions.

  The immediate need in these regions is to address the socio-economic barriers to aquaculture development.

Fish cage systems

The production of fish from cages is increasing globally. The technologies are now well developed in Europe, parts of South America (Chile in particular) and China.farming4 In SE Asia, cage farming of fish is advancing rapidly, in a wide range of species; the main limitations being the availability and high cost of feeds and shortage of seed. There is already considerable transboundary movement of fish seed and fingerlings in Asia, mainly for live fish markets in Hong Kong and China. Little is known of environmental impacts, although this trade is known to result in some destructive fishing techniques for fish fingerlings.

Each country has its own species, markets and issues that need to be addressed in the development of cage culture, but future expansion of this farming system is expected.

Inshore-nearshore cage farms:

Environmental impact minimization, or even positive impacts, can be achieved with inshore and nearshore cage farms. For example, combinations of fish cages with seaweed and shellfish culture can reduce nutrient and organic loading, combining cages and artificial reefs can contribute to stock enhancement and could have a long term potential for culture based fisheries.

There are a number of other technical issues that include:

  • Making better nets, (stronger, less prone to attack by predators, and coping with fouling (while reducing use of antifouling paints);
  • New designs, in particular deeper, larger and submersible cages;
  • Increasing scale requires new levels of risk management;
  • Equipment for sorting, handling, counting, biomass estimates.


Environmental management issues will be particularly important for the future development of cage culture. The issues to be addressed include:

  • Better knowledge about mortality and real number of fish in cages, better feeding regimes, with less waste of feed;
  • Thorough study of material and energy flows through cage systems.
  • Modeling of the environmental impacts (not only benthic deposition, but also nutrient release and dispersal)

  • Better knowledge about recovery processes, so as to estimate fallowing time;

  • Site rotation: better equipment for simpler mooring;

  • Models are lacking that relate to remote zones and interactions between nearby farms.

  • Improved management of coastal zones, access rights and ownership are required in many countries that have the potential for expansion of coastal aquaculture.