>>Recirculating Aquaculture Systems

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Recirculating Aquaculture Systems

Aquaculture Systems (RAS) are systems which re-use water with mechanical and biological treatment between each use. A recirculating system generally occupies very little area, requires less water than conventional aquaculture and provides a predictable and constant environment for the culture species.

RAS technology can be useful where land or water is limited, where water is of poor quality, if temperatures are outside the optimum range of the species to be cultured or if the species is exotic. It can also be usefully employed where ideal sites are unavailable or when the effluent stream needs to be controlled. A more intensive farming approach can be applied in recirculating systems than in open systems such as ponds, cages or flow through systems.The level of control provided by recirculating systems can provide a basis for improved risk management. The trade off however, is the increased dependence on technology and associated expense and expertise to manage it. Recirculation systems are expensive to purchase and operate and for this reason it is usually only economically viable to farm high value species in these systems.Recirculating systems represent relatively new technology with a wide variation in system design and quality available. When selecting a supplier or consultant, a potential grower should check their track record for the construction of recirculation systems, after sales service in technical and husbandry advice and ensure that the system will produce the tonnage of fish that has been originally specified.

Advantages of RAS

1. All aspects of the production environment may be controlled to achieve optimum growth
2. Low water consumption per tonne of fish produced
3. Impact on the external environment minimised by containing and treating wastewater
4. Production facility can be operational all year round
5. The ability to produce species outside their natural range

Disadvantages of RAS

1. High capital costs (a 20 tonne system can cost approx. $250,000 - $500,000 to establish)
2. High operational cost ($7-10 for each kilo of fish produced)
3. Constant maintenance by trained personnel required

Principles of Recirculation Systems

A recirculation system is a closed system with culture tanks, filtration and water treatment components. The culture species is grown in tanks and the water is exchanged continuously to guarantee optimum growing conditions. Water is pumped from the tanks through biological and mechanical filtration systems and then returned to the tanks.A good general knowledge of the principles of water chemistry and a good biological knowledge of the species being cultivated, including an understanding of disease prevention, identification and treatment, is essential in recirculation system management.

Species

Many species can be succesfully grown in Recirculating Aquaculture Systems due to the high level of control when compared to other systems. Some species which have been successfully grown in these systems are Murray cod, eels, jade perch, barramundi and silver perch. Other species with potential for culture in these systems are native catfish, sleepy cod and possibly salmonids.

Principal Components

Production tanks vary in size and shape however round plastic or fibreglass tanks between 5,000 and 15,000 litres in capacity are most commonly used. Smooth round tanks with conical bottoms are preferred as solids can be concentrated by circulating water and subsequently removed from a central drain.For the integration of fingerlings, nursery tanks incorporated into the main system or smaller floating cages within the main tanks are generally used. This facilitates closer management of juvenile stock.Quarantine tanks, isolated from the main production system are useful in minimising disease transfer risk, particularly when new stock arrives and can also ensure that any medication used on stock does not interfere with biological filtration of the main production system.

Fish require oxygen to survive. Recirculation systems usually facilitate stocking at very high densities. This is subject to the oxygen available and simple mechanical aeration systems may not be sufficient. Oxygen can be added to the system via liquid oxygen and/or an oxygen generator to maintain required oxygen levels of above 60% saturation. If the fish are stocked at very high densities, oxygen saturation of over 100% is sometimes required.

Biological Filters

Fish produce ammonia and nitrites as metabolic waste products. These waste products become toxic to the fish if allowed to build up above certain levels and need to be converted to harmless nitrates. This is done using a biofilter, which is a medium upon which nitrifying bacteria colonise and grow. Common biofilter types include gravel, rotating biological contactor, bead filters, trickle filters and fluidised bed filters. The recirculating water passes through this biofilter and as it does so the nitrifying bacteria convert the toxic ammonia and nitrites in the water into non-toxic nitrates via oxidation. This process is known as nitrification.

Mechanical Filters

Mechanical filtration in recirculation systems is necessary to remove solids such as faeces, uneaten feeds and biofilter floc. Many systems crash due to inadequate mechanical filtration. If excess solids are deposited on the biofilter there can be a reduction in bacterial action and a resulting elevation in ammonia and nitrite levels. Additionally, excess organic matter in the recirculating system may increase risk of disease development. There are several types of mechanical filter available such as screens, sand, gravel, bead filters or settling devices. Several mechanical filters may have to be incorporated to ensure complete removal of solid waste matter.

Temperature Controllers

Temperature requirements vary with different culture species and it is vital to maintain temperature within the optimal range for growth for the particular species to be cultured. Fish grow more rapidly, achieve optimal food conversion ratios and are less stressed and less prone to disease within this range. Heat exchangers, electric submersion heaters/coolers, or air injection can be used to achieve the right temperature.

Stocking Densities

The loading capacity of recirculating production systems depends on flow rate, oxygen availability, filter and heat exchange unit efficiency and the particular requirements of the culture species. These requirements vary between species. For example, under optimum conditions eels can be grown at stocking densities of >300 kg/m³ whereas Murray cod can be grown at >100 kg/m³. The higher the density that a species can be cultured without negatively impacting growth rates or fish health, the more viable recirculating systems are. Another important factor in the viability of recirculation systems is the market price per unit product.

Economic Considerations

The cost of establishing and operating a viable recirculating facility can be higher than expected. There are computer models available that can be used to assess the merits of developing a recirculating aquaculture venture (AquaFarmer feasibility software). The larger the system, the more economically viable it will be.Low cost, small scale entry into the industry is often recognised as a means of limiting financial exposure while gaining valuable experience. This can lead to complex equipment retro-fitting, higher production risk margins and technological short cuts that may be costly in the medium to long term. While there may be an incentive to de-construct component parts by adding or subtracting from established designs, in practice this should not be considered lightly. It must be recognised that recirculating systems involve complex water chemistry in a finely tuned balance and that deviation from proven designs increases venture failure risk significantly.