An overview of fish farming recirculating systems
Recirculating aquaculture systems, or RAS, are closed-loop production systems that continuously filter and recycle water, enabling large-scale fish farming that requires a small amount of water and releases little or no pollution.
Recirculating Aquaculture Systems (RAS)
A recirculating aquaculture system is an enclosed system where the only water replacement is the water lost to evaporation and cleaning. These systems are being deployed in developed countries such as the United States where coastal land costs and labor costs are very high. The majority of tilapia grown in the US is from these types of systems.
There are several advantages in using recirculating aquaculture systems over traditional fish farming systems:
lower water requirements – a properly designed and operating RAS replaces less than 5% of the total water volume on a daily basis.
lower land requirements – in locations where land costs are very high, some sort of RAS should always be considered since they can produce a large volume of aquacultured product from a relatively small area. The amount of land required is less than 1/20th of the amount required for traditional pond farming.
reduced labor requirements – a typical 100 metric ton per year RAS can be run by as few as two people, which is at least a fivefold reduction in labor usage versus traditional fish farming methods.
increased control over water quality parameters – having control over water temperature allows a RAS producer to grow species which could not normally be raised in a given geographic area. This can provide a key market advantage. The traditional fish farmer essentially has no control over water temperature and must grow a species suited to the local environment or be a seasonal grower. Other important water quality parameters such as dissolved oxygen can theoretically be maintained at optimum levels in RAS, which leads to higher growth rates.
lower risk of negative impact from adverse weather conditions – the risk of crop loss from a natural disaster can be eliminated in a properly sited and constructed RAS. The traditional fish farmer is more vulnerable to natural disasters and the effects of natural weather patterns. By rearing fish indoors, the RAS producer is not limited by inclimate weather. An early season cold spell can spell disaster for a traditional fish farmer who has waited a week too long to harvest.
lower risk of creating adverse environmental impacts – if left untreated, the discharge plume from traditional fish and shrimp farms is essentially a source of pollution of local water bodies. Recirculating systems treat and reuse the water and there is zero discharge to the local environment. RAS should be selected when an environmentally friendly solution to the growing demand for seafood is required.
increased biosecurity – a properly designed and managed RAS has complete control over biosecurity concerns, whereas a traditional open system is open to attack.
About 99.75 percent of the water in each unit is continuously cleaned and returned to the fish tanks. Manure filtered from the water during the recycling process is used as fertilizer on nearby farm fields. The nutrient-rich water can also be used to feed vegetables and herbs in large-scale aquaponics systems, which in turn filter the water for reuse.
One of RAS’s biggest benefits is its small “water footprint,” which opens the door to commercial fish production in areas with limited water resources. (The technology is proven for both fresh- and saltwater species).
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