FISH FARMING TECHNOLOGY
RAS Recirculation aquaculture systems
R
ecirculation aquaculture systems (RAS) represent an exciting, eco-friendly and unique way to farm fish. Instead of the traditional method of growing fish outdoors in open ponds, net cages or tanks, this system rears fish at high densities within an operator controllable rearing environment inside a building. Recirculation systems filter and clean the water for recycling back through the fish culture tanks. New water is added to the tanks only to make up for water loss due to splash out and evaporation and to replace that used to flush out fish waste materials to storage tanks. RAS designs have several system process technologies, all of which work together to ensure minimum loss of water, heat and of course fish stocks, while constantly cleaning and re-using the fish tank water. RAS farms can be designed to various levels of complexity, however all true water recirculation designs involve the use of equipment to remove and safely store wastes, clean and re-use water and maintain rearing conditions at or near optimum for the species of fish being raised. In contrast, many older style tank farming systems that have been used to grow fish are termed “open” or “flow through” systems
because the water makes only one pass through the tank and then is discarded. Although several improvements can be added to older designs to reduce water usage and create hybrid types of operations, these are not exactly true water recirculation farms.
Benefits of Water Recirculation Aquaculture System Designs
Fish farmed in any facility must be supplied with the correct conditions necessary to remain healthy and grow with a good Feed Conversion Rate (FCR). Fish need a continuous supply of clean water at an appropriate temperature and dissolved oxygen content that is optimum for good growth. Water recirculation farms accomplish these tasks. A mechanical and biological system is necessary to purify the water and remove or detoxify harmful waste products and uneaten feed. The fish must be fed a nutritionally complete feed on a daily basis to encourage fast growth and high survival. With that in mind, it is now possible to review the potential benefits of using a Recirculation Aquaculture Systems.
Low water Requirements
Because RAS farms recycle most of their water, they consume considerably less than other types of culture systems and are especially well suited to areas with limited water supplies. The required quantity of water needed to successfully grow fish varies with the species of fish selected, size of the farm system, and the unit process equipment design. As a general rule, a minimum water volume of one-five gallons is needed for every pound of fish reared and minimum new water flows of five-20 gallons per minute are needed to grow 50,000 - 75,000 pounds of fish per year. A properly designed and operated recirculation system requires a minimum daily input of water, just enough to clean the waste from the filters and to replace water lost to evaporation. This allows construction of fish farms in areas where ground water is limited and even opens the possibility of an operation being located in an urban area to use de-chlorinated municipal water. Such a production facility could be located close to the market. By comparison, a recirculation system which produces the same number of pounds of fish as 1000 acres of ponds (about 4.8 million pounds of fish) would require only about 4000 gallons of fresh water each day or 1.5 million gallons per year. However, to fill 1000 acres of ponds averaging just 5 feet in depth, once only, would require 1.6 billion gallons of water. That is just to fill the ponds and does not include any water added to maintain levels or freshen the ponds after the initial filling.
Less Land Requirements
Since fish in a recirculation system are reared in tanks, with oxygen being supplied and their metabolic wastes removed by constant recirculation of water, fish can be safely stocked and grown in high densities. Currently the goal, which designers are striving to attain, is 1 pound of fish per gallon of water. However, many people consider 0.5 to 0.75 pounds of fish per gallon of tank water as being acceptable. In pond aquaculture, the common maximum density is about 0.003 pounds of fish per gallon of water. Therefore, a recirculation system can be located in areas where large amounts of level land (to build ponds) are not available. The low land requirement also permits the facility to be located in areas where the soil cannot hold water or, again, in urban areas, or housed in vacant farm or warehouse buildings converted to fish farming.
Control of Water Temperature
The low water requirement of recirculation systems opens up
002 | INTERNATIONAL AQUAFEED | Fish farming Technology