Recirculatory Aquaculture System
Recirculatory aquaculture systems (RAS) are tank-based systems in which fish can be grown at high
density under controlled environmental conditions. These are the facilities that retain and
treat the water within the system. Water flows from a fish tank through a treatment process and
is then returned to the same tank, hence the term is used as recirculating aquaculture systems.
The system can be designed to be very environmentally sustainable, using 90-99 percent less
water than other aquaculture systems which can reduce the discharge of waste, and can combat
disease. RAS systems are also known as the zero water exchange system, as no water is discharged
from the system and no additional water is added after the system is initially filled.
Advantages:
RAS offer fish producers a variety of important advantages over open pond culture. These include:
- Maximize production on a limited supply of water and land. They maximize production in a
relatively small area of land and use a relatively small volume of water.
- Complete environmental control to maximize fish growth year-round.
- Complete and convenient for harvesting, quick and effective disease control measures.
- Since water is reused, the water volume requirements in RAS are only about 20% of what open
pond culture demands.
- RAS provides year-round production of fish for growers because of a relatively short growing
season (less than 200 days) or extremely dry conditions.
- They can be located close to large markets (urban areas) and thereby reduce transportation
costs.
- They offer a holistic solution to water use, water quality, and waste disposal.
- RAS are particularly useful in areas where land and water are expensive and not readily
available.
Biofiltration in RAS:
The biological filter (biofilter) is the HEART of the RAS. Biofilter composed of a media upon
which a film of bacteria grows. The bacteria perform the act of waste treatment by removing
pollutants.
The two primary water pollutants that need to be removed in RAS are:
(1) Fish waste (toxic ammonia compounds) excreted into the water
Uneaten fish feed particles.
The biofilter is the site where beneficial bacteria transform (detoxify) fish
excretory products, primarily ammonia into NO2 and NO3.
Pangasius Farming in RAS
Pangasius and Tilapia both are ideally suited for culture in RAS. Presently more emphasis is paid
to the Pangasius for different reasons. It exhibits fast growth when cultured in a good
environment. It is cultured due to its strong market demand, fast growth; few countries dominate
the culture production through the cultivation of Pangasius. Pangasius is now cultured in
several countries in the world like Thailand, Nepal, Pakistan, India, Bangladesh, Vietnam, Laos,
Myanmar, Indonesia, and Cambodia. Pangasius hypophthalmus contributes more than 90% in global
culture output. Pangasius is an air-breathing fish that can tolerate low Dissolve Oxygen (DO) of
the water. It can be cultured in fish ponds, concrete tanks, fish cages or fish pens.
Culture Protocol of Pangasius in RAS
- Fill the pond with water to about 1.5m to 2m deep, to provide a wide environment for the
stocks.
- Pangasius spp. can be stocked at a rate of 3-15 pcs/m2 depending on the culture environment.
Stocking should be done early morning or late afternoon. Survival rate of Pangasius spp. is
estimated to be 80-90%.
- Pangasius is omnivorous (eat both plants and animals) during their first year and become
herbivorous for the following years. Pangasius can be fed with pelleted feeds (recommended
for faster growth and better fish quality) at a rate of 2.5% of their average body weight
and will be adjusted bi-weekly. Feed Conversion Ratio (FCR) averages to 1.5:1, which makes
it suitable for culture.
- Water is important for all fish, so optimum conditions of certain parameters of water should
be the primary factor to be considered for fish culture. Water quality parameters like pH
(6.5 - 7.5), Dissolve Oxygen (DO) (0.1 mg/l), Temperature (25 - 30° C), Salinity (< 2 ppt)
and water depth (1.5 - 2m). These should be maintained to get a better yield for the
culture of Pangasius spp.
- Sampling is done to monitor the growth of stocks and to compute feeds to be given to the
stocks for the following days. This is also done to see if the stock reaches its marketable/
harvestable size. Since Pangasius spp. is faster grower fish, sampling is mandatory.
- Harvesting can be done in partial (selective harvesting) or total harvesting. Pangasius spp.
can be harvested after 4-6 months of culture period. It can reach the weight of 1-1.5
kilogram in 6 months.
Design Parameters
- Production capacity of Pangasius in RAS: 42.70 MT
- Capacity of each culture tank: 81.93cum
- Total number of Tanks: 8
- Total capacity of all tanks: 655.44 cum
