Recirculation in fish

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Seasonal planktonic diversity of Goverdhan sager lake

CREDIT SEMINAR ONApplication of Recirculatory System in Fish Culture



IntroductionRecirculation aquaculture systems (RAS) flows from a fish tank through a treatment process and is then returned to the tank, hence the term recirculating aquaculture systems.RAS technology has steadily developed over the past 30 years and is widely used for bloodstock management, in hatcheries and increasingly for salmon production. Recirculating systems filter and clean the water for recycling back through fish culture tanks.A filtering (biofilter) system is necessary to purify the water and remove or detoxify harmful waste products and uneaten feed. Recirculation systems also occupy very little area and require little water consumption compared to other forms of aquaculture.


Fig-1 recirculating aquaculture system

Biological filtrationFish Mechanical filtrationWater

Fig. Schematic of a recirculating aquaculture system consisting of shrimp culture system and water treatment system.


Recirculation systems are becoming increasingly popular as they provide a predictable and constant environment for growing fish. Species of fish that can be cultured within recirculation systems include barramundi, Murray cod, Silver perch, snapper and eels plus a number of other species.Recirculation systems occupy a very small area and allow the grower to stock fish at high densities and produce high yields per unit area.

Necessity of recirculationThe Recirculatory system is necessary to reduce the risk of disease/ parasite infections considerably.Oxygen can be replenished through aeration and most of the carbon di-oxide is dissipated, the removal of metabolic products especially ammonia involve more complex system.This system of farming highly improves survival and growth performance of fish due to high degree of control over the water quality.This system eliminates water quality problems. Recirculation of waste-loaded pond water reduces potential pollutants which assures the availability of quality water for fish farming where the source of fresh water is limited.

Application of recirculationThe use of RAS technology is already increasing in the Scottish salmon industry and further investment in this area will almost certainly be essential for the successful future of the industry. There is a long-term threat to the industry from RAS technology being adopted closer to major markets, but this should be seen as an incentive to continue to innovate for cost competitiveness using the natural resources available in Scotland.The first European RAS farmed salmon to be delivered to market had a 20-30% higher production cost compared to the most efficient cage farm in Norway. The USA, which relies almost entirely on imports to meet its demand for salmon, also has one of the largest markets for premium seafood products.

China and SE Asia also represent important emergent markets. Recent European salmon and sea bass RAS start-ups are already targeting these markets and this is central to their business plans.

Types of Recirculatory system1. Zero-water exchange system Zero-water discharge , sludge removal and fish culture. Each system includes the three numbers of one hectare fish grow-out pond, one hectare water treatment pond for culture of fish and bivalves. Cement cistern for sedimentation of phytoplankton, a sludge-setting pond, sludge-drying bed is removed from grow-out ponds through the setting pond. In this system, fish is stocked in each pond at the rate varying between 8,000-10,000 /ha.

In-pond treatment system

In this system, remove the excess algae and suspended solids for mussels.Four aerators are placed in such a way that the pond water is circulated and at the same time waste materials are concentrated at the centre of the culture pond.

Pond-in-pond Recirculatory system

This system consists of two ponds , the pond is utilized for intensive culture and the second one is for extensive farming. The intensive pond is somewhat deeper than the extensive one and is provided with paddle wheel aerators. Different species of fish such as carps, fresh water prawn and bivalves are stocked in extensive pond at lower densities.

Drainage canal system In this system, fish farm is circumscribed by a number of canals. These canals and farm ponds are filled up with water and the water allowed to stand for three months. Water of the drainage canal is then circulated to the pond via the supply canal. The canals are provided with a number of aerators for aeration of the water. Canals are also used as sedimentation beds. Fish production for this system about 8 tones/ha /carp.

Earthen pond systemThis is the most appropriate type of west-water treatment for developing countries. The system consist of a series of ponds in which bacterial and algal growth can occur in a symbiotic manner.Bacteria utilize the organic matter from which they produce inorganic nutrients which are used by algae. Algae, in turn, produce oxygen through photosynthesis. This oxygen is consumed by bacteria. This co-existence of bacteria and algae for the benefit of each other is termed as symbiosis.

Recirculation ComponentsA recirculation fish farming system comprises of a number of major components that are necessary for the management of the system. This includes both site and system components. Site ComponentsBuildingPump HouseThree Phase ElectricityEmergency GeneratorBulk Feed StoragePurging and Packaging Facilities

Recirculatory design

Growing tank

Fish tanks typically are rectangular, circular, or oval in shape.Rearing tanks range in size from 500 to 500,000 gallons capacity.Tanks can be constructed of plastic, concrete, metal, wood, glass, rubber and plastic sheeting, or any other materials that will hold water, not corrode, and are not toxic to fish.

Rectangular tank

Round tank

Sump of particulate removal device

A sump (clarifier tank) is used to prevent the excessive accumulation of fish excretory products and waste feed. Waste products increase the biological oxygen demand (BOD), decrease the dissolved oxygen content, lower the carrying capacity (density of fish) that can be reared, and may result in off-flavor in fish products.


The bacteria provide the waste treatment by removing pollutants. The two primary water pollutants that need to be removed are:- (a) fish waste (toxic ammonia compounds) excreted into the water (b) uneaten fish feed particles.The biofilter is the site where beneficial bacteria remove (detoxify) fish excretory products, primarily ammonia.

Types of biofilter

Other filters

Oxygen injection with U-tube aeration

Effective diffusion of pure oxygen gas into a liquid (water) can best be accomplished using a U-tube oxygenation, counter-current flow injectors, or micro-bubble devices (tubes or fine wet stones). The purpose is to dissolve much of the oxygen injected so that it is available to the fish, rather than wasted by bubbling out of solution to the atmosphere.

Water circulation pump.

The rotating biological contractor (RBC) has a water-wheel configuration consisting of plastic media attached to a central axle which spins slowly, moving the media through the water in the RBC containment vessel. Advantages of the RBC are that it is self-aerating and self-cleaning. Once established, it tends to be very stable and can operate for years without failure.

Advantages of Recirculation SystemsMinimum demand on limited water resources. The limited quantity of high quality water in aquifers and on the surface is an indication that water recirculation systems will become increasingly important as a means of meeting the demand for fish.Minimum environmental impact. Recirculation systems permit the concentration and removal of fish wastes so that water pollution can be controlled and minimized.Few government permits are required. Because of limited impact on environment, and limited withdrawal of waste water, few permits are required for aquaculture systems based on water recirculation.

Closed circulation systems can be located near markets. Suitable sites for other systems are dependent on location of suitable water or land resources. Recirculation systems can be located so that transportation costs and time between harvest can be minimized.Water quality and temperature can be maximized. Water temperature can be maintained at the optimum level for fast growth and optimum feed conversion.Minimize losses from environmental hazards such as predators, pollutants, and disease. Fish produced in closed recirculation systems are safe from environmental pollutants and many pathogens.Minimum space requirements for level of production. In comparison to other types of systems, protein production in closed systems require very little space.

Disadvantages of Recirculation SystemsHigh Capital Costs. Capital costs of buildings, pumps, tanks, heaters, etc. are higher than other systems of aquaculture.High Operating Costs. Closed systems require pumping water though tanks and filters. The operating costs of pumps are significant, and these costs may be the difference between profitable and non profitable fish farms.Vulnerability to Mechanical Failure. Pumps make fish farm vulnerable to breakdowns and blackouts which can result in catastrophic losses of fish.Difficulties with Fish Health Management. Disease outbreaks, once they occur, are difficult to manage. Pathogens in the system find refuge in the biological filter and are difficult to remove. Often the only option is to treat the fish in the tank, thus killing the bacteria in the biofilter.

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