Introduction
Aquaculture has been defined in many ways. It has been called as the rearing of aquatic organisms under controlled or semi controlled condition – thus it is underwater agriculture. The other definition of aquaculture is the art of cultivating the natural product of water, the raising or fattening of fish in enclosed ponds. Another one is simply the large-scale husbandry or rearing of aquatic organisms for commercial purposes. Aquaculture can be a potential means of reducing over need to import fishery products, it can mean an increased number of jobs, enhanced sport and commercial fishing and a reliable source of protein for the future.
Fish is a rich source of animal protein and its culture is an efficient protein food production system from aquatic environment. The main role of fish culture is its contribution in improving the nutritional standards of the people. Fish culture also helps in utilising water and land resources. It provides inducement to establish other subsidiary industries in the country.
The basic principle of composite fish culture system is the stocking of various fast-growing, compatible species of fish with complementary feeding habits to utilize efficiently the natural food present at different ecological niches in the pond for maximising fish production. Composite fish culture technology in brief involves, the eradication of aquatic weeds and predatory fishes, liming: application of fertilizers on the basis of pond soil and water quality, stocking with 100 mm size fingerlings of Indian major carps-catla, rohu, mrigal, exotic carps, silver carp, grass carp and common carp in judicious combination and density; regular supplementary feeding and harvesting of fish at a suitable time. Composite fish culture system is conducted by adopting three types of combinations viz., culture of Indian major caps alone, culture of exotic carps alone, and culture of Indian and exotic carps together. Fish production ranging between 3,000 to 6,000 Kg. per hectare per year is obtained normally through composite fish culture system. Development of intensive pond management measures have led to increase the fish yield further. Integated fish and animal husbandry systems evolved recently are the fish-cum-duck culture, fish-cum-poultry culture, fish-cum-pig culture, utilization of cattle farm yard wastes and recycling of biogas plant slurry for fish production. Advantages of the combined culture systems, number of birds/animals, quantity of manure required and fish production potentiality of the recycling systems are described. Fish culture in paddy fields is an important integrated fish cum agriculture system. Essential requirements of paddy fields to conduct fish culture, characteristic features suitable for culture in rice fields, constraints to culture fish in paddy fields due to recent agrarian practices, and improved fish-paddy farming methodologies are discussed. Freshwater prawn culture is a recent practice. Giant freshwater prawn Macrobrachium rosenbergii and Indian riverine prawn M. malcolmsonii are the two most favoured species for farming purposes in India. Breeding, hatchery management, seed productio, culture systems and production potentialities of the freshwater prawns are presented. Commercially important air-breathing fishes of India are the murrels, climbing perch, singhi and magur. Techniques of their seed production and culture systems are described.
1.2 Fresh Water Culture Systems
Cultivable organisms are cultured in different types of culture systems. Many culture systems are based on traditional ideas that have been used for years, but some encompass new and some times radical concepts that make them unique. There are three major culture systems – open, semi-closed and closed culture systems. Each has its special characteristics, advantages and disadvantages. The choice of system is largely dependent on the function of the organisms to be grown and the resources and ideas of the farmer.
1.2.1 Open culture systems
Open systems are the oldest and its farming is the use of the environment as the fish farm. Natural resources can be used as culture systems and organisms to be cultured are stocked in the water body. Capital expenses are low for the open culture systems. There is less management than in the other systems. The conditions are more natural and uncrowded in the culture environment, less time is required in monitoring the condition of the culture organisms in open systems. The disadvantages like predation and poaching are common. The growth rate and the uniformity of the product are variable compared to other systems. Cages, long lines, floats, rafts, trays and clam beds are examples of open system techniques.
1.2.1.1 Cage culture:
It is the culture of fish or other organisms in a river, lake or bays by holding them in cages. Cages are built of metal rods, bamboo mesh or PVC pipes and covered by mosquito cloth or nylon net.
Cage culture, in recent years, has been considered as a highly specialized and sophisticated modern aquaculture technique, receiving attention for intensive exploitation of water bodies, especially larger in nature, all over the world. In India, cage culture was attempted for the first time in case of air breathing fishes tikeH.fossilis and A. testudineus in swamps.
1.2.1.2. Pen culture:
Pens are the specially designed nylon or bamboo made enclosures constructed in a water body into which fish are released for culture. Such type of culture is referred to as pen culture.
1.2.1.3. Raft culture:
Rafts are generally made of bamboo poles or metal rods with buoys at the top for floating in the water. These are used in the culture of oysters, mussels and seaweeds in open seas.
1.2.1.4. Rack culture:
Racks are constructed in brackishwater areas and inshore areas for rearing oysters, mussels, seaweeds, etc.
1.2.2 Semi-Closed Culture Systems
In semi-closed culture systems, water is taken from natural sources or ground water and is directed into specially designed ponds and race ways. These systems offer an advantage over open systems in that they allow greater control over the growing conditions. A greater production per unit area is possible in addition to crop being more uniform. Water can be filtered to remove predators, diseases can be observed and treated more easily in semi-closed systems. The main disadvantages are more expensive and require more complex management. Ex:- ponds and raceways.
1.2.2.1. Pond Culture:
The majority of aquaculture throughout the world is conducted in ponds. Earthen ponds or reinforced concrete ponds are used for culturing the fish, shrimp, prawn, etc. in both freshwater and brackishwaters.
1.2.2.2. Raceway culture:
A series of earthen or cement tanks are constructed along the course of a river or stream and are used for fish culture. Raceway is a culture chamber that is generally long and narrow. Water enters at one end and leaves through the other end in most cases.
1.2.3. Closed Culture System
In closed culture systems, no water is exchanged and the water is subjected to extensive treatment. Extremely high densities of organisms may be raised under these conditions. Farmer has complete control over growing conditions in closed systems. The temperature is regulated, parasites or predators are not found and harvesting is simple. Food and drugs can be added efficiently into the system to grow quickly and uniformly. Fish or prawn culture in water recirculation systems is good example for closed systems.
1.2.3.1 Water recirculation systems:
Here the water is conserved throughout most or all of the growing season by circulating in the culture tanks after purifying it through biological filters. Closed recircu-lating water systems are being used primarily for experimental work and for the rearing of larval organisms in commercial or research facilities. Closed systems are generally comprised of four components; the culture chambers, a primary settling chamber, a biological filter (biofilter) and a final clarifier or secondary settling chamber for purification of water for reuse.
Source: Aquaculture