Mariculture – the farming of marine organisms for food - is the fastest growing sub-sector of aquaculture. Enhancing fish production from inland sector has limited scope and the major portion of the additional demand has to come from mariculture.
Sea cage farming is viewed as a major option for increasing the seafood production and has been expanding rapidly in recent years at global level. he ICAR-Central Marine Fisheries Research Institute (CMFRI) has projected that even if 1% of the inshore waters is used for cage farming, we can deploy 8,20,000 cages with a production potential of 3.2 million tonnes.
The National Fisheries Development Board (NFDB) has released the Guidelines for Sea Cage Farming in India with the objective to augment fish production in a sustainable manner through sea cage farming to meet the increasing demand for seafood.
Sea cage culture involves growing fishes in the sea while being enclosed in a net cage which allows free flow of water. It is a production system comprising of a floating frame of varying dimensions and shape, net materials and mooring system, to hold and culture a large number of fishes.
Advantages of Sea Cage Farming
Choosing a site in cage farming is crucial because it impacts the economic viability of farming. Site selected should provide optimum water quality to avoid stress conditions and achieve maximum growth of farmed fish, and ensure proper and secure installation of cages, logistic and other support for supply of inputs, harvesting and marketing of fish. Sea cage site has a direct bearing on operational cost, fish mortality, production and overall profitability of farming operation.
Exclusion of areas for cage farming : Sites which are active fishing zones and close to harbours/ fish landing centres and navigation channels, defense areas, marine protected areas, coral reefs, mangroves, areas under coastal management plan, points of industrial effluent discharge, sewage pollution, heavy freshwater discharge by rivers, presence of underwater pipelines, telecom cables, explosives dumping and areas of historic ship wreck are to be avoided during site selection.
A sea cage essentially comprises of cage frame and net cages. For sea cage fabrication the materials used should be sturdy enough to withstand rough sea conditions and weather. Cage frames are fabricated either with High Density Poly Ethylene (HDPE) or Galvanized Iron (GI) material to withstand rough conditions in the Indian seas. HDPE pipe PE100 or GI pipe (1.5”) B/C Class are preferred for cage frames. When GI pipe is used, all joints are double welded and the GI cage frame is epoxy coated and floated on air-filled HDPE barrels (200 litre, 8-10 nos.). Sea cages are preferably circular-shaped as they can withstand sea conditions better than rectangular or square shapes.
Specifications for 6 m Diameter HDPE Cage
The sea cage frame has two collar rings designed with flotation properties and a middle ring as catwalk in between them (Fig. 1). In case of HDPE the pipe-ends are joined by using a fusion welding process for plastics. The two collar rings for flotation are filled with polyurethane foam (PUF) or thermocol. The two collar rings, the middle catwalk ring and the handrail ring are all held in place by means of various support pipes, brackets and T-joints. The pipe used for handrail is without PUF inside and the brackets made of galvanized steel are corrosion-free and are fitted to the diameter of the pipes. The maximum height of handrail should be approximately 100 cm (shorter than the shortest person). The service systems (catwalk, handrail. etc.) are required for operation and maintenance activities like feeding, cleaning, monitoring, grading, etc. The minimum width for catwalk is approximately 60 cm. The brackets/ base supports, vertical and diagonal supports hold together the collar rings, catwalk ring and the handrail. They also give additional stability and sturdiness to the frame structure.
The ballast pipe is another support system required to maintain the shape and structure of the net bags (Fig. 2). Normally 1.5 inch (38 mm) diameter HDPE ballast pipe with holes at regular intervals, for the free flow of water, is used. Metal lines are inserted inside the pipe for increasing weight so that the ballast remains submerged in water.
Mesh size for the net cages must be selected according to the species of fish farmed and also to ensure good water exchange. Proper water flow enhances water quality, reduces stress, improves feed conversion and allows holding more fishes. Net cages should be as per the dimensions of the cage frame and depth of water at the site. They must be securely fastened to the cage frame. For sea cage farming, 3 types of nets are essential:
High Density Polyethylene knotted netting is preferred for net cages. The mesh size of the net cages depends on the size of individual farmed fish. Three sets of net cages of three different mesh sizes are required during the farming operation (18 mm, 25 mm and 40 mm). To maintain the cylindrical shape of the net cages, ballasts of appropriate weight should be used. Concrete blocks tied at suitable intervals can be used. Alternately, HDPE pipe of 1.5 inch (38 mm) diameter inserted with MS chain or wire rope of 10 mm thickness can be employed.
Cage culture in the sea requires a fish variety that meets criteria such as suitability for marketing, commercial importance, consumer acceptance, easy to culture, adaptability to the cage environment, acceptance of formulated feeds, faster growth rate and resistant to common diseases.
As per the above criteria, Cobia (Rachycentron canadum), Silver Pompano (Trachinotus blochii), Seabass (Lates calcarifer), Snappers (Lutjanus sp.), Groupers (Epinephelus sp.) and Spiny Lobster (Panulirus sp.) are highly suitable for sea cage farming.
Quality of fish seed is of vital importance for the success of grow-out culture in cages. Uniform size seeds appropriate for the mesh size of the fish net cage should be stocked to prevent their escape. This will also help in selecting the correct sized feed for fishes, avoid wastage of feed and reduce cannibalism. Seeds should be healthy, free from diseases and deformities.
The most vital issue for the expansion of the sea cage farming in India is the shortage of fish seeds. Presently seed of Cobia, Pompano, Seabass and Groupers are being produced in a few hatcheries in the country. Apart from these species, seeds of fishes like Mullets, Snappers, Milkfish, etc. collected from the wild can also be used for cage farming. To meet the growing demand from farmers, there is an urgent need to produce sufficient quantity of seed either through commercial hatchery production or by importing till we achieve self-sufficiency in seed production.
List of Marine Fish Hatcheries
Stocking appropriate size and number of fish seed in cages is very crucial for the success of cage farming. After allowing the hatchery produced spawn to grow for a period ranging from 30 to 60 days, fish seed can be stocked in cages. Nursery rearing of seed is essential for all species and it can be done as a separate activity, in land based nursery ponds or hapas held in ponds or in floating nursery cages, by individuals or groups at different localities to support sea cage farming with ready to stock fingerlings. Healthy, uniform-sized fingerlings should be procured for stocking in cages. The fingerling stocking details are given below:
|Species||Stocking Size (Length/ Weight)||Stocking Density (Nos./ m 3 )|
|Cobia||15 cm/ 35 g||8-10|
|Pompano||10 cm/ 35 g||30-40|
|Seabass||10 cm/ 30 g||30-40|
|Grouper||15 cm/ 40 g||15-20|
Any material used for feeding contains the following five principle constituents: (i) Protein, (ii) Carbohydrate, (iii) Fat, (iv) Minerals and (v) Vitamins. Proteins are essential for growth of the animal and a deficiency can lead to growth retardation. The nutrient requirements of marine carnivorous fishes (as %) are given below:
|Size of Fish||Moisture||Crude Protein||Crude Fat||Crude Fibre|
|Fry-Fingerling (1 - 20 g)||<12||>42||>5||<4|
|Juvenile (20 - 50 g)||<12||>40||>5||<4|
|Grower (50 - 300 g)||<12||>38||>5||<4|
|Marketable Size (>300 g)||<12||>35||>5||<4|
Marine fishes require higher protein (35-40%) feed for their optimal growth. Based on growth of the fish, size of the feed pellet should be adjusted. Normal feeding rate is 10% of the body weight for juvenile fishes which can be reduced to 3% body weight as farming progresses. A feed with an FCR of 1:2 is advisable. Only recommended ration should be given to fishes since overfeeding leads to wastage and environment pollution.
Ingredients used for making feeds can be classified as protein rich ingredients which are mainly fish and meat products of animal origin and oilcakes of plant origin. Energy rich ingredients are mainly cereals and cereal byproducts. Other than these there are non-conventional feed resources (NCFR) which are used in feed manufacture. Apart from these ingredients, mineral mixtures, vitamin mixtures and other additives such as oil, phospholipids, carotenoids are also added according to the needs. Non-nutrient additives such as synthetic binders, anti-oxidants and anti- fungals are also added.
With a fair knowledge of nutrients and the feed ingredients, the next aspect to be understood is the need for blending of the ingredients to have a nutritionally complete and balanced feed mix. A nutritionally complete feed can be made by mixing ingredients on a scientific basis which will be effective in producing the desired results in terms of fish production. The feeds should have the physical properties suitable for the fish to consume it with minimum loss of nutrients in water. Feed technology evolved from a dry mash to a wet ball and to a pellet. Now, pellets are produced such that they sink, slow-sink or float depending upon the feeding habit of fish. For cage farming floating or slow sinking pellets are found to be appropriate. Production of floating and slow-sinking pellets is done using extruders, which are available in India or can be imported.
During storage, feeds undergo deteriorative changes which not only lower their nutritive value but also affect their palatability and appearance. Feeds should be stored in dry ventilated warehouses away from direct sunlight at more or less constant temperature. All feeds should be used within the prescribed time (preferably within two months of manufacture) and inspected regularly. During long storage there may be changes such as fungal growth, degradation of vitamin potency and fat rancidity. Unnecessary handling may damage feed bags and reduce pellets to powder that is usually not consumed by fish and wasted. Pests like rats, cockroaches, etc., must be strictly controlled in the storage, to avoid contamination. Proper storage of feed is simple, but it is important to keep its quality high.
Feeding rates, frequency of feeding and time of feeding are important factors to be considered in cage farming. Feeding rates and frequencies are related to age and size of the fish. Fish larvae and fry need to be fed on a high protein diet more frequently. When fishes grow bigger, feeding rates and frequencies can be reduced. Feeding fish is a labour-intensive activity and the frequency has to be adjusted in such a way that it is economically viable. Generally, growth and feed conversion increase with increase in feeding frequency. Feed consumption is also influenced by time of day, season, water temperature, dissolved oxygen levels and other water quality parameters. Even though several feeding charts are available, it is better to construct one of your own with information on: Days of Culture (DOC), Fish Weight, Protein in Feed, Meal/Day, Feed Consumed as % of body weight, Average Daily Gain (ADG) and Feed Conversion Ratio (FCR). Indicative feeding charts for Cobia, Silver Pompano and Seabass are as follows:
Salinity above 25 ppt is essential for optimal growth of Cobia; whereas Silver Pompano tolerates even salinity of 10 ppt. Farming duration usually ranges from 7 to 8 months for Cobia and Silver Pompano while it is 6-8 months for Seabass, to attain optimum marketable size.
Cage Farm management involves optimizing production at minimum cost. Efficient management largely depends on the competence and efficiency of farm operator with regards to feeding rate, stocking density, minimizing loss due to diseases and predators, monitoring environmental parameters and maintaining efficiency in all other technical aspects. The entire structure including cage frame and mooring must be routinely inspected and necessary maintenance and repairs should be carried out. Bio-fouling clogs the mesh of net cages and thereby reduces rate of water exchange causing stress due to low oxygen and accumulated wastes leading to mortality of fish. The rate of fouling depends not only on the mesh size of the net cage, but also on temperature and productivity of the water at the cage site. Therefore, timely exchange of fish rearing net cage is to be done, by replacing with a fresh net cage, to ensure good water quality and facilitate faster growth. Also, as fish grow bigger, net cages of appropriate mesh size are to be used.
Disease outbreaks can occur as a result of intensive farming in cages. Infectious diseases are mainly due to waste accumulation, crowding, handling, variations in water quality parameters and bio-fouling. The most common disease that occurs in cages is Vibriosis caused by Vibrio a bacterial species. Cage abrasion can cause fin and skin damage to farmed fish. Occurrence of infections/ diseases can be minimized by selecting good site, optimal stocking density and careful handling of fish stock. Fish farmers should maintain a record of weather, water quality parameters, feeding rate, length and weight of fish sampled, fish behavioral changes, net cage exchange details, etc. These records provide useful information for analysis of health status.
Good management practices (GMPs) need to be adopted to satisfy consumer demand and expand the market by offering quality farmed products that meet food safety standards. Adoption of GMPs also helps fish farmers to achieve higher economic returns. Some of the key factors in GMP include:
Cage culture system releases waste to the aquatic environment in the form of uneaten feed, feces and other debris. These may accumulate beneath the cages leading to reduction in dissolved oxygen. The tendency of large quantity of waste to settle on the sea bed is greater at sheltered inshore sites. Continuous farming for several years at the same site in sheltered areas is to be avoided. Change of site after a number of crops should be done. Otherwise, semi-exposed/ exposed sites having good tidal flushing are selected to prevent accumulation of wastes at the bottom of the cage. Adequate spacing between cages and farms is also an essential management practice to reduce the spread of diseases. Indiscriminate use of antibiotics and their release into aquatic environment may result in development of antibiotic resistant bacteria. Cage culture can also introduce diseases, transmit parasites and cause changes in aquatic flora and fauna. Hence, carrying capacity of the environment, as per the nature of the site, has to be given prime importance while undertaking sea cage farming.
Harvesting can be done as per the market demand to ensure maximum returns. Partial harvesting of stock may be practiced, by harvesting the larger fish first, to avoid glut in the market and consequent fall in sale price. Records of harvest should be maintained at the site. It is necessary to have a post-harvest and marketing strategy while undertaking large scale sea cage farming activity.
The production centres should have facilities such as proper craft and gear to harvest fish, facilities for icing, holding and storage of fish, live-fish transport, linkages to post-harvest processing centres and market chains.
Cage culture in the sea involves working in a risky environment. Hence adequate safety measures should be taken to prevent accidents including loss of life. Life-saving equipment like life-buoys/ jackets, communication devices, and first aid boxes at the cage site should be made mandatory. All persons while working at the sea cage should wear life-jackets.
To avoid poaching or vandalism effective watch and ward arrangement should be made at the sea cage site. Also, to alert fishing craft and seafarers and to indicate the location of sea cages and their moorings, Maritime Signal Flags and Solar Beacon Lights may be positioned on the cages.
To access the complete guidelines including Economics of Sea Cage Farming, Financial Assistance, List of Cage Manufacturers, List of Feed Manufacturers, etc. click here.
Source : National Fisheries Development Board