Jyotish Barman
Scientist (Fisheries), ICAR Research Complex for NEH Region, Nagaland Centre, Medziphema
Availability of quality fish seed in required quantity and in appropriate time is one of the major factors that determine the success of fish farming. In general fish farming begins with the onset of monsoon, and hence a fish grower has to make its pond ready beforehand. The period from April to October is the best period for fish culture as the required environmental condition is congenial during this period for better and faster growth of fish.
However, as fish seed is not readily available at the required time, a fish grower has to wait till he gets the seed and in the process gets very less time for culturing fish. Since, seed produced from hatcheries cannot meet the timely requirement, a fish grower can undertake nursery rearing as a means of producing fish fry that can stocked in pond and at the same time earn some income from it. The nursery rearing involves culture of 3-4 days old spawn (5-6 mm in length) brought from a hatchery for a period of 15-20 days, during which they grow to a fry stage (20-25 mm in length) and can be stocked.
Fish seed of Catla, Rohu, Mrigal, Silver carp, Grass carp can be reared in nursery pond and made available. Any pond (earthen or cemented) ranging between 200 – 1000 sq m areas with an average depth of 1.0 m can be easily used for nursery rearing. In case of cemented tank, a soil bed of 15-20 cm is required. Since nursery phase is short, an individual can go for multiple cropping thereby ensuring a profit of 60-70 % per cycle. Further, the practice ensures availability of the desired species in required time and quantity.
Nursery pond management
Appropriate management of nursery pond is the guiding principle for success in carp rearing. Perennial pond are often infested with aquatic weeds and harbour harmful aquatic insects, predatory fishes, weed fishes and predatory animals like snakes, frogs etc., which create problems for survival of the carp seed. It is therefore necessary to follow a series of management measures to ensure better growth and survival of the carp seed.
Clearance of aquatic weeds
The first and foremost measure to be taken is to remove all aquatic plants growing in the pond. These weeds not only serve as shelter for many fish predators but also utilizes the available nutrients which otherwise would have helped in growth of natural food for the carp seed. Further, the weeds consume the available dissolved oxygen as well as restrict the entry of sunlight into the pond. As far as possible, the weeds should be removed manually as it is safer and convenient as the pond size is small
Eradication of unwanted fishes
Old ponds usually harbour miscellaneous fish species. This fish competes for food, space and oxygen with the stocked seed and therefore are considered unwanted in nursery ponds. Some of the predatory fish may also consume the spawn. These unwanted fishes have to be removed from the pond before stocking spawn. To ensure complete removal, dewatering and drying of pond bottom is a better option. However, if complete draining of water is not possible, then either repeated netting or application of suitable piscicides is recommended. The most commonly used piscicides are: i. Mahua oil cake: Recommended dose is 2000-2500 kg/ha (364-455 kg/bigha). Apply 3 weeks prior to stocking spawn. Toxicity last for 20-25 days. ii. Bleaching powder: Bleaching powder with 30% chlorine can be applied @350 kg/ha (64 kg/bigha). Toxicity lasts for 7-10 days.
Liming
Fish grows well in alkaline condition and since majority of the soil and water in northeastern regions are acidic in condition, application of lime is mandatory. Lime helps in correcting pH of the pond water, facilitates rapid decomposition of organic matter and reduces toxic effects of harmful compounds including disinfecting the environment to prevent disease. Lime should be applied after removal of predatory and weed fishes, depending on the water pH. The total lime required per year is given in table. Apply one-third of the total requirement, at least 15 days before fertilization.
Fertilization
Fish seed in their early stages prefers natural food (phytoplankton and zooplankton) and therefore it is necessary to fertilize the pond with organic or inorganic manure to facilitate for optimum production of the natural food organisms. i. Organic:
• Raw cattle dung (RCD) @ 10 tons/ha (1800 kg/bigha) or poultry droppings @3-5 tons/ha (545- 900 kg/bigha) should be applied (2/3rd of the does is applied 15 days prior to stocking and remaining after a week of stocking spawn).
• If mahua oil cake is used, application of RCD should be reduced to 5 tons/ha (900 kg/bigha). ii. In-organic:
• Following organic manuring, Urea@13-18 kg/bigha and single super phosphate (SSP)@18-27 kg/bigha should be applied.
iii. Mixed manuring
• A mixture of groundnut oil cake/Mustard oil cake (136 kg/bigha), RCD (36 kg/bigha) and SSP (172 kg/bigha) is very effective in production of natural food organisms. Half of the mixture should be applied 2-3 days prior to stocking and the remaining in 2-3 split doses after stocking.
Control of aquatic insects
After fertilization, the population of aquatic insect increases which not only compete with carp seed for food, but also causes extensive damage to the hatchlings. In order to control insects, soap-oil-emulsion should be applied 2-3 days prior to stocking spawn. The emulsion is prepared by mixing cheap soap (3 kg/bigha) with vegetable oil (10 kg/bigha). Kerosene oils @ 15-18 litres or diesel @14 litre/bigha can also be used as substitute for vegetable oil.
Stocking
Appearance of dark green colour of the pond water indicates luxuriant growth of natural fish food organism and the pond is ready for stocking. For stocking as far as possible, quality seed should be brought from a reputed or known hatchery.
• Stocking should be carried out in cool hours preferably in the morning or evening hour.
• Carp spawn brought from the hatcheries should be acclimatized to pond condition prior to stocking.
• In nursery, culture of single species is usually recommended.
• Stocking densities are:
• Earthen ponds : 3-5 million spawn/ha or 5 - 9 lakh spawn/bigha
• Cemented tanks: 10-20 million spawn/ha or 1.8 - 3.6 million spawn/bigha Supplementary feeding Since natural food alone cannot fulfill the daily food requirement of the growing seed, it is essential to provide external feed for proper growth. A mixture of groundnut oil cake/mustard oil cake and rice bran at 1:1 ratio mixed with 1% mineral mixture is usually recommended and should be provided two times in a day. Total feed requirement per day is given in the table.
Application of manure
Apply the remaining portion of RCD/Poultry dropping/mixed manure after one week of stocking the spawn
Control of birds
Spawns reared in nurseries are vulnerable to predatory birds and therefore to protect them, the pond should be either covered with suitable mesh size nets (suitable for cemented tanks) or tie strings over the pond surface from one end to the other in zigzag to scare the birds.
Harvesting
• After 15-20 days of rearing, the spawn reaches fry stage (20-25 mm in length) with survivality rate of 40-60%.
• Harvesting should be done in the cool hour preferably in the morning or evening.
• The harvested fry should be kept in hapa under shower condition for 2-3 hours to minimize feacal load before transportation.
Transportation
• For long distance transportation, the fry should be packed in oxygenated polythene bags filled with water 1/3rd by volume and remaining 2/3rd with oxygen.
Important notes:
• It is always advised to grow single species in one pond at a time. In this way the demand for particular species can be fulfilled without any mixing.
• As the rearing period is limited, multiple cropping is possible from the same nursery pond
• Earthen pond : 2-3 crops in one season
• Cemented tanks : 4-5 crops in one season
