1. Fish spawn rearing techniques – nursery pond

2. Three phases of carp culture (three-tier system) -
nursery phase (rearing three-day-old spawn to fry)
rearing phase (rearing fry to fingerling stage) and
grow out phase (rearing fingerlings to adult stage)
Nursery rearing – crucial
According to Alikunhi (1957)-
under Indian conditions a 4 ha farm should be divided into nursery ponds, 0.2 ha; rearing ponds, 0.8 ha and stocking ponds, 3 ha
Considering the enhanced rate of stocking and survival of spawn, fry and fingerlings (Barrackpore, 1973) the ratio of nursery to rearing to stock ponds should be 1:40:1280

3. The occurrence of large scale mortality of stocked spawn – major problem
Average survival of carp fry in nursery ponds does not normally exceed 30% (Basavaraja, 1994), which is attributed to poor nursery practices
The three-day-old spawn normally accept natural or artificial feed and can be transferred to nursery pond where predatory fishes and insects, algal blooms and poor water quality are major causes of mortality

4. Control of predatory and weed fishes
They not only compete with fish for food and space, but also directly prey on them
weed fish - include all species of uneconomic, small-sized fish that naturally occur or are accidentally introduced in ponds along with carp spawn
Weed fishes have high fecundity and mature in summer months and even breed in captivity, in the absence of rains
According to Alikunhi et al. (1952), even the young weed fishes directly feed on carp hatchlings

5. Removing unwanted fishes - repeated drag netting
Bottom dwelling fishes (murrels, catfishes and climbing perch) burrow themselves in mud and are difficult to caught
In the worse situation the nursery should be poisoned
Fish toxicants: (i) plant derivatives, (ii) chlorinated hydrocarbons and (iii) organo-phosphates
Chlorinated hydrocarbons - most toxic to fish; Organophosphates - less toxic to fish

6. Plant derivatives:
Derris powder, with 5% rotenone content, - most commonly used fish poison
At doses 4 to 20 ppm, most of the fishes, tadpoles and bottom dwelling organisms are killed
The toxicity lasts up to 4-12 days
The seed powder of Barringtonia acutangula kills a wide variety of fish at 20 ppm, with toxic effect lasting for 48 hrs
Tea seed cake ( ppm) can be used both as a piscicide and a fertilizer
Mahua oil cake (cake obtained after extracting oil from the seed of the plant, Madhuka latifolia) is widely used to control predatory and weed fishes in Karnataka

7. Mahua oil cake (toxic component : saponin, 4-6%) is used as a fish poison at ppm, killing fish within 8 hours and the toxicity lasts up to 96
Later, it acts as a pond fertilizer
Bleaching powder with 30% chlorine content, when applied at ppm, kills all varieties of fishes, including catfishes, murrels, weed fishes and carps
The cost of treatment at 25-30ppm is economical as it has the advantage of ready availability at low cost

8. (ii) Chlorinated hydrocarbons
(a) Aldrin: Aldrin at 0.2 ppm kills weed fishes and predatory fishes, while 0.3 ppm aldrin spares zooplankton
(b) Dieldrin : Dieldrin at 0.01 ppm effectively kills fishes and 0.5 ppm it kills prawns and insects
(c) Endrin: The most poisonous chemical. Endrin at ppm effectively kills all fishes, prawns and notonectids
(iii) Organophosphates
3 organophosphates, viz., thiometon, DDVP and phosphamidon have been found effective in killing fish
DDVP at 3-30 ppm kills most of the unwanted fishes and is recommended for control of trash fishes in nursery ponds

9. The duckweeds are removed after the blooms have been controlled
b) Fertilization
To stimulate the production of zooplankton, the most preferred natural food of carp spawn and fry
After liming ( kg lime per ha, depending upon soil pH), nursery ponds are treated with either organic manures such as cow dung (10,000 kg/ha) or poultry manure (5,000 kg/ha) and a small quantity of inorganic fertilizers (NPK at kg/ha)
Phytoplankton blooms can be controlled by sprinkling liquid cow dung or by covering the surface of the pond with the duck weed, Lemna
The duckweeds are removed after the blooms have been controlled
Figure: Application of cow dung as a manure in a carp nursery pond

10. c) Aquatic insects and their control
In addition to preying directly on carp spawn and fry, also compete with the latter for food
The members of the Orders Coleoptera, Hemipitera and Odonata are more important
Among members of the Order Coleoptera, those belonging to the families Dytiscidae, Hydrophilidae and Gyrinidae prey upon fish spawn
Dytiscicids, known as predacious diving beetles, are carnivorous both in adult and larval stages
Dytiscidae
Hydrophilidae
Gyrinidae

11. They commonly attack small fish, tadpoles and even dragonfly nymphs
The larvae of Cybister were found to devour 20 advanced fry (24-40mm) within 24 hours and can kill even mm carp fingerlings
Among members of the order Hemiptera, the backswimmers, giant water bugs and water scorpions cause a heavy mortality of carp spawn
The back swimmers are highly dangerous, attacking carp spawn, small fish, tadpoles and even small frogs
water scorpions
backswimmers
giant water bugs

12. Backswimmers - kill even 10-13mm long carp fry, in addition to preying upon plankton, mosquito and chironomid larvae and mayfly nymphs
Water bugs - predaceous and feed on frogs and fishes
Water scorpion and water stick insect attack carp fry and feed on them
Dragonfly (Order: Odonata) nymphs are predatory and feed on carp spawn
They (15 mm long) can swallow 7 carp spawn (6-7 mm) within 3 hours
The short and long bodied drangonfly nymphs consumed 133 and 125 carp spawn, respectively, in 24 hours
Dragonfly

13. Predatory aquatic insects multiply rapidly in a nursery pond
To ensure better survival of carp spawn, it is important to effectively clear the pond of its insect population, prior to stocking
Repeated dragging of pond with a fine-meshed net can remove most of the insects
For effective control of insects, insecticides are recommended
Common insecticides affect adversely the zooplankton and fish spawn
Recommended to use only selective poisons which are capable of killing only the insects, but not the spawn and fish food organisms

14. Spraying diesel or kerosene on the surface of pond is a well-known and a routine practice in malaria control
The CIFRI’s Substation, Orissa - spraying, on a non-windy day, an emulsion of oil and soap in the ratio of 56:18 kg/ha
The recommended dose of teepol B-300 which is readily soluble in water and easy to mix with oil, is 560 ml, emulsified with 56 kg of mustard oil
The Department of Fisheries of Maharashtra Government prescribed an emulsion prepared by mixing light speed diesel (1 l), the emulsifier hyoxide 1011 (0.75 ml) and water (40 ml) for every 200 m2 water surface

15. d) Stocking of nursery pond
Nursery pond is stocked with three day old spawn when adequate plankton appears in it
Nurseries with rich crop of zooplankton preferably rotifers and cladocerans- maximum survival of spawn
A minimum of 1.5 to 2.0 ml zooplankton in 54.5 ml water is essential for satisfactory stocking of the pond

16. In well manured nurseries 1. 0-2
In well manured nurseries million spawn has been stocked with satisfactory results, with a provision of artificial feed, at the pond culture Substation of the CIFRI
Sen (1974) stocked spawn at a high density of 10 million/ha and obtained survival as high as 66.6%
Later, Tripathi et al. (1979) stocked rohu spawn at an average rate of million/ha and reported an average survival rate of 80.73%, leading to the production of 8 million fry in a single crop, during days rearing
The stocking of a nursery pond is best done either in the morning or evening hours

17. e) Supplementary feeding
Stocked spawn feeds on microscopic zooplankton voraciously
A single specimen of live carp spawn ( mm length) consumes 3-34 cladocerons within an hour
The common feed - mixture of rice bran and oil cake of ground nut, coconut, mustard, etc.
Figure: Harvesting of fry from a nursery pond

18. The quantity of artificial feed to be given daily depends upon the weight of the stocked spawn (Alikunhi, 1957; Hora and Pillay, 1962)
Recommended feeding rates - 200, 300 and 400% of the body weight of spawn during the first 5 days, 6th - 10th day and 11th -15th day after stocking
Figure: A haul of carp fry from a nursery pond