Nursing of carp fry
Nursery practices for all species of carp are based on the need to provide post-larvae and fry with the right type of food, environmental conditions and protection for high survival rates and growth. The critical period is when the egg yolk is fully absorbed and the larvae start feeding. Zooplankton of the right size are the most efficient food at that time. By the use of nursery tanks or small ponds, greater control of water quality and predation by insects and their larvae, as well as predatory fishes, etc., can be achieved. One of the essential requirements in nursery farms is the eradication of the many pests and predators that infest fish ponds.
Although several types of nursery structures are used, including troughs and cement cisterns, the most commonly used in carp farms are specially constructed nursery tanks measuring over 100m2, and nursery ponds measuring up to 2000m2. In such structures the post-larvae or fry can be raised to the fingerling stage within a period of about one month.
Before transfer of the post-larvae or early fry for nursing, the tanks or ponds have to be properly prepared. Woynarovich and Horvath (1980) describe the use of soaked and decaying hay as a substrate under a layer of 5cm of water in the tanks to stimulate the growth of rotifers and Paramoecium, which will serve as food for the fry. Stocking is done at the rate of 1000–2000 per m2. From the third or fourth day after the release of fry, the tanks are regularly manured during early morning hours, at the rate of 1kg of fresh manure for every 10–15m2 water surface.
The water level is raised daily by 2–3cm. If available, artificial starter feeds can be given from the seventh day onwards. If not, the manuring is continued to provide natural food. A crop of advanced fry can be raised in such tanks in 10–15 days. If they have to be grown to a fingerling stage, the density is reduced by about 25 per cent.
When earthen ponds are used, the preparatory treatment consists of draining the pond bottom and drying it. After refilling the ponds, a suitable insecticide is applied to eradicate aquatic insects, etc. To obtain a good standing crop of rotifers, application of chemical fertilizers (1kg superphosphate, 1.5kg ammonium nitrate and 1.5kg carbomide per 100m2 pond surface) is recommended. When a dense growth of rotifers has appeared, the post-larvae or early fry are stocked in the pond. As Cyclops form a major enemy of the post-larvae, it is necessary to ensure their absence in the pond. Cladocerans like Moina and Daphnia may be introduced into the pond to multiply and be available for the mature fry to feed on.
In the rearing of Chinese carp fry to a size of 3cm, monoculture is preferred. The fry ponds are generally 1000–2000m2 in area and 1.2m deep. They are prepared by draining and application of quicklime at the rate of 750– 1125kg/ha, depending on the amount of silt at the bottom. Tea seed cake, derris powder or bleaching powder can also be used to eradicate pests and predators. The ponds are fertilized with green manure or organic manure supplemented with inorganic fertilizers. The optimal stocking rate is 1.5–2.25 million larvae, about 70 mm in length, per ha for a culture period of 15–20 days. The depth of water is raised from 50–70cm at the beginning of the culture period by 10–15cm every three to five days.
It is advisable to provide artificial fry feeds to obtain a rapid growth of the fry. A common fry feed is made of yeast (40 per cent), blood meal (25 per cent), fermented and pre-digested soya (20 per cent), fine quality fish meal (10 per cent) and soyoil (5 per cent), all finely ground and sieved through 100–150 mm mesh and fed at the rate of 0.5–1.0kg food per 100000 fry per day. After 10 days, the size of the feed particles can be increased to 400–500 mm. Green manuring is often adopted to increase the availability of
natural food in the pond. In Israeli fish farms, the fry are fed with ground cereal grains (sorghum, wheat, etc.) and at a later stage, after they reach about 10g in weight, with whole cereal grains.
The duration of fry rearing and the size to which they are grown before stocking in rearing ponds vary considerably. The most common practice appears to be to grow them in nurseries for about a month and then in grow-out ponds to market size. Some farmers transfer the fry to well-fertilized larger ponds (1–2ha) and grow them to advanced fingerling size at lower population densities. Some farmers hold the fingerlings in holding ponds at high densities (a standing crop of 10 tons/ha or more) to become available for late stocking. In such cases the fingerlings become stunted, but when introduced in rearing ponds grow rapidly under favourable conditions.
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