Good quality broodstock eligible for induced spawning are essential for seed production. Only when adults have reached 4 - 6 years old with a body weight of over 2.5 kg, as well being free from serious diseases and injuries, can they be acceptable for creating broodstock for induced spawning. Generally, broodstock are stocked by weight, at 1 500-2 250 kg/ha, with a female:male ratio of about 1:1.5.
Under artificial conditions, the pituitary glands of broodstock do not secrete sufficient hormone for their natural propagation in ponds. Artificial methods have been devised whereby such broodstock are injected with estrogenic agents such as LRH (Luteinizing Release Hormone) or LRH-A (Luteinizing Release Hormone-Analogue), fish pituitary gland (fish hypophysis), HCG (Human Chorionic Gonadotrophin), etc., so as to induce the fish to secrete its own gonadotrophic hormone, or to provide a direct substitute for this. The standard dose of the estrogenic agents varies:
- Fish hypophysis: 3-5 mg (DW)/kg of female broodstock (for male fish the dose is reduced by half).
- HCG: 800-1 000 I.U./kg of female broodstock.
- LRH-A: 10 µg/kg of female broodstock. The dose is given in two injections, 1-2 µg/kg in the first and the remainder in a subsequent injection after an interval of 12-24 hours. Only one injection is given to males, usually at the time of the second injection for females.
In order to facilitate production and operations, it is usual to ensure that the fish spawn in daytime. If the single injection method is adopted, it is given in the afternoon around 16.00; then oestrus will occur at about dawn on the following day and spawning will take place. If the double injection method is adopted, the first injection is most commonly given at 14.00-16.00 and the second at about 24.00. Choosing the most suitable season to induce spawning is one of the key factors in the successful artificial propagation of silver carp. Other success factors include gonad maturity, suitable weather conditions and water temperature and local phenology. A period when average water temperatures remain at 18 ºC or more for 10-15 continuous days is considered an appropriate time for spawning induction.
The environmental factors that affect hatching rate include a water temperature range of 22-28 ºC, with an optimum of 26 ºC. If it is lower than 17 ºC or higher than 31 ºC, embryonic development will cease, or be abnormal. After the emergence of the tail buds of the embryonic stage, oxygen consumption suddenly increases to more than twice of the amount of earlier stages. By the larval stage (68 hours after hatching) oxygen consumption reaches its highest peak, equivalent to 8-10 times that of the earlier stages. High dissolved oxygen levels are therefore highly important for the development of embryos and larvae.
Running water type hatching devices (hatching jars, vats and circular hatching tanks) are all designed in accordance with the characteristics of Chinese carp eggs and in order to fulfil the requirements of embryonic development. This enhances hatching rates and the availability of fry for stocking. Commonly, the volume of a hatching jar (vat) is about 250 l and the stocking rate is 2 000/litre. Circular hatching tanks are ring-shaped tanks built of cement or brick, with a size that depends on the scale of production. Small versions have a diameter of 3-4 m, while the larger type are 8 m in diameter. The rings are 60-100 cm wide and about 90 cm deep and the tanks may hold 7-15 tonnes of water. The stocking rate is in the range of 700 000-1 200 000 eggs/m³. Such tanks are suitable for comparatively large-scale production units.
Good water quality is very important to achieve a high hatching rate. Water that has been polluted by industrial activities or pesticides should not be used for hatching purposes. Small fish, tadpoles, shrimps and copepods are all very harmful to fish eggs and larvae. The degree of injury by predators is closely connected with egg density, the predator level, and the duration of contact with them. Predators may be eliminated by capture, filtration and chemicals such as quicklime, bleaching powder, rotenone etc.
The culture of fingerlings requires special care because fry are small and delicate, their feeding ability is weak, they do not adapt well to changes in external environment, and they are not expert in avoiding predators. Therefore, well-controlled intensive systems are necessary to maximize survival rate and to produce healthy fingerlings that will lay a solid foundation for high productivity at the grow-out stage.
The nursery stage refers to the period from 3-4 day old fry to the production of fingerlings that can be stocked into the grow-out enclosures. There are two stages in nursery production. Firstly, in the fry rearing stage, they are grown until they are 15-20 days post-hatch and have a body length of 2.5-3 cm; these are usually called 'summer seedlings' in China. Secondly, in fingerling production, these 'summer seedlings' are reared for a further 3-5 months, when they become 8-12 cm in body length, which are known as 'yearlings'.
After draining the nursery ponds, mud and wastes need to be removed and dikes repaired, following which they must be sterilized with chemicals or herbs before the stocking of fry. The main purpose of these procedures is to eradicate predatory and other wild fish, harmful aquatic animals and plants, parasites and their eggs, and pathogenic bacteria. The chemicals used may be quicklime (CaO), bleaching powder, tea-seed cake, rotenone, etc.
After pond clearing, the pond water is fertilized by applying a basal manure, in order to produce natural food organisms before the fry are stocked. The time of application and the amount of manure vary with the pond conditions and the type of fertilizer. Fermented manure and compost, if used, may be applied within 3-5 days before fry are stocked, at 2 250-4 500 kg/ha. The water should look greenish brown, indicative of rich plankton population.
The fry stocking rate has an impact on survival rate. If it is too high, survival will be low; however, it should not be too low either, or space will not be properly utilized and production costs will be unnecessarily high. The correct stocking density is 1.5-2.25 million/ha.
The routine management of nursery ponds consists of:
- Morning and afternoon inspections to observe fry activity and changes in water colour, in order to decide on the quantity of fertilizer and food to apply, or whether water replacement is necessary.
- Being careful to exclude harmful insects, frog eggs and tadpoles, etc., and to remove the weeds growing at the edges of the ponds.
- Observation, so that fish diseases can be prevented or treatment given.
- Training the fry, so that they are strong enough withstand transfer into fingerling ponds with minimal mortality. This training process consists of driving the fry into a net-cage with a net and concentrating them in it for about 20-30 minutes, during which they secrete a lot of mucus and their muscles become stronger. Then they are released back into the pond. Usually, this process is carried out once or twice before the fry are transferred into fingerling ponds.
A two-year-culture cycle is generally adopted in pond fish culture in China; the first year is for rearing fry into fingerlings and the second year is for rearing fingerlings into market-size fish.
Polyculture is very popular in the rearing of silver carp in China. The considerable skill of aquaculturists enables them to maximize unit production efficiently by using the characteristics of various species to utilize the whole water body efficiently.
Another popular system is continuous harvesting and stocking, sometimes referred to as 'catching and stocking in rotation'. This consists of stocking at a high density, partial harvesting of the larger fish, and the addition of new fingerlings; this keeps the carrying capacity of the pond high all the time. This also speeds up turnover and supplies fresh fish to the market in both summer and autumn. Silver carp and bighead carp are the main species used in this system; grass carp and a small amount of Wuchang fish are the next most used. If tilapia is polycultured, market-sized fish should be netted out, leaving the smaller ones, in order to stop them propagating in the pond.
Silver carp are typical phytoplankton feeders, consuming diatoms, dinoflagellates (Pyrrophyta), golden brown algae (Chrysophyta), yellow green algae, some green algae and blue green algae (Cyanophyta). In addition, detritus, conglomerations of bacteria, and rotifers and small crustaceans are major components of their natural diet. Generally, there is no need to provide formulated feed in silver carp culture.
In the continuous harvesting and stocking system the following aspects need care:
- Injuries and mortalities are liable to occur in summer and autumn harvesting if this operation is slow and careless, because water temperatures are rather high at that time and fish are active and consume a great deal of oxygen; they can not stand being herded together for long periods.
- Fish should be caught on cool days, when they are not gasping, never on humid or rainy days when the fish are gasping or likely to gasp. Feeding should be reduced the day before harvesting and wastes and grass residues that hamper harvesting operations removed.
- When the fish are confined, those that have not yet reached the marketable size must be returned to the ponds immediately and gently.
- Harvesting operations stir the pond bottom mud, resulting in turbidity and higher organic matter levels; fish also consume more oxygen after violent activity. It is therefore essential to replace a large amount of the water and to turn on aerators in order to increase the dissolved oxygen content.
Handling and processing
Silver carp are normally bought live, based on traditional consumption patterns in China. It is therefore essential to keep them alive from harvesting to marketing. Trucks and boats containing water are basically used as transportation tools in most areas.
The production costs for silver carp vary from country to country (and even place to place) and the scale of operations. The major factors are the cost of labour, culture facilities, water, seed, feed (fertilizer), power, and transport.