Seed supply
At present artificial propagation is the major source of mud carp seed, although natural seed is still available in the Pearl River. Seed collected from the wild is mainly used for maintaining the genetic quality of broodstock. Broodstock used for artificial propagation are usually raised in captivity from seeds from the wild or from breeding stations where good natural stocks are maintained.
Hatchery production
Broodfish of mud carp weigh about 0.8-1.4 kg (for example, a female mud carp weighing 850 g has an ovary weighing 136 g and has 204 000 eggs - usually an individual fish can spawn more than 100 000 eggs). Well-matured breeders are released into a spawning tank (round, cement, with diameter of 6-10 m and a water depth of ~2 m), after being injected with an inducing hormone (usually LRH-A). The oestrus of mud carp begins 4-6 hours after injection. The sex ratio of female and male of mud carp is 1:2. The mud carp often makes a low mating call 'ku ku', with many bubbles coming up to the surface of water. Water circulation is maintained throughout the spawning period.
Eggs are transferred to hatching raceways or jars, either manually or by gravity. Hatching raceways (which are round or ellipse-shaped structures) are commonly used for large-scale production. The width and depth of the raceways is normally 0.8 m and 0.8-1.0 m respectively. The inlets are mounted on the bottom of the raceways with openings in the same direction and at an angle of around 15 º to the bottom, to promote water circulation. Screens are mounted on the inner wall for discharging water during the operation. Water can be totally drained out through the outlet on the bottom. Current flow is maintained during the hatching period to keep the eggs and larvae suspended in the water column. This phase typically takes about 18 hours at 28 ºC.
Rearing fingerling
Nursery phase
Earthen ponds (usually 0.1-0.2 ha and 1.5-2.0 m deep) are used for the nursing of mud carp. Ponds are chemically treated after total drying, to eliminate all harmful organisms after total drying; normally quicklime is used for this purpose, at 900-1 125 kg/ha. Organic fertilizer - animal manures and/or plant wastes ('green manure') - is commonly applied, to increase the natural biomass of algae and zooplankton, 5-10 days before stocking, according to water temperature. The quantity of fertilizer is usually 3 000 kg/ha for animal manure or 4 500 kg/ha for green manure. Green and animal manures can be used simultaneously but the quantity of each is reduced accordingly.
Monoculture is practiced in the nursery stage, with a stocking density of 3 mm fry normally ranging between 4.5-6.0 million/ha, depending on the length of rearing and targeted size. The nursery phase usually takes 4-5 weeks in China. Organic fertilization is used to enhance the supply of zooplankton, normally ranging from 1 500-3 000 kg/ha once every 4-5 days for animal manure or green manure, depending on existing water fertility. Soybean milk may also be used as both direct feed and fertilizer to replace organic fertilizer in this phase of culture; if used, the normal quantity is 3-5 kg (dry soybean)/100 000 fish daily. This usually means production costs are high. A paste-form of soybean cake or other by-products from grain processing is applied from the 5th day after stocking, usually at a rate of 1.5-2.5 kg/100 000 fish daily. Sometimes a paste of water peanut, water lettuce and water hyacinth is used to replace the above-mentioned feed and fertilizer regimes at the rate of 25-40 kg/100 000 fish daily (0.5 percent of table salt needs to be added to the paste of water peanut to remove its saponin toxicity). Normal survival rates in nursery ponds are 70-80 percent, although they may reach over 90 percent under good management.
The fish usually reach a length of about 30 mm after 4-5 weeks of rearing. These are called summer-fingerlings in China and are ready for the fingerling rearing stage. Conditioning, through careful netting and holding the fish at high density for a while (several hours) is required before the transfer of summer-fingerlings to the fingerling pond; this enhances tolerance to transport stress.
Fingerling production
Relatively larger (0.2-0.3 ha) and deeper earthen ponds (about 1.5 m) are used for fingerling rearing. Contrary to the nursery stage, both monoculture and polyculture are practiced. Usually monoculture is adopted for summer-fingerlings from 30-60 mm in size. It is important to ensure good fingerling survival rates to select healthy and similar size fry for stocking; they should be active, uniform-sized, bright-coloured, complete-scaled, small-headed, and thick-backed, and able to swim strongly against water flow. A stocking density of 3.5-4.5 million/ha is used for monoculture. Mud carp fingerlings can be polycultured with other carp species except silver carp (
Hypophthalmichthys molitrix), which competes for the same food; the stocking density is usually 1-1.5 million/ha when it is the major species in the pond. For example polyculture ponds may be stocked with 1 050 000-1 500 000/ha of mud carp, and 45 000/ha of grass carp or 1 050 000-1 200 000/ha of mud carp, 15 000/ha of grass carp, and 30 000-45 000/ha of bighead carp.
Feeding is vitally important throughout the fingerling rearing period, mainly with commercially manufactured feeds. In monoculture, groundnut cake can be used at 0.3 kg/day/10 000 fish. Two months after stocking, 'tatsao' (a traditional Chinese combination of various wild herbaceous plants, compost and inorganic and organic manures that encourage the proliferation of plankton) is applied at 1 500-2 500kg/ha once every 15 days.
Fingerling rearing normally takes 4-8 months in China, but can be considerably shortened in warmer climates or if lower stocking densities are used. The normal survival rate should be above 80 percent.
Ongrowing techniques
The most commonly adopted ongrowing technique for mud carp is polyculture in ponds. Though this species grows slowly and does not reach such a large size as grass, bighead and silver carp, it can be reared in high density and its production rate is high. Mud carp may be stocked either as the major species or as a secondary species, together with other carps.
Polyculture as a major species
When they are one of the major species in pond polyculture, mud carp stocking density is 15 000-25 000/ha of 25-50 g fish. Grass and bighead carp are also stocked as major species; 1 200-1 800/ha of 250 g grass carp and 450-2250/ha of 500 g bighead carp are used. Other fish are stocked as secondary species, such as silver carp (375-750/ha of 250 g fish), tilapia (3 000-6 000/ha of 15-20 g fish), common carp (375/ha of 100 g fish), black carp (75/ha of 500 g fish), and bream (525/ha of 50 g fish).
After one year, the mud carp typically reach 125-200 g, with a production level of 2 000-3 000 kg/ha, accounting for about 24 percent of total production, which ranges from 7 500 to 10 000 kg/ha. The grass carp reach 1 000-1 500 g (~1 800 kg/ha), the bighead carp 1 000-1 500 g (~2 700 kg/ha), the silver carp 1 000g (~700 kg/ha), the tilapia 150g (~560 kg/ha), the common carp 900 g (~270 kg/ha), the black carp 2 000 g (~120 kg/ha), and the breams 250g (~100 kg/ha).
Mud carp are omnivores that feed on both natural food and commercial feeds. Although some farmers continue to use individual feed ingredients or mixtures of them in monoculture systems, the use of pelleted commercial feeds is becoming more popular. These feeds mainly consist of by-products from oil extraction (such as soybean cake) as the major protein source, together with grain processing by-products. The use of animal protein (fish meal) is very limited (<10 percent), so the feed is relatively cheap. Fertilization can reduce the quantity of feed needed by enhancing natural food availability.
Polyculture as a secondary species
Mud carp can also be stocked as a secondary species in ponds, in which case no special feeding is required; the mud carp stocking density is normally 7 500-9 000/ha of 25-50 g fish. Production can achieve 1 000-1 500 kg/ha in a year, usually accounting for 10-15 percent of the total. Mud carp can also be cultured as a secondary species in pens in shallow lakes; stocking and production levels are similar to pond culture.
Feed supply
Commercial feeds are readily obtainable in China. There are many manufacturers of freshwater fish feeds and some are specifically designed for mud carp.
Harvesting techniques
Mud carp, being a bottom-dwelling fish, are difficult to harvest effectively without draining the pond. Harvesting takes place at the end of the culture period firstly by netting at a reduced water depth, followed by total harvesting after drainage. Selective harvesting is also practiced by some farmers for balanced marketing. Selective harvesting is usually conducted in the early morning (because temperatures are relatively low and for morning sales) during late summer and autumn. Individuals of marketable size are selected after netting.
Handling and processing
Farmed mud carp is normally sold live or fresh. However, some is canned and a small quantity is processed by ready-to-eat food stores. This process consists of mincing the fish back muscles in a grinder and mixing the result with flour to form a fish cake or quenelle (dumplings). Mud carp are also dried and salted by some traditional inland fishers or farmers.
Production costs
This varies according to the culture practice used but is normally below USD 0.60/kg.