Experience Sharing on Rotational Culture of Giant Freshwater Prawn and Mandarin Fish
Macrobrachium rosenbergii is not tolerant of low temperatures. In late autumn and early winter, when the water temperature drops to 15°C, the pond must be drained and the prawns harvested for market in a timely manner. In this way, the Macrobrachium rosenbergii culture pond is actually only utilized for half a year (May to October). Mandarin fish do not completely stop feeding in winter and continue to grow, only with reduced feeding intensity and growth rate. Using the Macrobrachium rosenbergii culture pond to raise mandarin fish during the winter idle period takes advantage of cheaper fry and abundant forage fish at this stage. The mandarin fish are raised until April to May of the following year for market, when prices are relatively high. Implementing rotational culture of Macrobrachium rosenbergii and mandarin fish can maximize the use of the pond's idle period, increase pond productivity, and boost culture profitability.
1 Pond Conditions and Preparation
1.1 Pond Conditions and Cleaning. The pond requires an adequate water source, convenient drainage and irrigation, an area of 3 to 5 mu, a flat pond bottom, a slope ratio of 1:3 or more, and an effective water depth of 1.2 to 1.5 m. Before releasing the post-nursery prawn fry, the pond must be dried, cleaned, and sun-exposed, and a trench 1 m wide and 0.5 m deep connected to the drainage ditch must be dug as a prawn collection trench; 100 kg of quicklime per mu is used for dry pond disinfection, and new water is added the next day to a depth of 0.6 m.
1.2 Planting Shelter. Water spinach is planted on the pond slopes around the pond. It is generally sown at the end of April and transplanted to the pond slope at the 0.7 m water line in mid-May, with a planting spacing of 0.1 m. As the water level rises and the water spinach grows well, it extends 2 to 4 m into the pond, covering 1/5 of the total pond area, serving as habitat and molting environment for Macrobrachium rosenbergii.
1.3 Building an Intermediate Nursery Pond. In a wind-sheltered, sunny location of the pond, a rectangular small pond with an area of 50 sq m is excavated, with a water depth of 0.8 to 1.2 m. A bamboo frame is erected and covered with plastic film, and netting is hung inside the pond for prawns to rest on. One week before releasing the prawn fry (around April 15), 1 kg of bleaching powder is used for disinfection to kill wild fish and pathogens. Three days before releasing the fry (April 22), water is added and heated (a heat pump may be used) to maintain the temperature. When the water temperature is maintained above 22°C, Macrobrachium rosenbergii fry can be released.
2 Fry Release
2.1 Macrobrachium rosenbergii Fry Release. Around April 25, 100,000 desalinated fry (2,000 per sq m) are released in the intermediate nursery pond. During the nursery period, an electromagnetic air pump is used for aeration. In mid-May, when the weather is clear and the pond water temperature stabilizes above 20°C, the insulating film is removed. After the water temperature in the nursery pond matches that of the pond, the fry are caught with a dip net, counted, and released into the pond. Under normal conditions, after about 25 days of nursery, the survival rate of the fry is 60% to 70%, and the size reaches 2 to 3 cm. They are released into 3 to 5 mu (10,000 to 20,000 per mu). Twenty days after the fry are released into the pond, on June 15, 50 silver carp and bighead carp fingerlings each (either young-of-the-year or current-year summer fry) are released per mu to control plankton biomass and prevent the water from becoming too fertile, which would affect feeding and growth of Macrobrachium rosenbergii.
2.2 Mandarin Fish Fingerling Release. By September, most Macrobrachium rosenbergii have reached market size (60 prawns per kg) and can be harvested in batches for market ahead of schedule until the pond is drained in late October. Ten days after disinfection with quicklime, mandarin fish fingerlings are released at a size of 0.15 to 0.2 kg each, with a stocking rate of 300 per mu, totaling about 50 kg; the fingerlings should be disinfected before being released into the pond.
3 Culture Management
3.1 Feeding
3.1.1 Prawn Culture. Newly introduced early-breeding prawn fry in the greenhouse are mainly fed steamed egg custard, twice daily, 50 g each time; after 7 days, they are fed high-protein diets such as minced fish and soybean milk, at a feeding rate of 50% of the prawn fry body weight, three times daily in the morning, noon, and evening, with the evening feeding accounting for 60% of the daily total, adjusted as appropriate based on feeding and water quality conditions. After about 25 days, when transferred to the pond, they begin to be fed Macrobrachium rosenbergii complete pellet feed, supplemented with soybean cake, wheat flour, bran, small fish, and crushed snails spread on the pond bank, with animal-based feed accounting for more than 35%, and a daily feeding rate of 20% to 40% of prawn body weight; after two months, the feeding rate is 5% to 10% of prawn body weight. Fed twice daily, with 30% of the daily total in the morning and 70% in the afternoon. At the same time, some plant-based feeds rich in molting hormones and other trace elements, such as duckweed and silkworm excrement, are appropriately fed to meet the physiological growth needs of Macrobrachium rosenbergii.
3.1.2 Mandarin Fish Culture. In early November, when the mandarin fish are released into the pond, forage fish are also stocked at the same time, including inexpensive white crucian carp and bighead carp fingerlings and loach at 25 to 50 g each, initially at 150 kg per mu, gradually increasing thereafter, with 100 white crucian carp of 0.4 to 0.45 kg stocked in combination. Feeding is carried out in stages: before March, feed once a week on clear days; in March, feed twice a week, 15 to 20 kg per mu each time; in April, increase as appropriate based on weather conditions. Ensure the ratio of mandarin fish to forage fish is 1:(5 to 10). If forage fish density is too low, mandarin fish expend excessive energy chasing prey and cannot feed adequately, affecting growth; if forage fish density is too high, it easily causes oxygen deficiency and surfacing. To ensure the proportional growth of uneaten forage fish, an appropriate amount of rapeseed cake is also fed.
3.2 Fertilization and Water Quality Management
3.2.1 Prawn Culture. Five to seven days before releasing the nursery prawn fry into the pond, 100 kg of fermented organic manure is applied per mu to cultivate plankton and other natural food organisms; during the growth period, superphosphate is applied once every 20 days at 3 to 5 kg per mu, but it must never be mixed with or used simultaneously with quicklime. After the Macrobrachium rosenbergii are released into the pond, new water is generally added once every 8 to 9 days, raising the water level by 8 to 10 cm each week, reaching about 1.4 m after two months. During the high-temperature season, some new water is added every 2 to 3 days, and during consecutive low-pressure or overcast and rainy days, new water must be exchanged in a timely manner. During peak molting periods, the water level is appropriately lowered during the day to help raise water temperature and promote hardening of the new shell, and new water is added in the evening to benefit prawn molting and growth. The pond is inspected once each morning and evening to monitor water quality changes. If prawns are found crawling ashore, new water should be added immediately or an aerator used to increase oxygen, preventing fish and prawn death from oxygen deficiency.
3.2.2 Commercial Mandarin Fish Culture. The water body requires abundant dissolved oxygen and fresh water quality. In winter, new water is added once every 15 to 20 days, 10 to 15 cm each time; in spring, once every 7 to 10 days, 20 to 30 cm each time. Dissolved oxygen is maintained above 4 mg/L, and transparency is maintained above 0.4 m.
3.3 Disease and Pest Control
3.3.1 Prawn Diseases. Common prawn diseases include shell disease, black gill disease, and ciliate disease. Ciliate disease can be treated by spraying a mixture of copper sulfate and zinc sulfate (1:1) at 0.6 mg/L over the entire pond.
In terms of prevention and control methods, ecological prevention should be the primary approach. For example, from July to September, when prawns are in their rapid growth period, an appropriate amount of aquatic plants (such as Hydrilla verticillata) can be regularly transplanted into the prawn pond. On one hand, this provides habitat and shelter for Macrobrachium rosenbergii, preventing cannibalism and attacks by pests such as leeches, toads, loach, and wild fish; on the other hand, it improves the ecological environment and maintains the ecological balance of the prawn pond.
3.3.2 Mandarin Fish Disease Prevention. During mandarin fish culture, anchor worms may parasitize, but the harm is minimal. Five days before release into the pond, the pond water is sprayed with B-type insecticide at 0.5 mg/L over the entire pond; mandarin fish are bathed in salt water and malachite green solution before entering the pond; monthly spraying with Qiangli Miechongjing Type I at 0.25 mg/L is used for prevention. Because mandarin fish are sensitive to most insecticidal drugs, pay attention to changes in the pond fish when spraying. During the rearing process, forage fish must be continuously fed, but they need to be bathed in medication before being released into the pond.
4 Harvest and Profitability
4.1 Macrobrachium rosenbergii Returns. When raised according to this model until late autumn and early winter, throughout the entire rearing period, the feed input per mu is 380 kg, the feed conversion ratio is about 2.4, the prawn survival rate is about 75%, prawn yield can reach 160 kg, with an output value of 8,000 yuan, plus about 300 yuan from fingerling income; costs calculated at 6 yuan per kg amount to about 2,280 yuan, and electricity, pond rent, and other miscellaneous labor expenses total 1,000 yuan per mu, giving a net profit of more than 5,000 yuan per mu.
4.2 Mandarin Fish Returns. The mandarin fish are raised until the pond is drained in late April to early May. During the culture process, 600 kg of forage fish and 200 kg of rapeseed cake powder are fed per mu, producing about 150 kg of mandarin fish per mu (commercial mandarin fish of 0.4 kg or more account for 80%), and 70 kg of silver carp, with a total income of 7,800 yuan per mu; expenditures include 2,300 yuan for mandarin fish fingerlings, 2,400 yuan for forage fish such as crucian carp, and 1,000 yuan for feed, electricity, pond rent, and other miscellaneous labor expenses, giving a net profit of more than 2,000 yuan per mu.
5 Summary
The two crops per year combined yield a net profit of more than 7,000 yuan per mu, much higher than conventional culture profitability. It is a culture model worth further research and trial, but the following three issues must be noted in production.
5.1 Macrobrachium rosenbergii culture adopts early-breeding fry in a mother-child pond polyculture system, with a culture model of one-time full stocking, multiple harvests, and batch marketing. This can extend the growth period of Macrobrachium rosenbergii by 1 to 2 months, increase the market size of adult prawns, and spread out the sales period, avoiding concentrated marketing and increasing the selling price. The planting area of water spinach should not be too large, otherwise it will affect phytoplankton photosynthesis and cause oxygen deficiency in the water body.
5.2 Forage fish should be abundant and palatable. The supply of forage fish should be at least 4 times the weight of commercial mandarin fish. Mandarin fish yield depends on the quality of mandarin fish fingerlings: (1) uniform size, fish not injured, smooth surface, free of parasites and other diseases; (2) control the stocking rate, increase the stocking size, and improve the market rate.
5.3 Frequently add new water to maintain fresh water quality.