Overwintering techniques for freshwater pomfret fry
The freshwater pomfret (Piaractus brachypomus) is a freshwater aquaculture fish species introduced to mainland China for farming in the 1980s. Because of its good meat quality, fast growth, ease of extensive rearing and large market demand, it is deeply favored by the vast number of farmers. However, since the freshwater pomfret is a warm-water fish, its optimal growth temperature is 15-30°C; when the water temperature is below 12°C or above 36°C, it will freeze to death or be scalded. Therefore, the following key technical points should be well managed when overwintering freshwater pomfret fry.
1. Site selection. The pond chosen for overwintering freshwater pomfret must be able to maintain a water depth of more than 1.5 m, a minimum water temperature (in January) of more than 12°C, an area of 1000-2000 sq m, convenient water supply, ample sunlight, and ideally a site sheltered from the north wind.
2. Construction of cold-proof sheds. For ordinary ponds (meaning ponds without hot spring water), freshwater pomfret overwintering requires the construction of cold-proof sheds. At present, most farmers weld small galvanized pipes plus angle iron into a half-moon shape and fix them on the pond dikes; then they cover the frame with nylon film. The structure is generally placed on the north side of the pond along the north-south orientation, with the shed area accounting for about 1/2 or 2/3 of the whole pond. Straw or weeds can also be used to make straw mats of 100×130 cm each, and then the straw mats are covered over the water surface of the fish pond on the north side. The front end of the straw mat is 70 cm above the water surface, and the rear end is close to the water surface. Some straw is placed under the cold-proof shed. In this way, when a cold wave strikes, the fry can gather in the tangled grass to keep warm. Some farmers place water hyacinth in 1/2 or 1/3 of the pond, separated by bamboo poles, and do not let it grow freely over the whole pond. One of the above measures is mainly to prevent rain and snow, hail or snowflakes from falling directly into the fish pond, startling the fish school and causing injury. Of course, ponds with hot spring water do not need cold-proof sheds, and freshwater pomfret can overwinter safely.
3. Daily feeding and management. During the overwintering period of freshwater pomfret (that is, from late November to late February of the following year), whether feeding and management are done properly directly affects the survival rate of spring fry and overall economic benefits. Therefore, management should pay attention to:
1. Feed feeding. When the water temperature is above 20°C, the daily feeding amount should be controlled at 4%-5% of the fish body weight, fed in the morning and afternoon; generally, peanut kernel bran or feed containing more than 30% crude protein is used after soaking. When the water temperature is around 15-20°C, the daily feeding amount accounts for 2%-3% of the fish body weight, and feeding once in the afternoon is sufficient. When the water temperature is below 15°C, there is no need to feed. Feed should be placed at fixed positions inside the cold-proof shed, with fixed timing, fixed location, fixed quality and fixed quantity.
2. Water quality management. Because the winter temperature is low, metabolism in the pond is slow, so the pond water easily becomes clear, often with transparency greater than 30 cm. At this time, some fermented chicken manure or pig manure should be properly added to the pond to make the water quality become “green, tender and refreshing”; a transparency of about 20 cm is sufficient.
3. Disease prevention and control. During the overwintering period of freshwater pomfret, the main serious hazards are saprolegniasis and ichthyophthiriasis. (1) Saprolegniasis. This disease occurs because after mechanical injury or frostbite to the fish body, the zoospores of water mold invade the wound, absorb nutrients from the fish skin and germinate, and grow rapidly; one end of the hypha attaches to the fish skin tissue like a tree root, while most of the rest is exposed outside the body surface. The water mold hyphae are white cotton-like and float in the water, visible to the naked eye. After the fish body is parasitized by water mold, it will show restlessness. Because the hyphae absorb tissue nutrients from the fish epidermis, and bacteria infect the wound, the epidermal tissue necroses, and the diseased fish gradually die due to emaciation. Prevention and control methods: ① Before stocking fry, thoroughly clear the pond with quicklime, which can reduce the occurrence of this disease. ② When seining and catching, operations must be careful and meticulous so as not to injure the fish body. On cold winter rainy and snowy days, if individual fish are frostbitten, do not go into the pond to catch them and disturb the fish school. ③ A 0.1 ppm potassium permanganate solution can be splashed over the whole pond once every other day, and two consecutive treatments are sufficient. (2) Ichthyophthiriasis, also called white spot disease. The body surface of diseased fish has many small white spots, which can cause the fish body surface to secrete a large amount of mucus and form cysts at the parasitic sites; if the pathogen parasitizes the cornea, it can cause blindness in the fish; diseased fish appear restless, swim in groups around the pond edge, and constantly rub against other objects or jump out of the water. The fish become thin and black, the gill filaments are congested, breathing is difficult, and soon they die in large numbers. The pathogen of ichthyophthiriasis is Ichthyophthirius multifiliis, which is suitable for growth and development at a water temperature of 15-25°C. Prevention and control methods: ① The pond must be thoroughly disinfected with quicklime, and cement ponds, glass containers, plastic containers, etc. must also be disinfected with potassium permanganate or bleaching powder. ② It can be treated with dried chili pepper plus dried ginger slices. For each mu of water surface with an average depth of 1 m, 100 g of dried ginger slices can be used, plus 500-1000 g of clean water; after boiling, simmer for 10 minutes, then stop the fire for 20-30 minutes; then boil and simmer again for 10 minutes, then stop the fire for half an hour; add 250 g of dried chili pepper and 2000 g of clean water, then boil and simmer for 10 minutes; finally, mix the residue and juice with 10 kg of water and splash evenly over the whole pond once a day for two consecutive days to cure it.
4. Other matters. During the overwintering period of freshwater pomfret, try not to enter the pond and disturb the fish school, to avoid collision injuries; it is strictly forbidden to use drugs such as trichlorfon; otherwise the whole pond will die, and the loss will be irreparable.
4. Management in early spring. When the water temperature stabilizes above 20°C, the appetite of freshwater pomfret begins to increase; at this time, concentrate feed plus appropriate green feed should be fed. Generally, the concentrate feed is mainly peanut bran or soybean powder or compound feed containing more than 30% crude protein, and the daily feeding amount accounts for about 4% of the fish body weight; the green feed is forage grass or oilseed rape greens, and the daily feeding amount accounts for about 20% of the fish body weight; usually green feed is fed in the morning and concentrate feed in the afternoon. In addition, an appropriate amount of new water is injected into the pond to keep the pond water “green, tender and refreshing”, which is beneficial to the growth and development of pomfret fry and the sale and transportation of spring fry.