How to use floating grass beds for ecological disease prevention in freshwater pond aquaculture?
In order to control the occurrence of diseases, conserve and utilize water resources, improve the comprehensive three-dimensional and efficient utilization of ponds, and advance the development of healthy aquaculture, in recent years we have used floating grass beds in freshwater ponds to regulate aquaculture water quality for ecological disease prevention, and achieved remarkable results. This aquaculture technique is mainly applicable to monosex and polyculture ponds of soft-shelled turtle, loach, common carp, crucian carp, and whiteleg shrimp (Litopenaeus vannamei); it is not applicable to grass carp, red swamp crayfish, or Chinese mitten crab aquaculture ponds. The aquaculture technique is described as follows:
Pond conditions
Aquaculture ponds are divided into two types: solar greenhouses and natural ecological ponds. The area should not be too large (generally 500 sq m to 1500 sq m for greenhouses, and generally below 5 mu for natural ponds), to facilitate aquaculture management and harvesting. Ponds for culturing Yellow River Estuary soft-shelled turtle must have anti-escape walls built around the perimeter (about 0.5 m high) and feeding platforms. The anti-escape walls are made of 60 cm (or 80 cm) ceramic tiles buried 20 cm deep along the pond perimeter, with the four corners of the pond made arc-shaped to prevent the soft-shelled turtles from escaping. Loach culture ponds must have anti-escape nets built around the perimeter (about 0.5 m inside the water's edge) (the anti-escape nets are made of 10-mesh to 20-mesh netting, buried 50 cm deep, supported by bamboo poles or cement posts about 1.5 m long, with the netting above the water surface leaving a space greater than 50 cm) to prevent loaches from burrowing and jumping out to escape. About 15 days before fry and fingerling stocking, the pond must first be cleared. Pond clearing generally uses the dry method, applying 75 kg/mu of quicklime or 10 kg of bleaching powder dissolved in water and splashed over the entire pond.
Water source and fertilizing the water
The aquaculture water source is underground well water, which has good quality and no pollution. In spring, water temperature is low, so the initial water intake should not be too deep, generally about 0.5 m; afterwards, as water temperature rises, the water level is gradually increased. The purpose of fertilization is mainly to propagate organisms in the water for the fry and fingerlings to feed on and grow after being stocked into the pond. Ponds stocked with large-size soft-shelled turtle fry and fingerlings do not need fertilization. The fertilizers applied are divided into organic fertilizers, inorganic fertilizers, and biological fertilizers.
Fry and fingerling selection and stocking
When selecting fry (fingerlings), the general principle is to choose robust individuals with strong vitality, uniform size, and no disease or injury. The stocking size is determined according to individual needs. Yellow River Estuary soft-shelled turtle fry and fingerlings are stocked at 50 g/individual or above according to aquaculture needs, and loach fry and fingerlings should be stocked at 4 cm or above. Stocking time depends on the temperature in each area and the suitable temperature conditions of each cultured species. Before fry and fingerlings are stocked into the pond, they must undergo fish body disinfection. The purpose is to kill pathogenic bacteria and parasites carried by the fingerlings; this is an important step in fish disease prevention and control. Disinfectant drugs include 3%-5% salt solution for 10 minutes to 15 minutes, 10 ppm potassium permanganate for 10 minutes to 20 minutes, and 10 ppm povidone-iodine for 10 minutes. The stocking quantity varies with different fry and fingerling sizes and designed yields, and thus different stocking densities. Generally, 4 to 5 individuals/sq m are stocked for 50 g to 100 g/individual fry and fingerlings, and 2 to 3 individuals/sq m for about 250 g/individual fry and fingerlings. Loach is generally stocked at 50,000 to 60,000 fry and fingerlings per mu at 4 cm to 5 cm. At the same time, in order to fully utilize pond space, a diversified pond aquaculture model is adopted. In polyculture, the main cultured species generally accounts for 70%-80%, and the secondary cultured species accounts for 20%-30%.
Construction of floating grass beds
1. Aquatic plant species for floating grass beds: water hyacinth, water cabbage, and bamboo-leaf vegetable.
2. Functions of floating grass beds: During the growth process of aquatic organisms, leftover bait, excrement, and dead aquatic animals and plants produce large amounts of organic matter. The oxidative decomposition of organic matter produces large amounts of toxic and harmful substances such as ammonia nitrogen and hydrogen sulfide, which pollute the aquaculture water body, harm the growth of aquatic organisms, and even cause disease. Through the construction of floating grass beds on the water, the aquatic plants growing and reproducing on the grass beds absorb organic matter and harmful substances such as ammonia nitrogen and hydrogen sulfide from the pond aquaculture water environment through their root systems, purifying and optimizing the aquaculture water environment, reducing wastewater discharge and the occurrence of aquatic organism diseases, and achieving healthy and clean production.
3. Construction of floating grass beds for water hyacinth and water cabbage: At the four corners, one end, or inside the pond, frames are made with bamboo poles and fixed around the perimeter, covering 1/4 to 1/3 of the aquaculture water body. After May, water hyacinth and water cabbage are transplanted into the pond (introduced in April in greenhouses). Water hyacinth and water cabbage grow and reproduce within the frames by absorbing organic matter in the water and harmful substances such as ammonia nitrogen and hydrogen sulfide produced by the oxidative decomposition of organic matter, forming grass beds that purify water quality and prevent disease.
4. Construction of floating grass beds for bamboo-leaf vegetable: Bamboo-leaf vegetable, also known as water spinach, tong cai, or vine vegetable, with the scientific name Ipomoea aquatica, is an annual or perennial herbaceous plant. It is rich in various vitamins and mineral salts and is a very important vegetable in summer and autumn. Cultivating water spinach in the cultivation carrier on bamboo floating beds achieves the healthy growth of both bamboo-leaf vegetable and fish. It fully utilizes water space and resources, realizes the recycling of materials and energy, and improves benefits. At the same time, the growth of bamboo-leaf vegetable absorbs ammonia nitrogen, hydrogen sulfide, and organic matter in the water, purifying water quality and providing an excellent shaded environment and growth environment for aquatic organisms.
Bamboo-leaf vegetable is mainly raised on land by seed germination and then transplanted into floating beds in the pond.
(1) Bamboo-leaf vegetable seedling raising.
Land preparation: Select land for seedling raising, first turn the soil and level it. Around the leveled land, build a ring of soil embankments 40 cm to 50 cm high for irrigation. Dig furrows about 10 cm deep and 15 cm to 20 cm wide, with about 15 cm between two furrows. Apply sufficient base fertilizer. The base fertilizer can be organic fertilizer such as pig manure or chicken manure, or compound fertilizer. Decomposed organic fertilizer: 500 kg to 1000 kg/mu. Compound fertilizer: about 50 kg/mu.
Sowing: Before sowing, sun the seeds first. Find a good flat area, lay down plastic sheeting, spread the seeds on it, and sun them in sunlight for 5 to 6 hours, which can increase the seed germination rate. However, when sunning the seeds, turn them over once every 1 hour.
When sowing, evenly sprinkle the sunned seeds into the prepared furrows. After sowing, apply a layer of carbendazim (wettable powder) on the seeds to prevent soil-borne diseases and pests. Then cover with soil, using a hoe to cover a thin layer of soil not exceeding 2 cm, and level the furrows. Water thoroughly.
Seedling stage management: During the seedling raising period, the temperature for bamboo-leaf vegetable is generally between 15°C and 45°C. Water once a day, and water thoroughly each time. Seedlings will emerge in about 5 days. When the seedlings grow to about 10 cm, it is no longer just watering but flooding. This is mainly due to its "water-loving" characteristic. Do not fertilize during the seedling stage, but ensure the water demand of the seedlings. At the same time, thinning is not needed.
(2) Floating bed construction.
Floating beds are generally made of bamboo or PVC pipes. According to the length and width of the pond, cut the PVC pipes into sections 3 m to 5 m long and 0.5 m wide, plug both ends with caps, then tie the PVC into rectangular frames. Then fix small plastic pipes at intervals of about 20 cm onto the PVC pipe frame with nylon cable ties or clips, or fix netting onto the rectangular frame made of PVC. A floating bed made of PVC is then completed. PVC pipes can also be replaced with bamboo or other materials. After the floating bed is made, fix it on the water surface. Then transplant the seedlings onto the floating bed at a certain spacing.
(3) Seedling transplanting.
When the seedlings grow to 20 cm to 25 cm, they can be transplanted onto the prepared floating bed. The day before transplanting, fill the seedling-raising land with water. In this way, when pulling them out of the soil, the moist soil will not damage the seedling roots. Wash the soil off the roots of the pulled seedlings with clean water to avoid bringing soil bacteria into the water. If the pulled seedlings are not transplanted in time, they can be placed in a basin containing clean water for temporary storage. The water height should just cover the roots; in this way, they can be kept fresh for 7 days.
Planting on water: Insert the seedling roots into the small plastic pipes. Each plastic pipe holds only one seedling. The planting area of bamboo-leaf vegetable accounts for about 1/5 of the pond area. If the area is too large, bamboo-leaf vegetable grows quickly, and if not harvested in time, it will affect the light in the pond.
(4) Growth and harvesting.
Bamboo-leaf vegetable grows on the pond water surface and absorbs organic matter in the water, so no fertilization is needed. About 25 days after transplanting onto the floating bed, the seedlings can grow to 50 cm to 60 cm high, and then they can be harvested. No tools are needed for harvesting; simply pinch them off by hand at the second stem node above the water surface. During harvesting, as long as the roots of bamboo-leaf vegetable are not damaged, later growth is generally not affected.
Aquaculture management
1. Bait feeding. Bait feeding adheres to the "four fixeds" principle, namely "fixed quality, fixed quantity, fixed time, fixed location." The feeding method changes according to the living habits of the cultured species.
Fixed quality. The bait should be palatable and favored. For Yellow River Estuary soft-shelled turtle, it is advisable to feed special soft-shelled turtle feed, prepared fresh at the time of feeding. For loach feed, special compound feed is used. The protein content of the compound feed must meet the nutritional requirements of aquatic organisms at different growth stages, and the appropriate pellet size should be controlled according to fish body size.
Fixed quantity. During the fry and fingerling stage, the daily feeding amount should account for 5%-8% of fish body weight; during the adult fish stage, the daily feeding amount should account for 2%-3% of fish body weight. The daily feeding amount increases with the growth of fish body weight. The daily bait is fed in 2 to 3 portions (soft-shelled turtle fed 2 times, loach fed 3 times). The optimal feeding amount each time should be about 80% of the satiation amount.
Fixed time. The daily feeding time should be relatively fixed, choosing periods of the day with high dissolved oxygen for feeding, such as around eight or nine in the morning and three or four in the afternoon; avoid the high temperatures at noon.
Fixed location. Feeding must have a fixed location. For Yellow River Estuary soft-shelled turtle, feeding platforms should be set up on the sunny side of the pond using asbestos tiles or ceramic plates. There are two feeding methods for Yellow River Estuary soft-shelled turtle: one is to mix the soft-shelled turtle compound feed into a flatbread shape and feed it on the feed platform (feeding above water); the other is to make the soft-shelled turtle compound feed into pellets and feed them onto an underwater feeding platform (this method is suitable for cultivating Yellow River Estuary soft-shelled turtle fry and fingerlings in greenhouses with hardened pond bottoms). Loach has a relatively sensitive sense of smell and easily forms conditioned reflexes, so 1 feeding point can be set up in a pond for fixed-point feeding.
2. Water quality management. During the aquaculture process, in addition to water purification by aquatic plant floating beds, the following methods can also be used to regulate water quality according to water quality and management conditions. First, timely add fresh water. During production, due to water evaporation and a small amount of leakage, timely flush and add fresh water as appropriate. Second, use aerators. Ponds with conditions can use aerators to create convection in the pond water, increase dissolved oxygen, and release toxic substances in the water. Third, use beneficial microbial preparations. Beneficial microorganisms such as photosynthetic bacteria, Bacillus, and EM bacteria are harmless to humans and livestock, can effectively use organic pollutants in the water as their own nutrient source, and rapidly purify water quality to the maximum extent through photosynthesis; enhance the immune capacity of aquatic organisms and prevent diseases; and promote the growth and reproduction of zooplankton, making the water fertile without pollution.
3. Disease prevention and control. In the aquaculture process, disease prevention and control follow the principle of "prevention first, comprehensive prevention and control." Disease prevention runs through the entire production process, including pond clearing, drug disinfection before fry and fingerling stocking, and using the growth of aquatic floating grass bed organisms and microecological preparations to optimize the aquaculture water environment. At the same time, feeding management, pond patrol work, and disinfection of aquaculture tools should be carried out well.