Technical Key Points for Farming Rainbow Trout in Mountain Stream Water in Southern China

I. Farming Conditions
1. Selection of water source. Based on the living habits of rainbow trout, select mountain stream water with a good ecological environment and high forest coverage around it (over 80%), and the pond construction site should preferably have a drop in elevation. A large water volume is required, the water quality must meet fishery water standards, and the water temperature must be low, with the maximum summer water temperature not exceeding 24°C.
2. Construction of reservoir. Mountain stream water quality is easily affected by rainwater, causing turbidity and pathogen infection, so building a reservoir is relatively important; where conditions permit, water purification equipment can be installed. Generally, a dam is built by intercepting the water intake, then water is diverted from the bottom of the dam into the reservoir through a PV pipe with a diameter of 160 mm, and after sedimentation treatment in the reservoir it is used as farming water.
3. Construction of farming ponds. The entire farming process from fry to adult fish adopts the intensive flow-through pond farming method. The fish ponds are rectangular cement ponds with a concrete structure. Fry ponds are built upstream of the water flow, and adult fish ponds are built downstream. Fry ponds have an area of 30-100 sq m and a water depth of 0.3-0.5 m; adult fish ponds have an area of 50-200 sq m and a water depth of 0.5-0.8 m. A pond length-to-width ratio of (3-5):1 is more suitable, with a slope of 3%-5% from the front part of the pond to the rear part, which facilitates waste discharge and pond cleaning. Generally, parallel ponds are built; when series ponds are built, there must be a drop in elevation to achieve the purpose of oxygenating through falling water, and 2-3 rows are more suitable, as too many in series can easily spread diseases.
4. Setting up the inlet and drainage system. Independent inlet and drainage systems are set up at opposite ends of each fish pond. The inlet of a fry pond is about 0.5 m wide and the drainage outlet is 0.6 m wide; the inlet of an adult fish pond is about 0.8 m wide and the drainage outlet is 1.0 m wide. The inlet is equipped with 60-mesh silk screen to filter the farming water, and the drainage outlet generally has two layers: the first layer is a fish barrier grate, with the mesh size depending on the size of the farmed rainbow trout, based on the principle of not affecting the water flow and not allowing fish to escape; the second layer is a sluice plate to control the water level. At the tail of the fish pond, a "V"-shaped waste discharge groove or a long strip-shaped waste discharge ditch is opened, and PV pipes are buried underneath to discharge waste out of the pond.
5. Installing shade nets. Because the water temperature of mountain stream water is easily affected by climate changes and fluctuates, especially in summer when the water temperature rises quickly under direct sunlight, installing shade nets during the high-temperature season is particularly important.
　　
II. Fry and Fingerling Stocking
1. Pond treatment. Before stocking, disinfect the fish pond with chlorine dioxide at 0.5 g/cu m; for newly built fish ponds, before disinfection, first soak and clean them with oxalic acid or acetic acid for 7-10 days to remove alkali before use.
2. Fry and fingerling stocking. Eyed eggs of rainbow trout introduced from Beijing are centrally incubated at the municipal cold-water seedling breeding base to 3-5 cm, then transported to each farming site with oxygen-filled bags or fish baskets under cooling. Before the fingerlings enter the pond, the temperature difference needs to be adjusted to within 2-5°C; before entering the pond, they are soaked and disinfected with potassium permanganate (8-10 mg/L) or formalin (25×10-6) for 15-20 minutes to kill pathogenic bacteria and parasites carried on the fish body.
3. Stocking density. Rainbow trout stocking is generally carried out in two stages: during the fry rearing period (size 3-5 cm per fish), stock 500 fish/sq m; after intensive rearing to over 50 g, they enter the adult fish rearing period, with 50-80 fish/sq m stocked. During each stage of farming, ponds should also be divided in a timely manner according to the growth of the rainbow trout, and the density should be thinned.
　　
III. Feeding Management
1. Feed feeding. Feed can be self-mixed or compounded feed. Self-mixed feed is mainly animal-based feed, mainly including fish meal, meat and bone meal, yeast, and animal viscera, accounting for 50%-70%; plant-based feed includes soybean meal, corn flour, wheat flour, etc., accounting for 30%-50%, with a small amount of multivitamins, minerals, fish oil, etc. added; during the fry rearing period, the proportion of animal-based raw materials is appropriately higher, and during the adult fish farming period, the proportion of animal-based raw materials is appropriately lower. Compounded feed mainly refers to specially made rainbow trout feed. At appropriate times, add 1% antibiotics, 2% salt, and trace amounts of potassium iodide (0.02 g/kg feed) and cobalt dichloride (0.06 g/kg feed) to the feed. From July to September, mix Chinese herbal medicine liquids such as licorice and isatis root into the feed, which is beneficial for surviving the summer. The feed is made into a floating type, which is easy to observe and not easy to pollute the water quality.
The feed feeding amount during the fry rearing period accounts for 3%-5% of the rainbow trout body weight, with 3-6 feedings per day; during the adult fish rearing period it accounts for 2%-3% of the fish body weight, with 2-3 feedings per day. Specifically, it should also be flexibly controlled according to the fish body condition, water temperature, weather, etc., and the feed feeding amount should be adjusted once every 7-10 days according to the weight gain of the rainbow trout. Generally, feed is applied at the front part of the fish pond, and when feeding, the principle of slow-fast-slow should be grasped, while closely observing, so that the rainbow trout reaches 80% satiation.
2. Water quality management.
① Keep the water flow constant and unobstructed. During the fry rearing period, the water level is 30-50 cm, the flow velocity is 2-4 cm/s, and the flow rate is such that the pond water is exchanged 2-3 times per hour; during the adult fish rearing period, the water level is 50-80 cm, the flow velocity is 6-10 cm/s, and the flow rate should preferably exchange the water 5-6 times per hour.
② Frequently brush the fish barrier grate, discharge waste once a day, and thoroughly clean the pond and discharge waste once every 7-10 days.
③ For rainbow trout farming with mountain stream water, management during the rainy season must be especially strengthened to avoid muddy water entering; especially in the fry stage, rainbow trout are relatively sensitive to muddy water, which can easily cause fish death. Close the water source in a timely manner, turn on aeration equipment, or place an iron screen between the pond water surface and the water inlet, and place sponge on the iron screen to purify the water quality.
④ In summer high temperatures, mountain stream water is affected by air temperature, and on sunny days at 2-3 p.m. the water temperature often rises above 25°C. At this time, increase the water exchange volume, use shade nets, and where conditions permit, groundwater can also be pumped to lower the temperature, ensuring that the rainbow trout smoothly survive the summer.
3. Timely grading. After rainbow trout have been reared for 1 month, they should be graded and reared in a timely manner to keep the size uniform within the same pond, enhance the feeding rate of the rainbow trout, and improve feed utilization. Stop feeding for half a day before grading, and operations must be carried out with water to avoid mechanical injury and infection with fish diseases.
4. Daily management. As the rainbow trout grow, the silk screen or iron gauze on the fish barrier grate should be replaced in a timely manner so that the mesh size matches the size of the rainbow trout; damaged areas and holes should be replaced in a timely manner to strictly prevent the rainbow trout from escaping. Patrol the pond once each morning, noon, and evening to observe the activity of the rainbow trout, record the pond water temperature, and if abnormal feeding or activity of the rainbow trout is found, inspect the fish body in a timely manner and treat according to the symptoms as early as possible; remove dead fish immediately when found.