What are the precautions for rainbow trout cage culture?
Rainbow trout (scientific name: Oncorhynchus mykiss) is a fish of the family Salmonidae and the genus Oncorhynchus (Pacific salmon), a cold-water fish of the family Salmonidae, and a valuable species among pond-cultured fish. It is native to northern North America and the western coast of the Pacific, mainly living in low-temperature freshwater, and it has high requirements for the water quality of the culture waters. There is also a type called “steelhead” that lives in freshwater or seawater in different seasons. Rainbow trout is a food fish and has been introduced into many countries, but in some areas it has become an invasive species. For food purposes it is raised in reservoirs, weighing about 3 kg, and grilled rainbow trout is extremely delicious.
Rainbow trout prefers to inhabit clear waters with relatively low water temperature, high dissolved oxygen and abundant flow. The extreme temperature range for rainbow trout is 0-30℃, the suitable living temperature is 12-18℃, and the optimum growth temperature is 16-18℃. Below 7℃ or above 20℃, appetite decreases and growth slows; above 24℃ feeding stops, and afterward the fish gradually weakens and dies. It has high requirements for dissolved oxygen in the water. A dissolved oxygen level below 3 mg/L is the lethal point; below 4.3 mg/L, “gasping at the surface” appears and death begins. When dissolved oxygen is below 5 mg/L, the respiratory rate increases.
Points to note in rainbow trout culture:
I. Selection of culture waters
Rainbow trout likes to live in an environment with clear water quality, unobstructed water flow and sufficient water volume, and it likes countercurrent and high dissolved oxygen. Therefore, when choosing waters for cage culture, attention should be paid to abundant water sources, clear and pollution-free water quality, little annual change in water level, and the open water surface of medium and large reservoirs where the annual water temperature varies between 4℃ and 24℃, with water quality meeting fishery water standards; a water flow speed of 10 cm/s to 30 cm/s, since properly controlling water flow stimulation can induce normal movement of rainbow trout, thereby improving feeding and accelerating growth; water transparency greater than 2.0 m, since in general the higher the water transparency, the lower the degree of eutrophication and the better the water quality, the better the production performance of cage fish culture, and the higher the potential fish yield; an annual average minimum dissolved oxygen value of ≥6 mg/L, since when dissolved oxygen is below 5 mg/L the respiratory rate of rainbow trout increases and the fish feel uncomfortable, below 4 mg/L death occurs, above 6 mg/L they grow normally, and at 9 mg/L the growth rate is fastest.
II. Structure and installation of cages
1. Shape and materials of cages The shape of the cage is best rectangular. Placing a rectangular cage in the water body perpendicular to the flow direction gives fast water exchange and a large exchange volume, which can ensure the fresh water flow needed for rainbow trout culture. The materials for making cages must be firm and durable enough to bear the weight of the fish in the cage, not obstruct water exchange, corrosion-resistant, lightweight, harmless to the fish, and reasonably priced. The size of the cage can be made in different specifications according to the requirements of rainbow trout at different culture stages, because the daily management of rainbow trout cage culture is relatively complicated; to facilitate operation and management, it is best to use a floating frame operating platform. The floating frame operating platform requires a material that is solid, durable, good in buoyancy, resistant to wind and waves, easy to operate and easy to maintain. A frame platform made of steel and foam is relatively reasonable and also the most common; hanging the cage body on the frame platform makes an independent rainbow trout culture cage. The cage is best made of nylon netting, whose advantages are being firm and strong, with relatively thin and small knots, low resistance during water exchange, corrosion resistance, light weight, and being less likely to abrade the fish body. The cage cover should be able to completely cover the top of the cage and be movable: first, it can prevent intrusion and predation by carnivorous fish and prevent natural enemies and thieves; second, it can shade light and reduce the stress response of rainbow trout to light, while reducing the time rainbow trout are exposed to sunlight helps strengthen the disease resistance of the rainbow trout immune system, thereby improving production performance; third, because less light enters, it inhibits the growth of biofouling and reduces attachments on the cage walls, which is beneficial to water exchange.
2. Installation of cages Cages should be installed in open waters, avoiding narrow reservoir bays, lake inlets and stagnant water areas, and also avoiding areas with excessively rapid flow and excessive wind and waves. Narrow reservoir bays and stagnant areas cause insufficient water exchange in the cage, worsening the water quality inside the cage and possibly causing physiological stress responses in fish; excessively rapid flow and excessive wind and waves cause overly frequent water exchange, and fish swimming against the current consume physical strength, causing physical stress responses in fish, and the cages are easily damaged. The water level changes in the cage culture waters should be predictable. The minimum water depth at the cage installation site should be at least 5 m deeper than the bottom of the cage body, and the cage installation area should not have mixed aquatic plants or too many rocks. For convenience of management and safety, the cage installation area should be easily accessible to workers and frequently observable.
3. Arrangement of cages Cages are best arranged in a straight “line”, placed perpendicular to the flow, and must never be arranged in a “chessboard pattern”; water must not flow directly from one cage into another. Considering water level changes and the effect of wind and waves, each cage should be connected to adjacent cages with steel wire shackles, and the four corners of the outer perimeter of each row of cages should be fixed with iron anchors and steel wire ropes, to prevent them from being blown away by wind and waves or drifting away from the culture area due to water level changes.
III. Stocking of rainbow trout
1. Quality of stocked fingerlings The rainbow trout fingerlings stocked in cages should preferably weigh more than 50 g, be relatively uniform in size, and free of injury and disease. The main signs of healthy fingerlings are: the whole fish population has a uniform body color, no sores, unbroken fin rays, and they are not easy to catch.
2. Stocking quantity of fingerlings The stocking quantity of fingerlings depends on the expected total weight and average individual weight of rainbow trout in the cage at harvest, and also on the optimal fish carrying capacity of the cage. The optimal fish carrying capacity of a cage changes with the quality of the culture environment and differs from place to place. Within the range of optimal fish carrying capacity, a reasonable stocking quantity can be calculated by the formula: stocking quantity (fish/cubic meter) = total weight of fish per cubic meter at harvest / average body weight of each fish at harvest.
3. Stocking conditions and operational requirements Fingerlings must be fasted for 2 days before stocking and transport. The water temperature at the departure site and the destination should be roughly the same, with a maximum difference not exceeding 3℃. Rainbow trout stocking is best carried out when the water temperature is 8℃ to 10℃; the higher the water temperature, the more severe the stress on the fish body, easily causing death. Fingerling transport should be carried out in spring, preferably using an oxygenated live fish transport vehicle, and during transport always pay attention to water temperature changes and oxygen supply. Stocking should be done in the morning when the temperature is lowest, or during rain and cloudy weather. The entire process of putting fingerlings into the cage should be carried out in water, and operations should be careful to minimize stress on the fingerlings and prevent skin abrasion, stunning and even death. If the fingerlings are in good health and operations are careful, the survival rate of fingerlings can reach 100%.
IV. Feeding and storage of feed
In the process of rainbow trout cage culture, feeding high-quality pelleted feed is extremely important. A nutritionally complete full-price pelleted feed must be used, and the protein content in the feed should be above 40%. Fingerlings that have just entered the water have not yet formed feeding habits and must be domesticated in time, and the feeding amount should be reasonably adjusted only after they actively feed in groups.
During culture, an appropriate pellet size should be selected according to the fish body size, and the feeding amount should be adjusted in time according to changes in fish body weight. Feeding time should be chosen at dawn or dusk. Observe the feeding and digestion of rainbow trout every day, and strictly avoid overfeeding. Feeding should be careful, slowly fed toward the center of the cage, and try to avoid feed flowing out of the cage.
Fish feeding behavior is the best indicator of fish health. If fish feed actively, it indicates that their health is good. Therefore, to keep fish with a strong appetite, each feeding should be about eighty percent full depending on the weather, water temperature and digestion.
The quality of produced feed declines as storage time lengthens. The fresh feed needed by rainbow trout must be used up within 1 month, and too much feed should not be purchased at one time for storage. Not feeding spoiled feed or feed whose nutritional components have been destroyed due to improper transport and storage can effectively prevent nutritional diseases in rainbow trout. Therefore, feed storage should pay attention to moisture-proofing, sun-proofing and rodent-proofing, and ensure that the storage place is ventilated and dry.
V. Daily management
Uneven growth of rainbow trout is a common phenomenon. Screening should be carried out once a month, and fish should be graded and raised separately, which can improve culture efficiency. When the water temperature rises in summer, the cage depth should be increased and the culture density reduced, thereby increasing the activity space for rainbow trout. Persist in inspecting the cages every day, measuring water temperature, recording feed input and fish feeding conditions, and promptly removing sick and dead fish; when strong winds occur, wait for the wind to stop and immediately lift the cage for inspection to see whether there is cage damage, repair it immediately if found, and retighten the steel wires and shackle screws connecting the cages to ensure that every connection is firm. Always pay attention to water level changes in the culture area, and as the water level rises or falls, promptly take in or let out the iron anchors and steel wire ropes fixing the four corners of the operating platform. Attachments easily form around the cage body and obstruct water exchange; when many attachments are found, the cage should be immediately lifted and cleaned, and operations should be gentle and slow, not too forceful, otherwise the fish body surface will be abraded and diseases will be caused.
VI. Disease prevention and control
Rainbow trout generally is not prone to disease. Common diseases mainly include fin rot and nutritional diseases, and they generally occur in early spring. In early spring, as the water temperature gradually rises, the culture density should be appropriately reduced and fish should be screened and divided into cages in time, with careful operations, thereby reducing mutual aggression among fish and human-caused injuries. At the same time, add an appropriate amount of medicine to the feed each week and feed it mixed with the feed, which can improve the immunity of the fish population and effectively prevent the occurrence of fin rot. If fish with fin rot are found, they should be immediately removed, bathed in potassium permanganate solution, and then raised separately to prevent infection. Nutritional diseases are mainly caused by feeding unqualified feed; as long as the feed fed is guaranteed to be fresh and nutritionally complete, the occurrence of this disease can be prevented. As long as advance prevention is done and measures are appropriate, various diseases in rainbow trout cage culture occur less often.