Methods for the Prevention and Control of Fish Diseases in Rainbow Trout (Oncorhynchus mykiss) Reared in Brackish Water

Rainbow trout (Oncorhynchus mykiss) is a cold-water, euryhaline fish, noted for its delicious flesh and rich nutrition, with high economic value, and is one of the world's fine aquaculture species. Jingtai County is one of the most arid areas in central Gansu Province, lacking freshwater resources, but it has brackish water resources with a certain flow, suitable water temperature, and water quality meeting the standards for pollution-free aquaculture, making it suitable for rainbow trout farming. To broaden the channels for increasing farmers' income, we carried out trial farming of rainbow trout in brackish water. After several years of development and utilization, under the conditions of a salinity of 4.7 and a flow of 0.05-0.06 cubic meters per second, fry rearing was successful, and the adult fish yield per mu exceeded 10,000 kg, with obvious farming results. Moreover, the rainbow trout produced is even more delicious than freshwater rainbow trout, market demand is stronger, and farming prospects look promising. However, in the process of increasing development efforts and expanding farming scale, due to lack of experience in disease prevention and control, certain losses were caused to fishery production, affecting the rapid development of the brackish water rainbow trout farming industry. Exploring the causes of disease in rainbow trout under brackish water conditions and finding effective prevention and control methods has become a necessary subject for our research. Through years of effort, the author has made certain progress in disease prevention and control. The key points of relevant disease prevention and control are introduced below.

1. Focus on scientific rearing, adhere to prevention first, with prevention more important than treatment

Strictly follow the rainbow trout production technical operating procedures and rear scientifically. (1) The fish pond should be built with a reasonable structure to ensure smooth water flow; before stocking, the aquaculture pond should be fully sun-dried, and 100-150 kg of quicklime per mu should be used for thorough disinfection. (2) Stock fry of fine breed and robust constitution; the stocked fry should be uniform in size, swim agilely, be robust in constitution, free of disease and injury, and stocked at a suitable density. (3) Feed nutrition should be comprehensive, attention should be paid to hygiene, no spoilage, no monotony, reasonable processing technology, feeding at fixed times and in fixed amounts, and neither starving nor overfeeding. (4) Do good work in epidemic prevention, and strictly prohibit the introduction of pathogens. Before entering the pond, soak in 2%-3% salt water or 10 mg/L potassium permanganate for 10-20 minutes, then release into the pond. (5) Pay attention to cleanliness and hygiene, do not allow dirt to accumulate at the pond bottom, and keep the water body from becoming turbid or polluted. (6) Operate seining carefully so as not to injure the fish. (7) Pay attention to drying and disinfecting tools, and regularly disinfect the fish pond. Before disinfecting the fish pond, the pond must be cleared; during disinfection, observe carefully and strictly prevent fish deaths from oxygen deficiency.

2. Persist in pond patrol observation, and always pay attention to the conditions of fish school activity

During pond patrol every morning and at each feeding, pay attention to observing whether the activity of the fish school is normal, especially noting changes in body color and the number of dead fish. Whether activity is abnormal or not is the key to discovering fish disease: First, if the number of dead fish increases slightly, one should think of it as a warning sign of disease; second, if the appearance of the fish body turns black or yellowish, it is a warning sign of illness; third, if the number of fish swimming scattered away from the school increases and their response to the outside world is sluggish, it indicates that the fish school is abnormal; fourth, if the fish school's appetite decreases, it may be that feed quality is poor or has an off odor, or it may be a warning sign of insufficient dissolved oxygen, water pollution, or disease; fifth, if the fish behave very restlessly in the water, sometimes darting up and down, sometimes swimming wildly at high speed, one possibility is infestation by parasites, and another possibility is that the water contains toxic substances and the fish are poisoned. When symptoms of illness are found in the fish school, routine examinations of the body surface, gills, and intestines of dead fish can be conducted to provide a basis for further determining the cause. Persist in sampling and testing once every half month.

3. Persist in scientific and safe drug use

Safe drug use plays a key role in preventing and treating fish diseases. First, use environmentally friendly and improved fishery drugs; second, strictly prohibit the use of fishery drugs without the "three certificates" (production license, approval number, and production batch number); third, use fishery drugs with three effects (high efficiency, long duration, quick action) and three smalls (low toxicity, few side effects, small dosage); fourth, use drugs according to the symptoms, use drugs correctly, do not use drugs blindly or indiscriminately, the dosage must not be too large or too small, and strictly follow the instructions; fifth, drugs in feed must be mixed evenly and processed into pellets to feed the fish. Do not add antibacterial drugs and hormones to feed for a long time, and strictly prevent the fish from developing drug resistance.

4. Prevention and control of common diseases

In the rearing process of brackish water rainbow trout, diseases such as vibriosis, furunculosis, saprolegniasis, enteritis, and hepatic steatosis cause relatively great harm to fishery production. The prevention and control experience for these diseases is mainly introduced below. (1) Vibriosis. Diseased fish have protruding, cloudy eyes or eyes with blood streaks; bleeding at the fin bases, oral cavity, and anus; intestinal tract congestion, inflammation, and loss of elasticity; blood spots in the liver; enlarged spleen; and swollen kidneys. Prevention and control: mix sulfonamide drugs into bait and feed, adding 100 mg of drug per kg of fish body weight, and halve the dosage after the 2nd to 4th day. (2) Furunculosis. Irregular round blood blisters appear in the muscle tissue of diseased fish; other signs are the same as vibriosis. Prevention and control: add 100-200 mg of sulfonamide drugs or 80 mg of erythromycin per kg of fish body weight, process into bait, and feed continuously for 3-5 days. (3) Saprolegniasis. White filamentous material appears at the lesions of diseased fish; in severe cases, they swim with difficulty, do not feed, and swim alone away from the school. Prevention and control: soak the diseased fish in 3%-5% salt water for 5-10 minutes. (4) Enteritis. The anus of diseased fish is red and swollen, the intestinal tract is red and congested, with no food; in severe cases, the intestine is filled with pus, sometimes blood, sometimes yellow fluid, and the entire intestine is purplish red. Prevention and control: discharge waste and clear the pond, keep water quality clean; feed fresh feed, prepared just before eating; add 3%-5% garlic to the feed and feed for 3-6 days. (5) Hepatic steatosis. Diseased fish have a blackened body color, with the head even blacker, gill color becoming lighter, severe anemia, slowed swimming, sometimes the dorsal fin exposed above the water surface, spiral movement, and sometimes remaining motionless in slow-flowing areas. Upon opening the abdomen, the liver is yellow, yellowish-white, or light brown, sometimes congested, hardened, enlarged, or shrunken. The gallbladder is enlarged, yellow, and translucent. The end of the intestinal tract is ulcerated and congested, containing yellow mucus. Prevention and control: do not feed spoiled feed; change the feed formula, feed fresh feed and fresh animal-based feed, and feed less or no feed with high fat and carbohydrate content; add VE and VB1 and other vitamins to the feed; increase water flow, discharge sewage, and improve water quality conditions.