How can rural fish farmers effectively prevent and treat fish diseases during the overwintering period?

Fish overwintering is an important part of the freshwater aquaculture process. The success or failure of fish overwintering directly affects the economic returns of that year. However, for many years many farmers have attributed the cause of fish deaths during overwintering entirely to oxygen deficiency, so in management they mostly focus on aeration measures and rarely pay attention to the prevention and treatment of fish diseases. Yet over the years, many of the causes of mass deaths of overwintering fish have been due to outbreaks of fish diseases. Therefore, in the management of fish overwintering, one should not only pay attention to changes in dissolved oxygen but also to the prevention and treatment of fish diseases.

During the overwintering period there are as many as 50 species of pathogens affecting fish. According to our observations and research by relevant scientific research institutions, among parasites, Ichthyophthirius (ich), Chilodonella, and Bodo are especially harmful; among fungi, Saprolegnia is especially harmful; and among algal diseases, Prymnesium parvum is the most serious. When these diseases break out and spread, the mortality rate of overwintering fish can reach more than 60%, and some even suffer total loss. According to analysis, the main reasons for the spread of these diseases are as follows: First, the overwintering ponds and overwintering fish were not strictly cleared and disinfected, allowing large numbers of pathogens to be introduced into the overwintering water body. Second, the overwintering fish have too low a condition factor, are thin and weak, and have reduced immunity. Third, during seining operations, the fish bodies are seriously injured. Fourth, the stocking density of overwintering fish is too high. In view of the above reasons, the following effective measures can be taken for prevention and treatment:

1. Before stocking fish in the overwintering pond, apply 100 kg of quicklime per mu, using the dry method of pond clearing, to kill pathogens and wild miscellaneous fish.

2. Seining operations should be done carefully so as not to bruise the fish bodies. Before the fish enter the overwintering pond, they can be disinfected by soaking for 1-2 minutes in a medicinal solution prepared at a concentration of 7 ppm from a mixture of 3 parts trichlorfon crystals, 5 parts copper sulfate, and 2 parts ferrous sulfate.

3. Fish prepared for overwintering must have their autumn cultivation strengthened, and the feed structure should include a greater proportion of energy feeds (such as cornmeal). It is best to bring the fish condition factor to above 4. The stocking density should preferably not exceed 2 kg per cubic meter of water body.

4. Treat fish diseases promptly when discovered. For treatment, a mixture of copper sulfate and potassium permanganate in a 5:2 ratio can be used to make the pond water reach a low concentration of 0.3-0.4 ppm. After about 24-36 hours, the above-mentioned more harmful parasites can be killed. When applying the medicine, if the pond is ice-covered, more ice holes can be made and the medicine poured into the water; if there is no ice, it can be splashed over the whole pond. However, it should be noted that some farmers, after discovering fish diseases, still use the 5:2 mixture of copper sulfate and ferrous sulfate used during the high-temperature rearing period for treatment. This not only fails to produce a therapeutic effect but instead accelerates the death of diseased fish. Because ferrous sulfate is a reducing agent, although it can also reduce copper sulfate ions and play a role in killing parasites, under low-temperature conditions the reaction rate is very slow, and ferrous sulfate often causes the mucus on the fish body to slough off, increasing the content of organic matter in the water and reducing the efficacy of copper sulfate. At the same time, the increase in organic matter increases the oxygen-consuming factors in the overwintering water body, causing oxygen deficiency in the pond water and accelerating the death of diseased fish. Potassium permanganate is an oxidizing agent; it can not only kill harmful protozoa, fungi, and gliding bacteria outside the fish body, but also react with organic substances in the water to produce inactive compounds. While potassium permanganate oxidizes organic matter, it also increases the efficacy of copper sulfate. In addition, these two drugs remain in the water for a relatively short time, so they not only have no side effects on diseased fish but also do not cause pollution. This is the reason why potassium permanganate is used instead of ferrous sulfate during the overwintering period.