Case Analysis of Fish Disease Outbreak Prevention and Control in a Small Reservoir

In early October 2005, an outbreak of hemorrhagic disease occurred in the fish farmed in Tuonan Reservoir in our town. Through our active treatment, the disease was basically brought under control. The prevention and control situation is summarized as follows:

1. Basic information

Under normal conditions, Tuonan Reservoir has a water area of 100 mu, and 150 mu during the high-water period, with an average water depth of 3 m and a maximum depth of 6 m. Artificial aquaculture in this reservoir began in the early 1990s, mainly with conventional fish species, with supplementary stocking of freshwater shrimp, yellow catfish, and others. The reservoir water is clear and has no history of epidemic disease. In recent years, with the continuous increase in stocking density, the stocking structure of fish species has been rather unreasonable, external pathogens have invaded, fish species were not disinfected at the time of stocking, and in addition, the influence of comprehensive factors such as irrational drug use in the past two years has meant that fish diseases have occurred from time to time in this reservoir and shown an upward trend. In early October 2005, this led to a large-scale outbreak of hemorrhagic disease.

2. Symptoms of diseased fish

In this disease outbreak, silver carp and bighead carp, and crucian carp were the main species affected. The symptoms were as follows: the oral cavity, head, orbit, operculum epidermis, and fin base of the diseased fish were congested, the muscles on both sides were mildly congested, the eyeballs protruded, the abdomen was distended, and the anus was red and swollen; dissection of the diseased fish showed grayish-white gill filaments, sometimes purple, and in severe cases the ends of the gill filaments were rotten, with red or yellow ascites in the abdominal cavity, enlargement of the liver, spleen, and kidney, congestion of the intestinal wall, and no food in the intestine.

3. Treatment process

After this disease occurred, 50-100 kg of fish died daily. From the onset of the disease on October 2 to the 5th, no measures were taken. After on-site investigation and understanding, the author adopted the following treatment measures: first, Yujing High Copper was splashed over the entire pond at a dosage of 15 ml per mu at 1 m water depth; on alternate days, bromochlorohydantoin plus fish and shrimp strong chlorinated lime (1:1) plus Yudazhuang mixture was splashed over the entire pond, with the concentration of the first two drugs at 0.3 mg/L and Yudazhuang at 50 g per mu. After the drugs were used, the number of dead fish decreased day by day, and basically stopped after 5 days.

4. Recommended prevention and control measures

In view of the characteristics of general reservoirs, such as a relatively large water area, deep water level, high medication cost, and difficulty in prevention and control, the following comprehensive prevention and control measures are recommended.

1. Strictly disinfect fish species. Resolutely eliminate any mentality of taking chances, and insist on disinfecting the fish body with drugs when stocking fish species. A 3%-5% salt water solution or 10 mg/L potassium permanganate may be used.

2. Minimize mechanical injury when seining and transporting fish species.

3. If freshwater shrimp and river crab are polycultured in the water body, do not use insecticides. If they really must be used, first choose the mixture of copper sulfate and ferrous sulfate (5:2) or Yujing High Copper, and try to avoid causing major harm to shrimp and crabs.

4. If there is no history of medication use in the water body, it is recommended to routinely splash bleaching powder over the entire pond for prevention, which can greatly reduce medication costs and also gives good preventive results. If fungicides have been used many times, it is advisable to change the drug to avoid the development of drug resistance, and it is advisable to use bromine preparations or bromochloro preparations that have little impact on water and fish and few side effects. According to this treatment situation, treatment with dibromohydantoin plus bromochlorohydantoin (1:1) or fish and shrimp strong chlorinated lime plus bromochlorohydantoin (1:1) mixture gave very good results; when combined with Yudazhuang (VC, VE), the immunity of the fish body is enhanced and wound repair is accelerated, giving even better results.