Prevention and Treatment Methods for Explosive Hemorrhagic Disease of Silver Carp in Hill Ponds and Reservoirs

From August to September 2005, large-scale explosive hemorrhagic disease of silver carp occurred successively in small reservoirs such as Baiyuan Reservoir and Shachong Reservoir in Pingle County, Guangxi, with an affected area of about 590 mu. Within two days, 5386 kg of fish died, with a size of 600-1000 g per fish, and economic losses reached more than 20,000 yuan. The disease strikes fiercely, has a high mortality rate, and the condition is prone to relapse and difficult to control, so it should attract great attention from the vast number of aquaculture farmers.

1 Epidemic situation and disease characteristics

The epidemic season is from March to May each spring and from August to October each autumn, with August to September in autumn being the most prevalent; the disease is characterized by acute, explosive, mass fish deaths. If treatment measures are not taken promptly, within 3-5 d of onset, the mortality rate is 80%-90%, and can even reach 100%. When water quality deteriorates severely and the pH value is below 6.5, outbreaks are very likely to occur, and in waters where the disease has occurred, silver carp cultured the following year are also prone to recurrence of the epidemic.

2 Water quality environment

The water is clear and lean, with a grayish-white water color, transparency above 30 cm, pH between 6 and 7, scarce phytoplankton in the water, abundant organic debris, and abundant zooplankton, among which rotifers are dominant.

3 Symptoms

The phenomenon of "dark floating head" often appears. In the early stage of the disease, silver carp often float about 10 cm below the water surface and swim slowly, reacting sensitively to human shadows. When the disease is severe, they react slowly or not at all, appearing dull, floating at the surface day and night, and not sinking even under daytime sunlight.

External examination: In silver carp that have just died or are about to die, the outer surface of the operculum is rough and lacks mucus, the inner membrane of the operculum has blood streaks, the lower jaw has striped or scattered red spots, the lips protrude and are purple, the base of the anal fin is red, the caudal fin has white edges and no mucus, and in severe cases there is fin erosion. After removing the opercular bone, the gill filaments can be seen to be red and swollen, with increased mucus, and the gill filaments are dark purple, separated into lobes like the teeth of a wooden comb, with a layer of blue-white glossy membrane on the surface. The reticulated gill rakers are covered with red blood spots, and in severe cases are grayish-white and corroded into notches. Dissection of the diseased fish's abdomen shows that the gallbladder, liver, and kidney are faded in color, enlarged, and fragile; there is no food in the intestine, but it is full of mucus; the anus is swollen and red.

Microscopic examination: A white membrane is attached to the outside of the gill filaments, with wave-crest-shaped spore clusters. The ends of the gill lamellae are red and swollen, with blood sinuses, and after crushing, large numbers of single microsporidia appear. There is a large amount of fluid accumulation in the abdominal cavity, along with hemolysis.

4 Preventive measures

Thoroughly clean and disinfect. After fishing is completed in winter, remove excess sludge at the bottom of the reservoir and expose it to sunlight. In March to April before the rainy season arrives, taking advantage of the lower water level and smaller water body of the reservoir, quicklime can be used at 25-40 g per cubic meter for whole-pond splashing. For reservoirs where the disease has occurred, after 7 days it is best to use strong disinfectants such as dibromohydantoin and chlorine dioxide for one thorough disinfection, both at a dosage of 0.3-0.4 g per cubic meter.

Select fish fry with strong physique and uniform size for stocking. Before the fry are put into the pond, bathe them with 2%-3% saline or 15-20 g per cubic meter potassium permanganate for 10-15 min.

During the epidemic season of fish diseases, regularly use 20-30 g per cubic meter quicklime, dibromohydantoin, or trichloroisocyanuric acid at 0.3-0.4 g per cubic meter for whole-pond splashing. Because explosive hemorrhagic disease of silver carp often occurs together with parasitic diseases, before the end of April each year it is also necessary to use Shuangxiaoling at 0.03 g per cubic meter plus copper sulfate at 0.3-0.4 g per cubic meter for whole-pond splashing, or copper sulfate at 0.3 g per cubic meter plus 0.5 g per cubic meter crystalline trichlorfon for splashing.

5 Treatment methods

Use Baoxueping at 20-25 mg per mu for whole-pond splashing, then after half an hour use copper sulfate at 0.4-0.5 g per cubic meter for whole-pond splashing, and the next day use Baoxueping again for whole-pond splashing to strengthen the therapeutic effect.

Per mu, use bleaching powder at 1.5 g per cubic meter, salt at 1.5 g per cubic meter, urea at 0.75 g per cubic meter, and trichlorfon at 0.6 g per cubic meter. Mix and dissolve the four agents together, then splash over the whole pond, twice within 3 days.

Per mu, use dibromohydantoin at 0.25-0.30 g per cubic meter and splash continuously for 2 d. When the condition is relatively severe, after an interval of 2 d, use 0.3 g per cubic meter Qiangke 101 (super iodine quaternary ammonium salt) for 1 splashing.

6 Precautions

When using medication, the volume of the reservoir water body should be calculated accurately. The error requirement is within 10%, and the reservoir capacity can be determined based on the reservoir elevation and capacity curve chart (table).

The medication method should be scientific. The dosage of external disinfectants can be adjusted according to water fertility and water temperature, but must not exceed 20% of the conventional dosage; when applying, it must be splashed evenly, and more medication should be used in deep-water areas, so as to ensure the effectiveness of the medication while preventing phytotoxicity caused by improper medication and avoiding fish death accidents. After splashing copper sulfate, attention should also be paid to observing fish activity to prevent oxygen deficiency.