Common diseases and prevention of salmon and trout

Salmon and trout are the main cold-water fishes in the world, which are generally cultured in intensive and intensive way, so all kinds of fish diseases are easy to prevail, including viral fish disease, bacterial and fungal fish disease and parasitic fish disease.

1. Viral diseases

1. Infectious pancreatic necrotic disease

The pathogen is infectious pancreatic necrotic disease virus.

Vertical transmission: eggs and semen of parent fish with virus; horizontal transmission: feces, urine and secretions of diseased fish bring the virus into the water, the route of infection is Gill and mouth.

Most of the juvenile fish with symptom weight more than 5 grams are chronic, the speed of death is slow, the appearance of abdominal swelling, blackening of body color, exophthalmos, hyperemia and redness of fin base and abdomen, anus mostly dragging linear feces, slow swimming and poor feeding. Most of the fry weighing less than 5 grams are acute, the speed of death is fast, the activity of diseased fish is slow, the bottom sinks after swimming sideways or slowly, the abdomen expands, and the body color turns black.

The epidemic and harm of the disease mainly harms the fry and larvae of salmon fishes. 2-month-old seedlings often cause acute mass death, generally 7 weeks after feeding began to die, larger fry first disease, more than 20-week-old young fish generally no longer disease. The water temperature of the disease is 10 ℃ ~ 15 ℃, and the mortality rate can reach 80% ~ 100% when the water temperature is 10 ℃ ~ 12 ℃. The incubation period of the disease is related to fish size and water temperature. The bigger the fish, the longer the incubation period, and the higher the water temperature, the shorter the incubation period.

Prevent the reproduction of larvae of parent fish without pathogens, do not transport fish and eggs from the affected areas, use independent water bodies for spawning, hatching and culture, fish fry and fry should be placed in the upstream of fishing grounds to prevent horizontal transmission, prevent water pollution, disinfect sick and dead fish in time, thoroughly disinfect fish facilities and tools with high concentration of chlorine, and disinfect fish eggs with polyvinylpyrrolidone iodine 50 mg / L for 15 minutes.

The treatment pool generally controls the development of the disease by raising the water temperature.

two。 Infectious hematopoietic tissue necrosis

The pathogen is infectious hematopoietic tissue necrosis virus.

Vertical transmission: eggs and semen of infected parent fish are more likely to be transmitted by eggs; horizontal transmission: water pollution, etc. The virus mainly invades the fish through the Gill and digestive tract.

Symptoms the disease often causes sudden mass death in the fry and juvenile stage, and usually occurs 7-14 days after infection. The diseased fish swims slowly, rotates, sometimes sinks to the bottom, the body color becomes black, the abdomen is dilated, the fin base is congested, the anal suspension is opaque, mucous feces, and the lateral muscles show "V" shaped bleeding. Fish with chronic diseases often have protruding eyeballs.

The epidemic and damage are mainly harmful to salmon fry such as salmon, salmon, river trout and other fry in the same year. The incidence is the highest when the water temperature is 4 ℃ ~ 13 ℃ and 8 ℃ ~ 10 ℃. The mortality of fry under 2 months old is 100% when the water temperature is 10 ℃. The mortality rate of fry aged 2 months to 6 months is more than 50%, and that of fry older than 7 months is about 10%. In recent years, it has been found that there are also cases of rainbow trout in the range of 7 grams to 100 grams.

Prevent strict disinfection of breeding facilities and tools; fish eggs are soaked with polyvinylpyrrolidone iodine (containing 1% available iodine) 50 mg / L for 15 minutes, but the concentration can be increased to 60 mg / L when pH is alkaline, because the effectiveness of polyvinylpyrrolidone iodine is reduced under alkaline conditions; diseased fish are destroyed thoroughly; and fish fry ponds are located at the upstream of the hatchery.

The serious deterioration of the disease could be controlled by raising the water temperature of the diseased fish pond to 17 ℃ ~ 20 ℃. In addition, the feces dragged by the anus of fish with infectious hematopoietic tissue necrosis disease are thicker and longer than those of fish with infectious pancreatic necrosis disease.

3. Viral hemorrhagic septicemia

The pathogen is Egtevid virus in the family rhabdoviridae.

The way of transmission is water as the medium of transmission, vertical transmission through parent fish, and horizontal transmission through pond bottom silt and invertebrates.

The symptoms can be divided into acute, chronic and neurotic types. It generally changes from acute to chronic, and finally to nervous type. However, it is difficult to distinguish the three phenotypes, which are mainly characterized by sudden mass death and skin bleeding. In the acute stage, the fish showed blackening of body color, large amount of death, pale gills with bleeding spots, bleeding spots around the eyes, bleeding spots in adipose tissue, gonads, muscle and mesentery, light color of liver, thinning and hyperemia of kidney. After becoming chronic, the death decreases, the body color is darker, the eyeball is seriously protruding, and the anemia is more serious, especially the Gill and liver show severe anemia, the swim bladder and kidney are enlarged, and the abdominal cavity is filled with effusion, which makes the appearance of the diseased fish look like abdominal swelling. After turning to nervous type, the diseased fish rotates and sometimes sinks to the bottom after a sharp struggle. The internal organs are severely anemic and the symptoms of body surface bleeding are not obvious.

Epidemic and harm are mainly harmful to fish species and juvenile fish over 5 years old, and the disease occurs only when the fish body is more than 9 cm. Popular in late winter and early spring, the water temperature 6 ℃ ~ 12 ℃ is more common, 14 ℃ ~ 15 ℃ is less, and the cumulative mortality rate can reach 80%. It is easy to occur when the water temperature changes or the fish moves.

It is forbidden to transport fish and eggs from the affected area. Once the disease occurs, the whole fish will be destroyed. After 3 months of pond disinfection, healthy fish will be restocked. At present, there is no effective treatment.

II. Bacterial and fungal diseases

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