Detailed Explanation of Common Diseases in Salmon and Trout and Their Prevention Methods

Salmon and trout are major cold-water fish species cultured worldwide, generally raised through relatively intensive farming methods, which makes various fish diseases prone to outbreaks. These mainly include viral fish diseases, bacterial and fungal fish diseases, and parasitic fish diseases.

I. Viral Diseases

1. Infectious Pancreatic Necrosis

Pathogen: The pathogen is infectious pancreatic necrosis virus.

Transmission routes: Vertical transmission: eggs and milt of virus-carrying broodstock; horizontal transmission: feces, urine, and secretions of diseased fish introduce the virus into the water. The infection routes are the gills and mouth.

Symptoms: Juvenile fish weighing more than 5 grams mostly show the chronic form, with slower death. Externally, the abdomen is distended, body color darkens, eyeballs protrude, the fin bases and abdomen are congested and reddened, the anus mostly trails string-like feces, swimming is slow, and feeding is poor. Fry weighing less than 5 grams mostly show the acute form, with rapid death. Diseased fish move sluggishly, swim on their side or slowly rotate before sinking to the bottom, the abdomen is distended, and body color darkens.

Epidemiology and harm: This disease mainly affects fry and juvenile fish of Salmonidae. Fry at 2 months after first feeding often suffer acute mass mortality. Death generally begins to appear 7 weeks after first feeding. Larger fry are affected first, and juvenile fish over 20 weeks of age generally no longer develop the disease. The disease occurs at water temperatures of 10°C to 15°C. At water temperatures of 10°C to 12°C, mortality can reach 80% to 100%. The incubation period of the disease is related to fish size and water temperature: the larger the fish, the longer the incubation period; the higher the water temperature, the shorter the incubation period.

Prevention: Do not use pathogen-carrying broodstock to reproduce offspring; do not transport fish and eggs from affected areas; use independent water bodies for spawning, fry hatching, and rearing; fry and fingerlings should be placed at the most upstream part of the farm to prevent horizontal transmission; prevent water source contamination; diseased and dead fish should be destroyed promptly; thoroughly disinfect aquaculture facilities and tools with high-concentration chlorine preparations; disinfect fish eggs with povidone-iodine at 50 mg/L for 15 minutes.

Treatment: Affected ponds generally control the progression of the disease by raising the water temperature.

2. Infectious Hematopoietic Necrosis

Pathogen: The pathogen is infectious hematopoietic necrosis virus.

Transmission routes: Vertical transmission: eggs and milt of virus-carrying broodstock, with transmission through eggs being more likely; horizontal transmission: contamination of water sources, etc. The virus mainly invades the fish body through the gills and digestive tract.

Symptoms: This disease often causes sudden mass mortality during the fry and juvenile stages, generally with onset 7 to 14 days after infection. Diseased fish swim sluggishly, rotate, and sometimes sink to the bottom; body color darkens, the abdomen swells, fin bases become congested, opaque mucus-like feces hang from the anus, and the muscles on the sides of the body show "V"-shaped hemorrhages. Chronic cases often have protruding eyeballs.

Epidemiology and harm: It mainly affects fry and fingerlings of the year of salmonid species such as @#@241@#@, chum salmon, and river trout. The disease occurs at water temperatures of 4°C to 13°C, with the highest incidence at 8°C to 10°C, and outbreaks stop above 15°C. For fry under 2 months of age at a water temperature of 10°C, mortality reaches 100%; for fingerlings 2 to 6 months old, mortality is greater than 50%; for fingerlings older than 7 months, mortality is about 10%. In recent years, cases have also been found in rainbow trout weighing 7 grams to 100 grams.

Prevention: Strictly disinfect aquaculture facilities and tools; soak fish eggs in povidone-iodine (containing 1% available iodine) at 50 mg/L for 15 minutes, but when the pH is alkaline the concentration can be increased to 60 mg/L, because povidone-iodine is less effective under alkaline conditions; thoroughly destroy diseased fish; set fry ponds at the most upstream part of the hatchery.

Treatment: Raising the water temperature of ponds containing diseased fish to 17°C to 20°C can control severe worsening of the disease. In addition, the feces trailing from the anus of fish with infectious hematopoietic necrosis are thicker and longer than those of fish with infectious pancreatic necrosis.

3. Viral Hemorrhagic Septicemia

Pathogen: The pathogen is the Egtved virus in the family Rhabdoviridae.

Transmission routes: Water is the transmission medium, and vertical transmission occurs through broodstock. In addition, pond bottom mud and invertebrates serve as horizontal transmission routes.

Symptoms: There are three forms: acute, chronic, and nervous. Generally, the acute form turns into the chronic form and finally into the nervous form. However, the three phenotypes are not easy to distinguish clearly. The main manifestations are sudden mass mortality and skin hemorrhage. In the acute phase, fish show darkened body color and high mortality; the gills are pale with hemorrhagic spots; hemorrhagic spots can be seen around the eyes; hemorrhagic spots appear in internal fat tissue, gonads, muscles, and mesentery; the liver is pale, and the kidney becomes thin and congested. After turning chronic, mortality declines, body color becomes darker, eyeballs protrude severely, and anemia becomes more severe, especially with severe anemia shown in the gills and liver; the swim bladder and kidney swell, and the abdominal cavity fills with fluid, making the diseased fish appear to have a distended abdominal cavity. After turning nervous, diseased fish perform rotating movements and sometimes sink to the bottom after violent struggling. Internal organs show severe anemia, and external hemorrhage symptoms are not obvious.

Epidemiology and harm: It mainly affects fingerlings and juvenile fish over 5 years old, and fish generally develop the disease only when larger than 9 cm. It is prevalent in late winter and early spring. More cases occur at water temperatures of 6°C to 12°C, and fewer at 14°C to 15°C. Cumulative mortality can reach 80%. It is prone to occur when water temperature changes or fish are moved.

Prevention: Do not transport fish and eggs out of affected areas. Once an outbreak occurs, destroy all fish on the farm, disinfect the ponds, and after 3 months restock with healthy fish. There is currently no effective treatment.

II. Bacterial and Fungal Diseases