A Detailed Guide to the Main Diseases of Large Yellow Croaker and Their Prevention and Treatment Methods

I. Bacterial Diseases

1. Vibriosis

Pathogen: Vibrio parahaemolyticus.

Symptoms: The appearance of ulcers on the body surface of diseased fish is the typical feature of this disease. In the early stage, there are petechiae, ecchymoses, and irregular red patches on the body surface, mostly seen on the abdomen and tail. In severe cases, there is bleeding and ulceration of the lower jaw, congestion and redness at the fin base, missing fin rays, and rotting of the caudal peduncle muscle, forming hemorrhagic ulcers. The liver, spleen, kidney, and intestine are all congested, the liver is enlarged and earth-yellow in color, and there is pale yellow mucus in the intestinal tract. Microscopic examination of the tissue at the lesion site reveals slightly curved bacteria.

Epidemiology: This disease is the most common among the diseases of large yellow croaker. It occurs from March to October each year, and is common after June, with frequent occurrences in fish weighing about 80 grams. The disease course is long, infection is rapid, and mortality is high.

Prevention and treatment methods: 1. Oral administration of antibacterial medicated feed such as norfloxacin and ofloxacin, adding 2-4 grams per kilogram of feed, for a course of 3-7 days. 2. Soak and wash with formalin at 200 mL/cubic meter plus norfloxacin and other antibacterials at 20 grams/cubic meter in oxygenated seawater for 20-30 minutes. 3. Hang 1 tablet of "Fulü Type I" per cage, and at the same time orally administer "Fish Health No. 1" or "Fish Health No. 1-2000", using the dosage according to the instructions.

2. Bacterial Enteritis Disease

Pathogen: Aeromonas punctata intestinal type.

Symptoms: In the early stage, there are no obvious symptoms on the body surface of diseased fish. The typical symptoms in the later stage are red and swollen anus, abdominal distension, and ascites. Dissection reveals congestion and inflammation of the intestinal tract appearing purplish-red, poor elasticity of the intestinal wall, no food or a small amount of food in the intestinal tract, and a large amount of yellow mucus.

Epidemiology: The peak epidemic occurs in summer and autumn. The disease course is short and mortality is relatively high; mortality can reach more than 80% within 3-5 days.

Prevention and treatment methods: Prevention is the same as for vibriosis. Treatment: Do not use medicated bathing; first stop feeding, then feed a small amount, and add an appropriate amount of antibiotics for treatment. 1. Oral administration of medicated feed with norfloxacin or berberine hydrochloride, adding 2-4 grams of the drug per kilogram of feed, for a course of 3-5 days, with the dosage doubled on the first day. 2. Hang 1 tablet of "Fulü Type I" per cage, and orally administer medicated feed of "Fish Health No. 1" or "Fish Health No. 1-2000" together with "Special Vc for Aquaculture", adding 20 grams of "Fish Health No. 1" or "Fish Health No. 1-2000" and 2 grams of "Marine Fish Multivitamin" per kilogram of feed, for a course of 3 days. In severe cases, medicated feed must be fed continuously until cured, and then fed for 2 more days to consolidate the efficacy.

II. Parasitic Diseases

1. Amyloodiniosis (also known as Amyloodinium disease)

Pathogen: Amyloodinium ocellatum, also known as ocellate amyloodinium.

Symptoms: The parasites mainly infest the gills, skin, and fins of fish. Many small white spots are visible to the naked eye on the gills; the skin tissue on the body surface becomes inflamed, bleeds, and even ulcerates. Diseased fish are restless, dart up and down, and rub their bodies against the netting.

Epidemiology: The epidemic seasons are March to June and September to October. From March to June, mainly broodstock, fry, and juvenile fish are affected, with a disease onset water temperature of 18-25°C. From September to October, it mainly occurs in large yellow croaker cultured in earthen ponds; both seedlings and grow-out fish can be infected, with a disease onset water temperature of 25-29°C. The disease onset is acute, spreads quickly, and is highly harmful, often causing mass mortality of large yellow croaker. In low-salinity estuary areas, the severity of the disease is relatively mild. The occurrence of this disease is related to the pathogen being introduced directly into nursery ponds by copepods or sand-filtered water, as well as the cross-use of nursery tools.

Prevention and treatment methods: Reduce the seawater specific gravity to below 1.01 to prevent reproduction of this pathogen. Main treatment methods: 1. Soak and wash diseased fish in freshwater for 5-10 minutes, and repeat once after 3-4 days. 2. Soak and wash in freshwater with copper sulfate at 10 grams/cubic meter for 10-15 minutes, for 4 consecutive days. Or soak and wash in oxygenated seawater with a mixture of copper sulfate and ferrous sulfate (5:2) at 10 grams/cubic meter for 10 minutes.

2. Cryptocaryoniasis (also known as marine fish white spot disease)

Pathogen: Cryptocaryon irritans, also known as "marine ich."

Symptoms: The parasites mainly infest the body surface, gills, cornea, and oral cavity of fish. In the early stage, the fish rub against the pond bottom, pond walls, or netting due to irritation and itching, or jump up and down at the water surface. In the middle stage, mucus secretion increases at the infested sites, and many small white spots are visible to the naked eye; in severe cases, the body surface is covered with a white film, and some fish become blind in one or both eyes. Diseased fish stop feeding, have difficulty breathing, and die from suffocation, or die from secondary infection. Microscopic examination of the white spots on the body surface or gill mucus generally confirms the diagnosis when 5-8 parasites are present per field of view.

Epidemiology: The peak epidemic is from May to August. Seedlings, grow-out fish, and spawning broodstock can all be infected, with the highest incidence at the seedling stage. In cage culture, it is most likely to occur in sea areas with no water flow or poor water flow, poor water quality, rich organic matter content, and high-density culture. In pond culture, it is most likely to occur in ponds with low water exchange rates and severe pond water pollution.

Prevention and treatment methods: Reduce stocking density, disinfect regularly, and improve water quality. 1. Soak and wash in freshwater for 5-10 minutes. 2. Soak and wash in oxygenated seawater with formalin at 200 mL/cubic meter for 20 minutes; or bathe with 20-25 grams/cubic meter for 20-24 hours, then completely change the water or transfer to another pond. 3. Soak and wash in freshwater with copper sulfate at 10 grams/cubic meter for 10-15 minutes. 4. Use miechongling at 60-100 grams/cubic meter; in the early morning, pull the netting to one side and lift it to about 0.5 meters above the water surface and splash evenly, maintaining this for 20-30 minutes, or scoop the fish into a canvas bag for soaking, but aeration by an aerator is required.

3. Petalosomiasis (also known as white spot disease)