What are the prevention and treatment methods for mullet diseases?

Mullet belongs to the order Mugiliformes, genus Liza, and its external morphology is similar to that of the grey mullet, but its adipose eyelid is not well developed. It lives in coastal waters and estuaries and can also live in freshwater, being a eurythermal and euryhaline fish. Under natural conditions, it scrapes and feeds on diatoms and decaying organic matter, and also eats benthic organisms. In pond aquaculture, artificial compound feed can be used. Among cultured species, there is a saying of "grey mullet in the south, mullet in the north." In Jiangsu and areas to the north, mullet farming is mainly practiced.

In recent years, the momentum of using freshwater ponds to culture mullet in coastal areas has developed rapidly, shifting from polyculture to primary culture. With the increase in stocking density, various diseases have also increased day by day, and in some areas, diseases have even caused total loss of harvest. In response to the problems encountered in the production process, we conducted experiments and summarized them. The diagnosis and prevention methods of common diseases in the mullet farming process are introduced as follows:

1. Saprolegniasis

Epidemiology and symptoms: Saprolegnia mainly occurs in early spring and winter, and can grow and reproduce at water temperatures of 5-26°C. Any injured or frostbitten fish can be infected. The wounds of diseased fish are grayish-white with cotton-like hyphae. The diseased fish have a darkened body, are restless, and rub against solid objects in the pond. Some diseased fish show raised scales at the parasitic sites.

Prevention and treatment methods: (1) Strengthen feeding management to improve the fish's own resistance and avoid scale loss and frostbite. (2) For injured fish, soak and wash with a mixed aqueous solution of 400 g/cubic meter salt and 400 g/cubic meter baking soda. (3) Sprinkle thioether sarafloxacin over the entire pond. (4) Sprinkle "Shuimeijing" over the entire pond. (5) Oral antibiotics to prevent bacterial infection and rapidly control the condition.

2. Trichodiniasis

Epidemiology and symptoms: It mainly occurs at the fry and fingerling stages, and can be seen year-round, but is most common from May to August. After onset, the fish body color darkens, a large amount of mucus is secreted, and gill rot occurs. Generally, microscopic examination can confirm the diagnosis.

Prevention and treatment methods: (1) Thoroughly clear the pond with quicklime or bleaching powder. (2) A mixture of copper sulfate and ferrous sulfate (5:2), sprinkle 0.7 g/cubic meter over the entire pond. (3) Bathe in a 10-20 g/cubic meter potassium permanganate aqueous solution for 10-30 minutes, or bathe in 8-10 g/cubic meter of a copper sulfate and ferrous sulfate (5:2) mixture. (4) Use 20-30 kg of chinaberry branches and leaves per mu at 1 m water depth, boil the juice, and sprinkle it. (5) Use 3-5 kg of fresh Chinese chives per mu at 1 m water depth, mash them, add 100 g of salt, and sprinkle over the entire pond. (6) Orally administer "Yuchong Kexing" at 1 g per kg of feed.

3. Dactylogyriasis

Epidemiology and symptoms: This disease is mainly prevalent in late spring and early autumn, with a suitable temperature of 20-50°C. It mainly parasitizes the fish's body surface and gill filaments. A small amount of parasitism shows no obvious symptoms. Under microscopic examination, many worms can be seen wriggling on the gill filaments. The gill filaments swell and produce a large amount of mucus.

Prevention and treatment methods: (1) Sprinkle 0.3-0.7 g/cubic meter of crystalline trichlorfon over the entire pond. (2) Use "Ruchongjing" (albendazole) according to the dosage in the instructions. (3) Mebendazole orally at 50 mg per kg of fish body weight each time, once a day, for 2-3 consecutive times.

4. Gill rot disease

Epidemiology and symptoms: This disease is prevalent from April to October, when the water temperature is above 20°C. It is divided into bacterial gill rot and gill rot caused by parasites. Diseased fish have a darkened body color, slow response, and reduced appetite. In severe cases, they swim alone away from the group at the water surface. There are green, gray, white, or wax-yellow spots on the gill lamellae, and the gill lamellae have abundant mucus and often adhere to mud. In severe cases, the inner epidermis at the center of the operculum becomes congested and rots into a round shape, hence the saying "open skylight." Microscopic examination of gill filament tissue: if parasites are present, it can be diagnosed as gill rot caused by parasites; if no worms are seen but columnaris flexibacter is seen, it can be confirmed as bacterial gill rot.

Prevention and treatment methods: For gill rot caused by parasites, the parasites must first be killed, and then the gill rot treated. (1) Sprinkle bleaching powder at 1 g/cubic meter over the entire pond, or sodium dichloroisocyanurate or trichloroisocyanuric acid at 0.3-0.5 g/cubic meter, once a day, for 2-3 consecutive days. (2) Double quaternary ammonium salt drugs such as BKC and other surfactants, 2-3 times, once every other day; the effect is particularly remarkable. (3) Chlorine dioxide disinfectant, use according to product instructions. (4) In high-temperature seasons, reduce the use of dichloro and trichloro, and use povidone-iodine solution instead, once a day, for 2-3 consecutive times.

5. Hemorrhagic disease

Epidemiology and symptoms: It mainly occurs from April to November, and can occur when the water temperature is 9-36°C, but in mullet farming it is mainly from June to September. This disease is easily confused with hepatobiliary syndrome. In the early stage, diseased fish show mild congestion symptoms in the oral cavity, jaw, operculum, orbit, fins, and both sides of the body, with a small amount of food in the intestine. In severe cases, the congestion symptoms intensify, with exophthalmos, abdominal distension, ascites in the abdominal cavity, enlargement of the liver, spleen, and kidney, intestinal congestion, gray-white gills and purple gills, accompanied by swollen gills and gill rot symptoms.

Prevention and treatment methods: Use a combination of oral and external prevention and treatment methods. External use generally includes: (1) Trichloroisocyanuric acid 0.3-0.5 g/cubic meter; for 3 consecutive days, once a day. (2) Dual-element chlorine dioxide 0.2 g/cubic meter, once every other day, for 2-3 consecutive times. (3) In high-temperature seasons, use unitary chlorine dioxide and povidone-iodine alternately for 2-3 times. Oral use generally includes: (1) Per kg of powdered feed, use enrofloxacin 1 g, Vc 2 g, and liver-protecting drugs 5-10 g. (2) Per kg of powdered feed, add povidone-iodine powder 5-10 g and Vc 2-4 g.

6. Hepatobiliary syndrome

Epidemiology and symptoms: Mullet hepatobiliary syndrome mainly occurs in summer. The symptoms shown by mullet are not obvious, and there are also symptoms similar to hemorrhagic disease. In the early stage, diseased fish have no obvious symptoms, the intestine is full of food, and there is no intestinal congestion, only purple gills and white gills, with sporadic deaths occurring in the pond. With the arrival of summer high temperatures, the mortality rate intensifies, and the symptoms become obvious: purple gills, body surface congestion, liver appearing earth-yellow or purple, mottled liver, and gallbladder enlargement appearing black or dark green, but most have no ascites.

Prevention and treatment methods: (1) Before entering the high-temperature season, regularly use water quality improvers and substrate improvers, etc. (2) In high-temperature seasons, reduce the use of external insecticides. (3) When the disease occurs, reduce the use of chlorine preparations, and use double quaternary ammonium salts, povidone-iodine, glutaraldehyde, etc., at the minimum dosage according to the product instructions. (4) Orally, per kg of powdered feed, add enrofloxacin 1 g, Vc 2-4 g, liver and gallbladder protecting drugs 5-10 g, and Sanhuang powder (use according to the instructions).