Prevention and Control Methods and Technical Key Points for Common Diseases of Large Yellow Croaker

I. Cryptocaryon irritans disease (white spot disease)

Symptoms: A large number of small white spots appear on the body surface. In severe cases, the fish body is covered with a layer of white film. Due to the destruction caused by the parasites, secondary bacterial infection is triggered, resulting in inflammation and necrosis of the fish epidermis, scales easily falling off, fin rays rotting and splitting. When parasitizing the gills, it destroys the gill filaments, and after bacterial infection, it causes gill rot. When the parasites invade the fish's cornea, it causes inflammation and blindness. Diseased fish have reduced appetite, become thin, and swim erratically. The parasites can be identified by microscopic examination.

Prevention and control methods: (1) Stocking density should not be too high; (2) Soak diseased fish in malachite green solution at a concentration of 0.4×10E-6 to 1×10E-6 for 5 to 10 minutes, and soak again the next day; (3) Bathe in formalin solution at a concentration of 60×10E-6 for 15 to 20 minutes; for earthen ponds, splash the entire pond at a concentration of 20×10E-6 to 30×10E-6; (4) Prepare Shachongling with freshwater at a concentration of 50×10E-6 and soak for 5 minutes.

II. Brooklynella disease

Symptoms: It mainly parasitizes the gills of fry. When heavily parasitized, the gills appear grayish-white and are covered with many debris. It also occasionally parasitizes the body surface, forming irregular white patches. The pectoral fins extend vertically from the sides of the body. Diseased fish often float on the water surface and swim slowly. It can be identified by microscopic examination.

Prevention and control methods: (1) Soak in furazolidone solution at a concentration of 20×10E-6 in freshwater for 2 to 5 minutes. (2) Bathe in formalin solution at a concentration of 200×10E-6 in seawater with aeration for 20 minutes; alternatively, when the water flow is slow, lift the net clothing to a depth of 0.5 m, dilute formalin 50 times with seawater, and directly splash it into the cage, with the entire operation lasting 20 to 25 minutes. (3) Bathe in copper sulfate solution at a concentration of 2×10E-6 in seawater for 10 to 20 minutes (use with caution for small fry).

III. Trichodiniasis

Symptoms: When a small number of parasites are present, there are no obvious symptoms. In severe infections, it can cause increased mucus at the parasitized sites, and the fish swim slowly and die from breathing difficulties. Generally, there are no specific symptoms.

Prevention and control methods: (1) Soak in copper sulfate for 30 minutes; (2) Splash the entire pond with a mixture of copper sulfate and ferrous sulfate at a concentration of 0.7×10E-6 (5:2) for 2 consecutive days; (3) Soak in formalin solution at a concentration of 200×10E-6 to 250×10E-6 for 5 minutes.