How to prevent and treat common diseases in ayu (sweetfish) farming?

I. Vibriosis

1. Pathogen: mainly Vibrio anguillarum.

2. Symptoms: depending on the species of Vibrio infecting the fish, it can be roughly divided into three types: yellow-liver type, pointed-intestine type and ulcerative type. Sick fish swim weakly, have darkened body color, protruding eyeballs and skin hemorrhages that in severe cases extend deep into the muscle layer; there is congestion and then ulceration on the inner surface of the operculum, the base of the fins and the tip of the snout; the anus is red and swollen; the gills are pale; the liver of sick fish is enlarged, in some cases entirely faded to a yellowish-earth color, in others partially faded and congested; blister-like structures filled with bloody fluid appear in the liver of sick fish; the gallbladder is enlarged, and the sac is filled with orange-yellow or brown-green bile; the kidney loses its elasticity, and in some sick fish the kidney is enlarged or shows some white block-like masses, sometimes with clear ascites accumulating. Ayu suffering from vibriosis generally have mixed infections with other pathogens, but small fish with the acute form may die without any symptoms.

3. Prevention and treatment: (1) Do not feed oxidized or spoiled feed; feed high-quality complete compound feed, and pay attention to feeding frequency and feeding method; keep the water clean to eliminate bacterial reproduction; (2) splash the whole pond with 0.2 ppm to 0.5 ppm chlorine dioxide, once a day, for 2 consecutive days.

II. Streptococcosis

1. Pathogen: Streptococcus.

2. Symptoms: sick fish swim slowly, scatter in slow-flowing areas, float on the water surface or hang head-up and tail-down; when dying, sick fish either swim wildly at intervals or turn belly-up; body color darkens, the body becomes thin and weak, gill color fades, showing an anemic state; eyeballs are congested, protruding, cloudy, and may even fall out; there are pinpoint hemorrhages on the ventral side, the anus is red and congested, the intestine is inflamed, and ascites accumulates.

3. Prevention and treatment: (1) Avoid excessive stocking density, strengthen feeding management, feed high-quality feed, add vitamins at the appropriate time, and pay attention to pond cleanliness and disinfection; once disease occurs, stopping feeding for 2 to 3 days helps delay the condition; (2) for every 100 kg of ayu, mix 5 g of antibiotics and 1 g of compound vitamin B into the feed and use for 5 to 10 consecutive days, with good results; (3) thoroughly clean the pond and change the water, keep the pond water at 60 cm, and soak with 3 ppm bleaching powder (available chlorine content 18%) for 30 minutes.

III. Enteritis

1. Pathogen: Aeromonas punctata.

2. Symptoms: the intestine is congested and inflamed, and the anus is red and swollen. In the early stage of the disease, when the intestine is cut open, the intestinal wall is seen to be congested and inflamed, there is no food in the intestinal cavity, or only a small amount of food in the posterior section of the intestine, and there is relatively much mucus in the intestine; in the late stage of the disease, the entire intestine appears red, the intestinal wall has poor elasticity, and there is no food in the intestine, only pale-yellow mucus.

3. Prevention and treatment: (1) On rainy days when water temperature drops, reduce the amount of bait; when the condition is not serious, it can recover naturally after 1 to 2 days without feeding; (2) when the condition is serious, add norfloxacin 1 g/kg to 2 g/kg and vitamin C 2 g to 3 g to the feed, mix well and feed, 3 times a day, for 3 to 5 consecutive days.

IV. Trichodiniasis

1. Pathogen: Trichodina, mainly harming fry and fingerlings. It occurs year-round across the country, and ponds with turbid water and abundant organic matter are more prone to it. The suitable reproduction temperature is 20°C to 28°C, and it is more prevalent from April to July.

2. Symptoms: at the fry and fingerling stage, Trichodina parasitizes the body surface, and when the fish are larger it parasitizes the gills. When only a small number of parasites are present, there are no obvious symptoms; in severe cases it infects the skin, fins and gills. Trichodina uses its denticular ring to abrade the epidermal cells of the ayu's body surface and the epithelial cells of the gill filaments and fin rays, stimulating the fish to produce large amounts of mucus. Sick fish do not feed, swim wildly in groups along the pond, and jump in the water; this can destroy and slough off the epithelial cells of the gill filaments and body surface, leading to bacterial invasion and causing mass mortality of fish.

3. Prevention and treatment: (1) Before stocking fish, thoroughly clear the pond with 150 ppm to 200 ppm quicklime or 10 ppm bleaching powder; (2) splash 0.3 ppm "Yuchongqing" over the whole pond, and change the water after 10 hours; (3) splash the whole pond with a 0.3 ppm mixture of copper sulfate and ferrous sulfate (5:2), change a large amount of water after 12 hours, and repeat once after a day; (4) splash the whole pond with a decoction of 5 ppm neem (chinaberry) bark; (5) at night, splash the whole pond with 1 ppm to 2 ppm potassium permanganate.

V. Argulosis (fish louse disease)

1. Pathogen: fish lice (Argulus).

2. Symptoms: fish lice parasitize the body surface and gills of ayu, sucking the fish's blood, causing the fish to gradually become emaciated. When feeding, they use their oral sting and mandibles to pierce the fish's skin everywhere, creating many wounds that then become infected with microorganisms and cause death. The venom secreted by the lice when sucking blood with the oral sting irritates the fish, making them extremely restless in the water, swimming wildly and jumping.

3. Prevention and treatment: (1) Before stocking, clear the fish pond with quicklime while retaining the water, to kill adult fish lice, larvae and egg masses in the water; (2) soak with a 30 ppm diluted formalin solution for 12 hours, for 3 consecutive days.

VI. Saprolegniasis

1. Pathogen: Saprolegnia. This fungus is not selective about the species of aquatic animals; any injured one can be infected, while uninjured ones are never infected. It is a secondary infection in aquatic animals.

2. Symptoms: in the early stage of infection, symptoms are not obvious; afterward the hyphae invade from the wound, and cotton-like hyphae become visible to the naked eye. Sick fish float on the water surface, swim slowly, have reduced appetite, and finally die of weakness. It mainly occurs at the stage when fry have just been put into the tank.

3. Prevention and treatment: (1) Try to avoid injuring the fish during handling operations; (2) splash the whole pond with a 0.28 ppm mixture of copper sulfate and ferrous sulfate, change the water after 24 hours, once a day, for 2 consecutive days.

VII. White skin disease

1. Pathogen: Pseudomonas dermoalba.

2. Symptoms: in the early stage, small white spots appear on the caudal peduncle or the base of the dorsal fin, then spread and enlarge rapidly, causing the rear half of the fish to become entirely white; when the condition is severe, the caudal fin of the sick fish completely rots away, the head points down and the tail up, the body is vertical to the water surface, and death follows soon after. It is mainly caused by infection after injury to the fish due to improper handling.

3. Prevention and treatment: (1) Avoid injuring the fish; soak with 0.4 ppm strong chlorine for 20 minutes while also administering antibiotics orally; (2) crush 2 ppm to 4 ppm Chinese gallnut, soak in hot water, and splash the whole pond with both the residue and the liquid; (3) splash the whole pond with 1 ppm to 2 ppm bleaching powder.