Methods and Technical Key Points for Disease Prevention and Control in Largemouth Bass

I. Prevention and control of common fish diseases:
Largemouth bass has strong disease resistance and generally rarely becomes ill. However, at the fingerling stage it is susceptible to saprolegniasis and infestation by trichodina, chilodonella and other protozoa, which can cause death in severe cases. Therefore, prevention should be the priority, combining prevention with treatment.
1. Medicated baths can be given before fingerlings are stocked:
(1) Soak in 3-5% saline or methylene blue and other drugs for about 15 minutes before stocking in the pond, which can prevent trichodina, saprolegniasis and scale protrusion disease.
(2) Soak in 15-20 ppm potassium permanganate (i.e. add 15-20 g potassium permanganate to 1000 kg water) for about 15 minutes, which can prevent infestation by protozoan parasites.
2. Regularly splash drugs. Use 0.7 ppm copper sulfate and ferrous sulfate mixture or insecticide preparations such as miechongjing to kill parasites, then disinfect with bactericides such as chlorine dioxide.
Each time after seining, the fish body will be somewhat damaged, so 0.2 ppm chlorine dioxide should be splashed for disinfection; if saprolegniasis infection occurs, never use malachite green; instead use methylene blue or a mixture of salt and baking soda splashed on the pond for treatment. When the pond water is found to be too fertile and water quality deteriorates, appropriately splash quicklime.
3. Regularly administer medicated feed orally. Starting from June, once a month, mix the drugs into largemouth bass feed, with 3-5 days as one treatment course. Oxytetracycline 5-10 g per day per 50 kg fish, furazolidone 5 g per day per 50 kg fish. In addition, frequently adding aquaculture-specific vitamins can improve disease resistance.
　　
II. Main characteristics of explosive diseases and their prevention and control techniques
1. Main characteristics of explosive diseases:
(1) The main symptoms of the disease are putrefaction on both sides of the body surface, accompanied by congestion at the base of the fin rays; sick fish float and swim slowly at the water surface, and cannot dive underwater when startled. Microscopic examination of the lesions reveals no parasites; dissection reveals inflammation of the internal organs, a small amount of ascites, and an enlarged, yellowed, necrotic liver.
(2) The disease strikes fiercely; once sick fish are found, it will cause massive outbreaks and death within three days. Prevention and control are relatively difficult; external disinfection with bactericides combined with oral antibiotics and antiviral drugs has shown no obvious effect.
(3) The disease mainly affects fingerlings and adult fish in high-density cage culture; in affected waters the incidence rate in cages often reaches 100%, and the mortality rate is also very high, even wiping out the entire stock.
2. Analysis of the causes of the disease
After analysis, we preliminarily believe that the disease is caused by nutritional deficiency, decreased disease resistance and secondary bacterial infection. Because:
(1) Largemouth bass cage culture mainly feeds frozen fish chunks; since frozen fish contain large amounts of “vitamin-degrading enzymes”, eating them will cause vitamin deficiency in largemouth bass; in addition, when feeding fish chunks, if they are not thoroughly thawed, the largemouth bass will suffer indigestion after eating them, and during storage and transportation of frozen fish, histamine produced by protein decomposition combines with free amino acids to produce “putrescine”, which damages the internal organs.
(2) The cage placement in the culture waters is unreasonable, with too small spacing between cages; the cage placement area has been used for culture for many years, and the substrate is poor; the culture density inside the cages is too high, making cross-infection and disease outbreaks likely.
3. Prevention and control methods:
(1) It is best to feed fresh fish chunks; if frozen fish chunks are used, they must be thoroughly thawed and disinfected with 3% saline or furazolidone before feeding.
(2) Frequently add aquaculture-specific vc and multivitamins, with an addition amount of 1-3‰ of the weight of the feed fish.
(3) Cage placement and stocking density should be reasonable, and the cages and culture waters should be disinfected regularly.
(4) Regularly thoroughly clean the netting to avoid dirt, residual feed and attachments blocking the mesh, which would cause poor exchange of water flow inside and outside; avoid dead fish and residual feed sinking to the bottom of the cage and causing pollution.
(5) After the disease occurs, aquaculture-specific vc and multivitamins can be administered orally, combined with external disinfection using chlorine dioxide and trichloroisocyanuric acid; results can be seen in three days. The specific method is: add 25 g each of aquaculture-specific vc and multivitamins per 50 kg fish per day to improve the fish body's own disease resistance. Splash chlorine dioxide inside the cage at a concentration about 3 times the conventional dosage; splash trichloroisocyanuric acid at 0.3 mg/l to disinfect the culture waters outside the cage.