Is the mass fish die-off caused by lack of oxygen

Characteristics of tilapia hemorrhagic disease: The epidemic season is long, with June to September being the outbreak season of the disease. When the disease occurs, the water temperature is mostly above 25°C. The mortality rate is high; during the epidemic period, the incidence rate can reach 50%-70%. According to reports, the mortality rate can reach 30%-90%, or even total loss. The disease has acute and chronic types. Acute cases mostly die within one week, while chronic cases last as long as two to three weeks or even two months. The affected water areas are extensive; both pond culture and reservoir cage culture can be affected.

Symptoms: In practice there are generally two types. The first type mainly shows congestion and redness on the body surface, mostly occurring on the lower jaw, operculum, fins and other parts; some fish have protruding eyeballs and intraocular congestion. Dissection may reveal intestinal congestion, enlarged spleen, kidney and gallbladder, and the liver changing from normal reddish-brown to earthy yellow or grayish-white. This symptom can cause acute death, and once the wrong medicine is used in the early stage, treatment will become more difficult. The other type mainly shows local congestion on the operculum and body surface, sometimes also around the orbits, abdominal distension, and pale yellow ascites accumulated in the abdominal cavity; the liver is enlarged and pale, the kidney and gallbladder are enlarged, and the intestine is congested. It differs from the typical symptoms of Streptococcus infection, in which diseased fish have protruding eyeballs, congestion around the eyeballs, and congestion and redness or severe bleeding on the inner side of the operculum. Causes: In recent years, the stocking density and culture yield of tilapia have increased substantially; pond water quality has deteriorated severely, the bottom sediment has aged, and water quality adjustment is not timely during the growing season (in some places the water cannot even be changed all year round), and ammonia nitrogen and nitrite levels in the pond water seriously exceed standards, leading to aging pond water and massive reproduction of pathogenic bacteria in the water; the feed lacks essential nutritional components, and feeding spoiled and poor-quality feed during culture causes organic lesions in organs such as the liver and kidney, reduces the body's resistance, and causes secondary bacterial infection; some farmers do not use the right medicine and misuse insecticides and pesticides; furthermore, long-term inbreeding of tilapia leads to serious degradation of its excellent genetic traits, or the parent tilapia produced by hybridization has not been sufficiently purified and rejuvenated, resulting in an insufficient male rate in the hybrid offspring line and germplasm degradation, and germplasm degradation inevitably weakens disease resistance; in addition, the uniformity of pond-cultured species is also one of the reasons for the annual outbreak of the same disease.

Prevention: It is recommended that the stocking density be appropriate, and that silver carp, bighead carp and crucian carp be polycultured in suitable amounts to remove leftover bait and regulate water quality. Select high-quality, robust fingerlings and handle them carefully to prevent injury to the fish. Thoroughly clear the pond and disinfect it, remove excess sludge, and transport it far away from the culture site. Change water and add fresh water at the right time, regulate water quality, and keep the water fresh, which can reduce the occurrence of fish diseases. Pay attention to nutrition; the feed should be hygienic and high quality, and do not feed spoiled or unclean feed. During periods when fish diseases are prone to occur, use drugs (such as the Yujunqing series) for regular prevention, and mix antibacterial drugs such as Yubikang and allicin into the feed to make medicated bait for feeding.

Treatment: Splash Xinbomie over the whole pond at 200-250 ml per 667 sq m per meter of water depth, once a day, for two consecutive times. If the water quality is poor at the time of disease onset or insecticides and other chemicals have already been misused, and this has led to reduced feeding, first splash the pond detoxification treasure series, and after stabilization, disinfect once. Focus on mixing feed with oxytetracycline 0.3%-0.5%, Kaiwei Jiedu San 0.4%, and Baogantai 0.5%; if ascites occurs, sodium cholate can also be fed separately. In addition, sulfadiazine is also used in production, and the effect is remarkable.