When white spot disease (WSSV), Taura disease (TSV) and other viral diseases ravage the shrimp industry in China, few farmers will pay attention to the hepatopancreatic problems of shrimp. However, in recent years, various diseases caused by hepatopancreas enlargement, atrophy, blurring and discoloration have caused serious losses to farmers, and protecting the liver has become a difficult problem that cannot be ignored in shrimp culture.
Why is the hepatopancreas of prawn so fragile in recent years? There are mainly the following reasons.
The first is the cultivation of prawns. At present, shrimp culture only pursues the resistance to viral diseases and growth rate, while ignoring the strengthening of digestive system and the improvement of immunity. In the case of energy conservation, the enhancement on the one hand will inevitably lead to the weakening of other aspects.
The second is the deterioration of the breeding environment. At present, the agricultural wastewater in China has exceeded the industrial wastewater, which means that the whole agriculture, including aquaculture, is at the expense of environmental damage. Moreover, the deterioration of the ecological environment in China has reached an alarming level.
Thirdly, with the increase of culture density and the extensive use of compound feed, the positive roles of phytoplankton, zooplankton, prawns and water bodies in the culture process are ignored or unilaterally replaced. It is precisely because of many objective reasons that protecting the liver has become a routine topic throughout the whole breeding process.
Correctly and effectively protecting the liver of Penaeus vannamei needs to start from the following aspects:
1. Comparison between normal liver and abnormal liver
The liver of normal white shrimp is generally yellow-brown, which is often affected by the color of food. There is a white membrane at the lower end of a healthy liver, which is connected with the stomach through the white membrane. Abnormal white shrimp liver turns red, white, swollen or atrophied, and the tissue is soft and inelastic. Microscopically, the diseased hepatocytes are unevenly arranged, and some or most of them are fatty and oil-drop degeneration.
The process of liver lesions can be observed with naked eyes, and the colors are usually yellowish brown (normal), light red, crimson (swelling, early stage), slightly white (beginning to shrink) and completely white (erosion, late stage). This disease can be effectively prevented and treated at an early stage. Once in the late stage, the liver will become weak, atrophy and erosion, and white shrimp often suffer from enteritis, yellow gill disease, black gill disease, rotten gill disease, erythrozoonosis, leukoplakia and other diseases. Viral diseases, in particular, are difficult to treat, and in the best case, only mild and healthy prawns can be preserved, and the losses are often relatively large.
Second, the correct and effective liver protection process
1, strengthen nutrition-provide internal material basis for protecting liver.
Protecting the liver with white shrimp should start with shrimp seedlings. Feeding opening material with high nutritional value, such as shrimp slices or yeast, on the second day after sowing can ensure the nutritional needs of shrimp, which is beneficial to the healthy development of liver and lays a good nutritional material foundation for the normal transition period. With adequate and comprehensive nutrition, shrimp can synthesize substances needed for liver development. Many times, the absorption of nutrients can promote each other. For example, some vitamins are beneficial to the absorption of mineral elements, so nutrition must be comprehensive.
2. Liver transplantation stage-successful liver transplantation is the key to successful breeding.
The liver transition period of shrimp usually refers to the process from eating natural bait to eating artificial feed. Because of the change of eating habits, the burden on the liver is increased, and the liver is in the process of rapid growth and development. Oral administration has a good liver protection effect, but the consumption of shrimp after cutting is still very small, and the effect of oral liver protection drugs on promoting liver metastasis is generally not significant. It is suggested that 1-2 days should be taken before and after shrimp cutting (the first shrimp is generally about 20-25 days, and the second shrimp 18-20 days). On the premise of strengthening and improving the pond environment, liver-protecting products with little irritation and easy absorption by white shrimp should be mixed into the feed, and bile acid is the best and has the most obvious effect on promoting liver transformation. At this stage, the change of liver markers is formation. Adding bile acids to aquatic animal feed or aquaculture water can improve the immunity and disease resistance of aquatic animals. Bile acid can effectively prevent and treat hepatopancreatic inflammation of shrimp, which has been verified in other aquatic animals. Bile acids can help create an environment that can kill and digest some bacteria and viruses in the gastrointestinal tract of fish. Bile acid has a strong inhibitory effect on acute and chronic inflammation, and also has obvious antibacterial effect on gram-positive bacteria and Staphylococcus aureus cultured in vitro. Liu Yufang et al. (1998) conducted antibacterial experiments with bile acids extracted from the gallbladder of grass carp. The results showed that bile acid had obvious inhibitory effect on three kinds of gram-negative bacteria (Staphylococcus aureus, Dactylogyrus flavipectus and Bacillus subtilis) in grass carp intestine.
3, regular liver protection-to prevent recurrence, healthy liver needs long-term maintenance.
After liver transplantation, the consumption of prawns increased. Although the liver has adapted to the artificial compound feed, it is still burdened with many important tasks such as digestion, detoxification and immune regulation, and it is often prone to unhealthy symptoms such as liver redness and anorexia. It is suggested to mix bile acids to protect the liver every 10 day (from liver transplantation to selling shrimp) to prevent the recurrence of liver diseases, maintain the normal physiological function of the liver, and strengthen digestion, absorption and detoxification.
In addition, practice has proved that a healthy liver is usually not easy to produce "white stool", so the prevention of "white stool" of prawns should start with liver maintenance.
4. Strict material control-reducing the burden on the liver is conducive to the maintenance and stability of the liver.
Many farmers' friends are a little eager for success in the process of breeding, often feeding too much, which greatly increases the burden on the liver of prawns, pollutes the water quality and sediment in the pond, and prawns are prone to stress and even get sick. This is why prawns often die first in the shrimp stealing pond. Therefore, strict material control can effectively reduce the burden of prawn liver, which is particularly important for maintaining the liver.
5, environmental improvement-to provide a good environmental foundation for the development and growth of the liver.
Compared with fish, the growth of white shrimp has high requirements on the environment, even almost harsh, especially on the liver. Many environmental changes are often first manifested in shrimp whiskers, shrimp tails and liver. A good environment is an important external condition for the smooth and rapid development of prawn liver and the success of liver transplantation. Therefore, it is necessary to improve the water quality and sediment environment regularly, and often cultivate water bodies, so that shrimp ponds are always in a benign material cycle and form a dynamic and stable ecosystem.