Crayfish are good at "taking things as they are", and rivers, lakes, reservoirs, rice fields and ponds can all be the shelter of crayfish. When the water temperature rises in spring, crayfish often move in shallow water. When it is hot in midsummer, it lurks in deep water. In winter, crayfish spend it in caves to prevent other animals from attacking.
Crayfish not only has strong feeding ability, but also has the habit of gluttony and competition for food. In the case of high reproductive density or hunger, crayfish will have a "tragedy" of killing each other, and the newly shelled crayfish and young shrimp often become the "plate food" of "strong" lobsters.
From June to August every year, it is the most "plump" time for crayfish. At this time, the crayfish shell is hard and thick, and it is also the best time for people to catch and enjoy it.
This summer, although crayfish swept across the river, diners were always a little uneasy. Rumors about crayfish's preference for dirty living environment greatly affected some people's appetite. It was even rumored on the Internet that crayfish was used as a "heavy metal cleaner" for cleaning sewage by sewage treatment plants. If these "cleaners" were put on the table, the consequences would be unimaginable.
Professor Cheng Yongxu is an expert in the reproduction and nutrition physiology of crustaceans. He believes that the rumor that crayfish grow better in dirtier places is unfounded. On the contrary, crayfish have certain requirements for the growth environment. If the environment is too dirty, it will affect its growth.
Crayfish have strong feeding ability and reproductive ability, and have low requirements for growth environment. Crayfish is an omnivore, which mainly eats animals. Small fish, shrimp, plankton, benthos and algae can all be used as its food. Heavy metals and oil stains are not the feeding objects of crayfish. It is rumored that in order to speed up the growth of crayfish, it is not feasible to artificially put metal oil and domestic garbage into the waters where crayfish grow.
Professor Cheng also said that artificial breeding of crayfish can make use of natural resources such as rice fields and lakes. When choosing breeding sites, it is necessary to choose places with sufficient water sources and no pollution in the surrounding environment. In rice field breeding, attention should be paid to changing water in time and preventing pests and diseases. The dirtier the better. It is best to plant some shallow water plants in the waters where crayfish are cultured, which can provide a hidden place for crayfish to inhabit and shell, and shelling is an important stage for crayfish to grow up. In addition, shallow water plants can improve the water environment.
Technicians from Peking University Taipu Company, who are mainly engaged in technical research in aquatic products, fine chemicals and other fields, said that when raising crayfish, organic substances such as animal droppings can be put into the waters first to cultivate plankton as bait for crayfish, but these things are not food for crayfish.
Li Jianhua, deputy director of the sewage treatment plant under Tianjin Chuangye Environmental Protection Company, confirmed to the reporter that the technology of using crayfish as "cleaner" in sewage treatment is unheard of. Sewage treatment technology has a history of 100 years, mainly using microorganisms to decompose organic matter in water. The operation and management personnel of the environmental protection company also said that they often learn advanced sewage treatment technology from universities, but they have never heard of the technology of using crayfish to treat sewage in China, and they have never heard of this experiment. In the process of sewage treatment, heavy metals in sewage are mainly removed by precipitation. Zhang Mingxu, a researcher at the Institute of Water Environment of Shanghai Academy of Environmental Sciences, believes that it is impossible to use crayfish as a tool for sewage treatment. The sewage treated by sewage treatment plants is too dirty, so crayfish may not survive.
Pest?
Because crayfish have the habit of burrowing up and down the water surface, and the caves are inclined downward, with a depth of about 30 cm, the large number of crayfish may pose a threat to the safety of lakes, reservoirs, rivers and dams. Professor Cai Shengli believes that crayfish is still a harmful animal. When the middle and lower reaches of the Yangtze River were flooded in 1998, the dangers of many dams were related to the destruction of crayfish.
Chinese experts have investigated the life history and habits of crayfish in Dongting Lake area. The investigation found that crayfish have seriously affected the flood control dam facilities in the lake area because some escaped crayfish individuals dug holes on the dam to survive.
In addition, as an invasive organism, crayfish can survive in temporary water bodies, and have a wide range of feeding habits, and the population is established very quickly. They pose a great threat to fish, crustaceans, aquatic plants, rice, etc. in the same water area, and also directly endanger the artificially propagated young mussels. Experts are trying to find ways to control the excessive invasion and spread of crayfish and protect the ecological environment of Dongting Lake.
Professor Cheng Yongxu believes that in recent years, people's demand for crayfish has increased, and many people have realized the economic benefits of crayfish fishing. The increasing catch naturally reduces the number of crayfish, which, in a certain sense, plays a certain role in protecting the ecology.
Cause a bad disease?
Recently, some media reported that a famous comedian in China suffered from paragonimiasis because of eating crayfish, and the delicious crayfish suddenly became disgusting. Chang Zhengshan, from the Institute of Parasitic Diseases Prevention and Control of the Chinese Center for Disease Control and Prevention, explained that several conditions need to be met for crayfish to infect paragonimiasis. At present, no cases of paragonimiasis directly related to eating crayfish have been found in Shanghai.
The transmission of paragonimiasis needs several links. First, infected cats and canines discharge feces containing eggs of paragonimiasis into water, and the eggs hatch into cercaria in the water, and the cercaria develops into cercaria in the first host such as Oncomelania hupensis and Oncomelania hupensis, and the cercaria discharged into the water enters into the second host such as crayfish, hairy crabs and crabs, and becomes metacercariae. If crayfish infected with metacercariae of paragonimiasis are not completely killed in the cooking process, people may be infected with paragonimiasis if they eat such crayfish.
Chang Zhengshan said that if there is no intermediate host, there is no condition for paragonimiasis to spread. According to the survey, the waters around Shanghai are safe. However, the crayfish in Shanghai market not only come from around Shanghai, but also come from Zhejiang, Anhui, Hunan, Hubei and other provinces, which are endemic areas of paragonimiasis. He said that for this reason, the food hygiene supervision department should supervise and manage the quality of crayfish, but the detection amount is so huge that it is really not easy to operate in reality. Researcher Chang Zhengshan suggested that eating crayfish should not be too much.
Professor Cheng Yongxu introduced that there may be paragonimiasis in other aquatic products, and it should be ensured to be thoroughly cooked when processing aquatic products. He believes that it is difficult for health management departments to inspect all aquatic products. At present, relevant departments mainly inspect and restrict some species that have been proved harmful to human health, such as clam and puffer fish.
Zhenhua Gu, director of the Office of Food Hygiene Supervision Department of Shanghai Municipal Health Supervision Institute, revealed that at the end of July and the beginning of August, the Health Supervision Institute conducted a sampling survey on the catering units dealing in crayfish in Shanghai. The crayfish sold in the Shanghai market did not exceed the national standards in terms of environmental pollutants, pesticides and other items, and the crayfish eaten were not found to have health problems. The results of this spot check will be announced on another day.
Professor Wang Xichang from the College of Food Science, Shanghai Fisheries University mainly studies food nutrition and hygiene. He believes that, like other aquatic products, if crayfish are cooked according to normal cooking methods to ensure that crayfish are cooked thoroughly and parasites are completely killed, it will not cause diseases, but it is best not to be greedy. In addition, because of the high content of protein in crayfish, protein will do great harm to human health after rotting, so crayfish must be eaten at one time.
Experts believe that the quality of aquatic products mainly depends on the culture environment. In heavily polluted waters, heavy metals may remain in the viscera and head of shrimp. However, Professor Cheng Yongxu believes that with the increase in the demand for crayfish, the proportion of artificial culture will become larger and larger, which will make the quality of crayfish more guaranteed. Professor Cheng also suggested that relevant departments should actively guide farmers to develop crayfish farming, such as crayfish farming in the waters where water bamboo is planted and adjusting the economic structure.
European and American countries are the main consumers of crayfish. Because of its delicate meat and cheap price, crayfish has long been the main ingredient in the daily diet of European and American countries. In French cuisine, there are often dishes made of crayfish. Sweden is an avid consumer of crayfish. Every year, a three-week crayfish festival is held in Sweden. The whole country not only eats crayfish, but also people draw pictures of crayfish on tableware and clothes. The scene is very grand. In the Swedish market alone, the annual consumption of crayfish reaches 1.5 to 2 tons.
Professor Wang Xichang introduced that the content of protein in crayfish is relatively high, accounting for about 16%-20% of the total, and the fat content is less than 0.2%. Moreover, the fat contained in crayfish is mainly composed of unsaturated fatty acids, which is suitable for human body to absorb. The content of trace elements such as zinc, iodine and selenium in shrimp meat is higher than other foods, and its muscle fiber is tender and easy to digest and absorb.
In addition to shrimp meat, shrimp skin that is usually discarded by people also has high utilization value. The head and shell of crayfish contain 20% chitin, which can be extracted after processing. According to the technicians of Peking University Taipu Company, chitin can be used in food, chemical industry, medicine, agriculture, environmental protection and other fields, and can be made into health care products, which have the effects of improving human immunity and resisting cancer. In addition, crayfish can also be used as medicine, which can eliminate phlegm and relieve cough, and promote the wound healing after operation.
It is understood that in recent years, the industry of extracting chitin from shrimp shells in Jiangsu and Zhejiang has gradually become large, and health products made of chitin have gradually been recognized by people.
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