Crayfish not only has strong feeding ability, but also has the habit of gluttony and competing for food. In the case of high breeding density or hunger, crayfish will have a "tragedy" of killing each other, and crayfish and young shrimp that have just been shelled often become "good meals" for "strong" lobsters.
From June to August every year, it is the most fat time for crayfish. At this time, the crayfish shell is hard and thick, which is also the best time for people to catch and enjoy.
This summer, although crayfish swept across the country, diners were always a little uneasy. The rumor that crayfish prefer dirty living environment has greatly affected some people's appetite. It is even rumored on the Internet that crayfish is used as a "heavy metal cleaning agent" for cleaning sewage by sewage treatment plants. If these "detergents" are put on the table, the consequences will be unimaginable.
Professor Cheng Yongxu is an expert in crustacean reproduction and nutritional physiology. He thinks that the rumor that crayfish grow better in dirty places is unfounded. On the contrary, crayfish have certain requirements for their growth environment. If the environment is too dirty, it will affect their growth.
Crayfish is an omnivore, mainly eating animals, such as small fish, shrimp, plankton, benthos, algae and so on. Heavy metals and oil pollution are not the feeding objects of crayfish. It is said that in order to speed up the growth of crayfish, it is not feasible to artificially put metal oil and domestic garbage in the waters where crayfish grow.
Professor Cheng also said that crayfish can make use of natural resources such as rice fields and lakes. When choosing a breeding site, you must choose a place with sufficient water and no pollution in the surrounding environment. In rice field cultivation, attention should be paid to changing water in time and controlling pests and diseases. The dirtier the water, the better. It is best to plant some shallow water plants in crayfish culture waters, which can provide hidden places for crayfish to inhabit and shell. Shelling is an important stage of crayfish growth. In addition, shallow water plants can improve the water environment.
The technicians of 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 water 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 reporters that the technology of using crayfish as a "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 managers of environmental protection companies also said that they often learn advanced sewage treatment technology from universities. But I have never heard of the technology of treating sewage with crayfish in China, and I 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 crayfish cannot be used as a tool for sewage treatment, and the sewage treated by sewage treatment plants is too dirty, so crayfish may not survive.
Pests?
Crayfish have the habit of digging holes up and down the water surface, and the holes are inclined downward, with a depth of about 30 cm. The abundance of crayfish may threaten the safety of lakes, reservoirs, rivers and dams. Professor Cai Shengli believes that crayfish is still a harmful animal. 1998 During the flood in the middle and lower reaches of the Yangtze River, many dams were destroyed by crayfish.
Experts from China 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, with a wide range of feeding habits and rapid population establishment. They pose a great threat to fish, crustaceans, aquatic plants and rice. In the same water area, it also directly endangers artificially propagated young shellfish. Experts are trying 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. More and more catches naturally reduce the number of crayfish, and in a certain sense, it plays a certain role in protecting the ecology.
Cause a bad disease?
Recently, some media reported that a famous comedian in China got paragonimiasis because of eating crayfish, and the originally delicious crayfish suddenly became disgusting. Chang Zhengshan, Institute of Parasitic Disease Prevention and Control, Chinese Center for Disease Control and Prevention, explained that crayfish are infected with paragonimiasis in several ways. 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 dogs discharge feces containing paragonimiasis eggs into water, and the eggs hatch into cercaria in the water, and the cercaria develops into cercaria in the first host such as snails and snails. The cercariae discharged into the water enter the second host such as crayfish, hairy crabs and crabs and become metacercariae. Crayfish infected with paragonimiasis cysts will be eaten if they are not completely killed during cooking.
Chang Zhengshan said that if there were no intermediate hosts, there would be no conditions for the transmission of paragonimiasis. According to the survey, the waters around Shanghai are safe. However, crayfish in Shanghai market not only come from Shanghai, but also from Zhejiang, Anhui, Hunan, Hubei and other provinces, all of 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.
Professor Cheng Yongxu said that there may be paragonimiasis in other aquatic products, so it should be cooked thoroughly when processing aquatic products. He thinks it is difficult for the health management department to check all aquatic products. At present, the relevant departments mainly check and restrict some species that have been proved to be harmful to human health, such as Scapharca subcrenata and puffer fish.
Zhenhua Gu, director of the Food Hygiene Supervision Department of Shanghai Health Supervision Office, revealed that at the end of July and the beginning of August, the Health Supervision Office conducted a sampling survey on the catering units operating crayfish in Shanghai. The crayfish sold in Shanghai market, environmental pollutants, pesticides and other items did not exceed the national standards, and no hygienic problems were found in the crayfish eaten. The results of this sampling survey will be announced on another day.
Professor Wang Xichang from the College of Food Science of Shanghai Fisheries University mainly studies food nutrition and hygiene. He believes that crayfish, like other aquatic products, if cooked according to normal cooking methods to ensure that crayfish are thoroughly cooked and parasites are completely killed, it will generally not cause disease, 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, so crayfish must be cooked and eaten once.
Experts believe that the quality of aquatic products mainly depends on the breeding 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 increase, which will make the quality of crayfish more secure. Professor Cheng also suggested that the relevant departments should actively guide farmers to develop crayfish breeding, such as crayfish breeding in the waters where water bamboo is planted and adjusting the economic structure.
European and American countries are the main consumers of crayfish. Crayfish has long been the main ingredient in the daily diet of European and American countries because of its tender meat and low price. In French cuisine, there are often crayfish dishes. Sweden is an avid consumer of crayfish. Sweden holds a three-week crayfish festival every year. People not only eat crayfish, but also draw crayfish patterns on tableware and clothes. The scene is very grand. Only in the Swedish market,
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 absorption. The content of trace elements such as zinc, iodine and selenium in shrimp meat is higher than other foods, and its muscle fiber is fresh and tender, which is easy to digest and absorb.
In addition to shrimp meat, shrimp skin, which 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 can improve human immunity and resist cancer. In addition, crayfish can also be used.
It is understood that in recent years, the industry of extracting chitin from shrimp shells in Jiangsu and Zhejiang provinces has gradually expanded, and health products made of chitin have gradually been recognized by people.