Crayfish not only have strong feeding ability, but also have the habit of gluttony and competition for food. In the case of high reproductive density or starvation, "tragedy" of crayfish killing each other will occur. Freshly shelled crayfish and juvenile shrimps often become the "dinner" of "strong" lobsters.
Every year from June to August, crayfish are at their most plump. At this time, the crayfish have hard shells and thick flesh, and it is also the best time for people to catch and enjoy them.
Although crayfish are sweeping across the country this summer, diners are always a little uneasy. Rumors about crayfish’s preference for dirty living environments have greatly affected the appetite of some people. The Internet There are even rumors in the Internet that crayfish are used as "heavy metal cleaners" in sewage treatment plants to clean up sewage. If these "cleaners" are put on the dining table, the consequences will be disastrous.
Professor Cheng Yongxu is an expert in crustacean reproduction and nutritional physiology. 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 very strong feeding and reproductive abilities and have very low requirements for the growth environment. Crayfish are omnivores and mainly eat animals. Small fish, shrimps, plankton, benthic organisms, algae, etc. can be used as its food. Heavy metals and oil pollution are not the food targets of crayfish. According to rumors, 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 artificially cultured crayfish can make use of natural resources such as rice fields and lakes. When choosing a breeding location, you need to choose a place with sufficient water sources and a non-polluted surrounding environment. Pay attention to timely water changes and prevent and control pests and diseases. Dirty is not always better. It is best to plant some shallow-water plants in the waters where crayfish are cultured. This can provide a sheltered place for crayfish to inhabit and shed their shells. Shelling is an important stage in the growth of crayfish. In addition, shallow-water plants can also improve the water environment.
Technicians from Peking University Taipu Company, which is mainly engaged in technical research in the fields of aquatic drugs and fine chemicals, said that when cultivating crayfish, organic matter such as animal feces can be put into the water first to cultivate plankton as a Crayfish bait, but these things are not food for crayfish.
Li Jianhua, deputy director of the sewage treatment plant affiliated to Tianjin Chuangye Environmental Protection Company, confirmed to reporters that the technology of using crayfish as a "cleaning agent" in the sewage treatment process is unheard of. Sewage treatment technology has a history of more than 100 years and mainly uses microorganisms to decompose organic matter in the water. Operations managers of entrepreneurial environmental protection companies also said that they often learn advanced sewage treatment technology from universities, but they have never heard of domestic technology using crayfish to treat sewage, nor have they heard of any experiments in this area. In sewage treatment, During the process, heavy metals in sewage are mainly removed through precipitation. Zhang Mingxu, a researcher at the Water Environment Research Institute of the Shanghai Academy of Environmental Sciences, believes that it is impossible to use crayfish as a tool for sewage treatment. The sewage treated by the sewage treatment plant is too dirty, and crayfish may not survive.
Pests?
Since crayfish have the habit of making caves above and below the water surface, and the caves are inclined downward with a depth of about 30 centimeters. The large reproduction of crayfish may have a negative impact on the safety of lakes, reservoirs, rivers and river dams. threaten. Professor Cai Shengli believes that crayfish are still considered harmful animals. When the middle and lower reaches of the Yangtze River suffered floods in 1998, the dangers of many dams were related to the damage of crayfish.
Chinese experts have investigated the life history and living habits of crayfish in the Dongting Lake area. The investigation found that crayfish have seriously affected the lake area because some escaped crayfish individuals dug holes in the dams to survive. flood control dam facilities.
In addition, as an invasive organism, crayfish can survive in temporary water bodies, have a wide range of food habits, and can establish populations extremely quickly. They pose a great threat to fish, crustaceans, aquatic plants, rice, etc. in the same waters, and also directly harm artificially bred young clams. 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 people’s demand for crayfish has increased in recent years. Many people have realized the economic benefits of fishing for crayfish, and the amount of fishing has continued to increase, which has naturally reduced the number of crayfish. In a certain sense, it plays a certain role in protecting the ecology.
Causing serious illness?
Recently, media reported that a famous domestic comedian contracted paragonimiasis from eating crayfish, and the fragrant crayfish suddenly became disgusting. Chang Zhengshan of the Parasitic Disease Prevention and Control Institute of the Chinese Center for Disease Control and Prevention explained that several conditions need to be met for crayfish to be infected with paragonimiasis. Currently, no cases of paragonimiasis directly related to the consumption of crayfish have been found in Shanghai.
The transmission of paragonimiasis requires several steps. First, infected cats and canines discharge feces containing paragonimiasis eggs into the water. The eggs hatch into miracidia in the water, and the miracidia develop into cercariae in the first host such as snails and Oncomelania snails, and are excreted The cercariae entering the water enter the bodies of second hosts such as crayfish, hairy crabs, and river crabs, and turn into metacercariae. If crayfish infected with paragonimiasis metacercariae are not completely killed during the cooking process, humans will eat such crayfish. You may be infected with paragonimiasis.
Chang Zhengshan said that without an intermediate host, there would be no conditions for paragonimiasis to spread. According to surveys, the waters around Shanghai are safe. However, the crayfish on the Shanghai market not only come from around Shanghai, but some crayfish come from Zhejiang, Anhui, Hunan, Hubei and other provinces, which are endemic areas for paragonimiasis. He said that for this reason, the food hygiene supervision department should supervise and manage the quality of crayfish, but with such a large amount of testing, it is indeed not easy to operate in reality. Researcher Chang Zhengshan suggested that you should not eat too much crayfish.
Professor Cheng Yongxu said that paragonimiasis may also be present in other aquatic products, and you should ensure that they are thoroughly cooked when processing aquatic products. He believes that it is difficult for the health management department to inspect all aquatic products. At present, the relevant departments mainly inspect and restrict some species that have been proven to be harmful to human health, such as clams and puffer fish.
Gu Zhenhua, director of the Food Hygiene Supervision Section Office of the Shanghai Municipal Health Supervision Institute, revealed that at the end of July and early August, the Health Supervision Institute conducted a sampling survey of catering units selling crayfish in Shanghai. The crayfish sold in the Shanghai market In the inspection of environmental pollutants, pesticides and other items, none exceeded national standards, and no health problems were detected in the crayfish eaten. The results of this random inspection will be announced at a later date.
Professor Wang Xichang from the School of Food Science and Technology of 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, ensure that the crayfish is cooked thoroughly and completely killed. Dead parasites generally do not cause disease, but it is best not to eat them greedily. In addition, due to the high protein content of crayfish, the rotten protein is very harmful to human health. Therefore, the crayfish must be eaten at one time after being cooked.
Experts believe that the quality of aquatic products mainly depends on the breeding environment. In seriously polluted waters, heavy metals may remain in the internal organs and heads of shrimps. However, Professor Cheng Yongxu believes that as the demand for crayfish increases, the proportion of artificial breeding will become larger and larger, which will ensure the quality of crayfish. Professor Cheng also suggested that relevant departments should actively guide farmers to develop crayfish farming, such as cultivating crayfish in waters where wild rice is grown, and adjust the economic structure.
European and American countries are the main consumers of freshwater crayfish. Due to their delicate meat and low price, freshwater crayfish have long become the main ingredient in the daily diet of European and American countries. In French cuisine, there are often dishes using freshwater crayfish as raw materials. Sweden is an avid consumer of freshwater crayfish. Every year, Sweden holds a three-week Crayfish Festival. Not only do the whole country eat crayfish, but people also draw freshwater crayfish patterns on tableware and clothes. The scene is very grand. In the Swedish market alone, 1.5 to 2 tons of freshwater crayfish are consumed annually.
Professor Wang Xichang said that crayfish have a high protein content, 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 easy for the body to absorb. The content of zinc, iodine, selenium and other trace elements in shrimp meat is higher than that of other foods. Its muscle fibers are delicate and easy to digest and absorb.
In addition to shrimp meat, shrimp skin, which is usually discarded by people, also has high utilization value.
The heads and shells of crayfish contain 20% chitin. After processing, chitin can be extracted from the chitin. According to technicians from Peking University Taipu Company, chitin can be used in food, chemical industry, medicine, agriculture, environmental protection and other fields. It can be made into health care products and has the effect of improving human immunity and anti-cancer. In addition, crayfish can also be used as medicine, which can reduce phlegm and relieve cough, and promote wound healing after surgery.
It is understood that in recent years, the industry of extracting chitin from shrimp shells in Jiangsu and Zhejiang has gradually grown in scale, and health care products made from chitin have gradually been recognized by people.