This is the key factor in the success of hairy crabs, which moult 21 times in their lifetime!
The Chinese mitten crab (Eriocheir sinensis) is commonly known as the Chinese mitten crab. In 2019, China produced 778,000 tons of eriocheir crabs. In recent years, with the improvement of Chinese mitten crab breeding technology, the breeding output has gradually increased to about 300 jin/mu, and the yield of producers with high breeding level can reach 400-500 jin/mu. Molting is an important physiological phenomenon of Eriocheir sinensis. After hatching, the larvae of Eriocheir sinensis in the Yangtze River system need to molt 21 times in their life, including 5 times in the seedling stage. The crab molted 11 times in culture stage. Adult crabs molt 5 times in culture stage. In the process of adult crab culture, there are many problems and great influence in the stage of slough four or five shells, which will determine the benefit of this season.
Chinese mitten crabs moult four or five shells, usually four in July and five in August. During these two periods, the farmers have taken many measures to improve the success rate of molting, such as increasing water exchange and well water injection at high temperature, opening oxygen generators and increasing the configuration of oxygen generators. Even so, there are always unsatisfactory places in the annual breeding. This paper deals with the relationship between temperature change and aquaculture in Xinghua area in recent 3 years, and provides some new ideas for aquaculture practitioners.
1. The optimum growth temperature of Eriocheir Sinensis The suitable growth temperature of Chinese mitten crab is 22-30 degrees. When the ambient temperature of the crab is higher than 30 degrees, its growth rate will be affected, and the high temperature will be accompanied by the pH value exceeding the limit, which is usually over 10, which will have a great impact on the gills and hepatopancreas of the Crab. When the water temperature is higher than 33 degrees, the growth and feeding of Eriocheir sinensis basically show stagnation. If the water temperature is high for a long time, the crab will become weak, which may lead to disease outbreak and increase mortality.
High temperature and aquaculture in Xinghua region in 2003 and 2002 According to our statistics, 8-90% of the overall Chinese mitten crab farming was profitable in 2019, and only 4-5 percent in 2020. The most important four and five shells of Chinese mitten crab are between July and September. Dead gRASs, low oxygen, high pH and other problems are most likely to occur during this period.
From July to September, the period of high temperature entered. In July 2020, there was 1 day with average daily temperature above 30 degrees Celsius, in July 2019, there were 11 days with average daily temperature above 30 degrees Celsius, and in July 2021, there were 6 days with average daily temperature above 30 degrees Celsius. Meanwhile, there were 16 southerly days in July 2020, 25 in 2019 and 20 in 2021 (see Table 1). This means that there are fewer southerly winds, fewer high-temperature days, and more rainy days, which means that cultivation is prone to hypoxia. The large loss of Chinese mitten crab in 2020 is greatly related to the low Dissolved oxygen.
■ Climate and High Temperature days in July-August 2003 (Table 1)
The change of Dissolved oxygen in 40 ponds in Taizhou in 2019 and 2020 for two consecutive years was analyzed. According to the daily mean value, it was found that the Dissolved oxygen in July 2020 was lower than that in 2020, the number of southerly winds in July 2019 was 9 days more than that in July 2020, and the high temperature above 30 degrees Celsius was only 1 day in July 2020. Overcast rain, low temperature, low oxygen and molting had great influence on the growth of Eriocheir sinensis.
From July to September in 2021, there were 6 days with average daily temperature higher than 30 degrees Celsius, but there were 20 days with southerly wind in July. The situation of low oxygen in 2021 was greatly improved compared with that in 2020. Online monitoring facilities also found that the overall low oxygen in 2021 increased by 1mg/L compared with that in 2020 (possibly related to the increase in the configuration of oxygen generators).
3. Optimum temperature and breeding The optimum temperature for the growth of Chinese mitten crab is between 22 ℃ and 30℃. According to the average accumulated temperature from April to August in 2019, 2020 and 2021, the optimum accumulated temperature in 2021 reaches 2448℃ on August 26, 2020 is 2137.5℃, and 2019 is 2061.5℃ (see Table 2). It is speculated that the overall size of The Chinese mitten crab in 2021 will be larger. The effective accumulated temperature is high, the growth rate of The Chinese mitten crab is fast, and the resistance of the body needs to be strengthened. The body will be sensitive to the stimulation of the external environment, especially when the south wind is less in August 2021, the change of the water environment will have an impact on the Chinese mitten crab. In the process of breeding nutrition supplements become a limiting factor.
■ Accumulated average daily temperature between 22 and 30 in three years (Table 2)
4. Influence of high temperature in 2021 on breeding In 2021, from July 10 to July 16, there will be seven consecutive days of average daily temperature above 30 degrees Celsius, which is the highest temperature between 2014 and 2021. The second 10 days of July will be the critical stage for the Chinese mitten crab to moult its four shells, which will be a great test for the Chinese mitten crab growing faster in the suitable range (22-30℃ accumulated temperature and 2448 ℃). High temperature accompanied by high pH value will affect some Chinese mitten crabs, the body immunity will be reduced, recovery will be slow after molting the shell, intermittent death phenomenon.
(To review the overall breeding situation of these eight years, shuibiezi disease occurred in 2014, and the low temperature continued in August this year. Shuibiezi appeared in 2015 and 2016, and continued to be influenced by Shuibiezi. In 2017, there were 35 days of ultra-high temperature in July-August, and the highest temperature in July reached 38-39 degrees and lasted for 6 days (see Table 3). This year saw the largest loss of Chinese mitten crabs in 8 years. In 2018 and 2019, the overall breeding yield specifications were high, and the overall incidence was not high, and the benefit was good. Shuibiezi phenomenon rarely appears, aquaculture experts may be affected by the 2017 high temperature, crab species has been reshuffled related. In July 2020, there was continuous precipitation, and the size of Eriocheir sinensis was also large, due to severe hypoxia loss). Current specifications should have an advantage over 2020, but there are still low oxygen problems.
■ Data table of maximum temperature above 38 ℃ in Xinghua region in 2020 (Table 3)
After the fourth shell in 2021, it began to be cloudy and rainy, and the requirements for pond management increased. At present, there are many soft-shell crabs, and they are not easy to cure. Rainy days, rotten gRASs, low oxygen. After the start of Autumn, The temperature difference between morning and evening becomes larger, the surface water temperature decreases, and the convection occurs from time to time, which further increases the pressure of the Chinese mitten crab.
■ Average daily temperature trend in July of 2008
After August, The temperature trend of 2021 is similar to that of 2019, with The temperature dropping slowly and the overall temperature is relatively stable. At this time, the most important thing is Dissolved oxygen and the body's energy and protein supplement, and the adequacy of nutrition will determine the quality and market time of Chinese mitten crab. The fast growing Chinese mitten crab is sensitive to external stimulation, so it is important to keep the water environment stable. ■ Average daily temperature trend in August of 2008 (Blue Spirit shipment chart - factory aquaculture equipment) The picture 5. Suggestions on late-stage management of Chinese mitten crab breeding in 2021 1. Early listing and quality improvement Early listing and high quality are very advantageous for the price of Chinese mitten crab. Figure 4 shows the price of male crab over 175 grams from 2013 to 2020, and it is found that the price before September 22 every day is the highest in a year (the data co