River crabs are also known as hairy crabs, clear water crabs, hairy crabs, freshwater crabs, the real scientific name is the Chinese river crab (Eriocheir sinensi), which belongs to the large family of the square crab family - river crabs, and other members of this large family include.
River crabs have a wide distribution range in China, north to the Yalu River, south to the Leizhou Peninsula, all the rivers and lakes connected with the ocean are basically produced. There are differences in the quality of river crabs produced in the north and south of China, and the quality of river crabs produced in the Yangtze River Basin is the best. Some scholars believe that after morphological research, not all the river crabs produced in China are Chinese river crabs, and some are Chinese river crabs (distributed in South China).
Yangcheng Lake hairy crab is famous throughout the country, the hairy crab produced here has typical characteristics, strong physique, delicious meat, so it is also known as "Chinese golden velvet crab". So, how do you grow a delicious, high-quality river crab? That is precisely what we are going to discuss today.
< h1 class = "pgc-h-arrow-right" > to understand the biological characteristics of river crabs, the Chinese river crab</h1>
Appendages of river crabs: Many people think that river crabs have 5 pairs of appendages, but this is not the case. The river crab has a total of 8 pairs of appendages on the chest, of which the first 3 pairs are jaw feet, which are used to assist in feeding; the fourth pair is a powerful claw foot, which is covered with villi, the main function is to catch food and attack defense; the remaining 4 pairs are slender foot, mainly playing the function of crawling.
▲ The appearance of the Chinese Mitten Crab
Belly of river crab: The abdomen of river crab is flattened, consisting of a total of 7 sodomes, commonly known as "crab umbilicus". The difference between female crabs and male crabs is mainly manifested in the abdomen, the crab navel of the male crab is more pointed, and the 1st-2th section evolves into a mating device; the crab umbilicus of the female crab is relatively round, called the umbilicus, the 2nd to 5th section has appendages, and the bristles on the top have the function of adhering to the fertilized egg. In addition, the mature sperm nest is called crab paste, the mature ovaries are called crab yellow, and the nutritional value of crab paste and crab yellow is very high, which is an important indicator to measure the market value of commercial crabs.
▲Comparison of the male and female appearance of Chinese river crab (female crab on the left, male crab on the right)
River crab feeding habits: In the natural environment, river crabs are typical omnivorous crustaceans, like to feed on snails, shrimp and aquatic insects, etc., but also eat plant bait, such as salt laver, duckweed, bitter grass and other aquatic organisms, is a typical opportunist. By dissecting wild river crabs, the researchers found that the stomach of river crabs is dominated by aquatic plants, which is because "meat food" is not easily obtained in the natural environment. But even if you eat vegetarian for a long time, the river crab will not starve to death, but the growth rate will slow down.
River crab adaptation to water temperature: river crabs can survive in the water temperature environment of 1 ~ 35 ° C, the best growth temperature is 20 ~ 28 ° C, when the water temperature is lower than 10 ° C or higher than 30 ° C, the feeding amount of river crabs will be significantly reduced, below 6 ° C will stop feeding, and even death.
<h1 class = "pgc-h-arrow-right" > the migration of river crabs: it's all about reproduction</h1>
Crab farmers often say two sayings, namely "the west wind sounds, the crab feet itch" and "crabs stand in winter, walk without a trace", both of which describe the reproductive migration of river crabs. From December to March of the following year, sexually mature male and female river crabs will invariably return to the estuary area to mate and lay eggs. That is to say, only in the inland rivers that communicate with the ocean can the Chinese river crab reproduce naturally.
▲The life history of the Chinese velvet crab
River crabs generally live in fresh water and fatten for 6 to 18 months, spending 1-2 winters before reaching sexual maturity, at this time large-sized river crabs are called "yellow crabs". In the autumn, the yellow crab continues to grow, after completing the last molting shell in life, the gonads begin to develop rapidly, at this time the gender characteristics of the river crab are becoming more and more obvious, the color of the carapace changes from the original yellow to dark green, and the river crab at this stage is called "green crab".
Green crabs mostly weigh between 100 and 200 grams, and they "go with the flow" and return to the sea in order to complete the most important mission in their lives - reproduction. In the estuarine area, as long as the water temperature is above 8 ° C and the salinity is between 8 and 33, the river crab will begin to mate. The fertilized eggs of the river crab are about 0.3 mm in diameter, the sauce is purple, like a bunch of small grapes, and the egg group composed of 300,000-500,000 fertilized eggs will be tightly adhered to the bristles of the female crab's abdomen, which is also known as the "egg-holding crab".
Under the protection of the female crab, the hatching rate of the fertilized eggs of the river crab is generally as high as 90%, and the fertilized eggs become flea-like larvae after hatching (Note: the early development of the river crab goes through three stages). Flea-like larvae have the habit of clustering and lighting, and can only become big-eyed larvae after 5 molts, which is also often referred to as "crab seedlings". The crab seedlings are fierce, have a mixed diet, have strong crawling ability and swimming ability, and after another molting (that is, the 6th molting) they enter the crab stage.
▲▲ Flea larvae and big-eyed larvae of river crabs
Note: River crabs undergo about 15 molts in their lifetime, and the frequency of molting in the early stages is fast, and the interval between molts in the later stages will become longer and longer. Like other shrimp and crabs, river crabs can only grow through molting, and in water bodies with abundant bait and clean water quality, river crabs molt with high frequency and grow fast, while otherwise they grow extremely slowly, and even do not molt for a long time.
< h1 class="pgc-h-arrow-right" > how did the river crab "darken the chen cang" and be listed as an invasive alien species in Europe and the United States and other countries? </h1>
River crabs are undoubtedly the most famous edible crabs in China, but when they go abroad, the treatment of river crabs is very different. As early as 1995, residents of Greenwich, England, caught Chinese velvet crabs in the Thames. In countries such as Europe and the United States, it is illegal to transport live Chinese velvet crabs, and they are worried that river crabs may spread to other waters, posing a threat to local aquatic life.
▲The "wanted list" of the river crab
This fear is not unfounded.
By comparing the size of the eggs of important economic fish, scientists have found that a variety of fish, including rainbow trout and salmon, may be potentially threatened by Chinese river crabs. Salmon trout such as rainbow trout and salmon are known to migrate to freshwater rivers to spawn, and the flooded velvet crabs will prey on fish eggs in large quantities, which will have a potential adverse impact on the local fishing economy.
▲The particle size of various fish eggs is compared with the size of river crabs
River crabs were first brought to Europe and the United States, most likely through commercial cargo ships, probably in 1900. In the ballast water of the ship, there may be fertilized eggs or hatching larvae of the Chinese velvet crab, and its body length is only between 1.7 and 5 mm, which is difficult to detect. When they reached the port, the water in the tank was discharged, and the larvae of the velvet crab successfully landed in a "foreign country".
In addition to "smuggling", artificial introduction is also an important reason for the Chinese velvet crab to "cross the ocean". The Great Lakes of the United States have introduced Chinese velvet crabs many times, but perhaps due to "unsatisfactory water and soil", Chinese velvet crabs have not yet successfully settled in the Great Lakes. River crabs were first introduced in Germany in 1912, and by 1936, the number of river crabs began to increase dramatically, flooding, and Germany removed 220 tons of river crabs from the Vissy River alone. Large numbers of river crabs damage fishing nets and local hydroelectric dams, causing economic losses of up to €80 million.
<h1 class="pgc-h-arrow-right" > the difficulty of river crab farming: how to raise high-quality river crabs with large heads, good quality and uniform specifications? </h1>
At present, the specifications of commercial river crabs are above 100 grams, and the larger the specifications, the higher the market price, while the small river crabs that do not reach 100 grams are low and pitiful (such as small crabs). It can be said that the way out of river crab farming is high quality and large specifications, so it is very important to do the following 6 points.
1. Larval rearing of river crabs
The so-called juvenile breeding refers to river crabs, which undergo 5 molts during this period. If the water temperature is controlled under ideal conditions of 20 to 22 °C, it generally takes 18 to 22 days of incubation time.
Before stocking the larvae, the breeding pond should be thoroughly disinfected to cultivate high-quality bait organisms such as triangular brown finger algae, algae, chlorella, and flat algae, so that the density reaches 150,000-200,000 /l. These phytoplankton will provide the first ration for the larvae of the river crab, which is of great significance for improving the survival rate and metamorphosis rate of the larvae of the river crab.
2. Beware of seedlings killing each other and improve the survival rate
River crabs have the habit of mutual cannibalism at the larval stage (large seedlings eat small seedlings), in order to reduce the residual food rate, it is necessary to synchronize the development of larvae in the same nursery pond to avoid polarization of size. Therefore, we must do the following 2 things:
(1) Try to choose egg-holding crabs with consistent development of fertilized eggs, which can be determined by observing the color of the egg population.
(2) In the nursery pond can put a few more crab cages, observe the number of crab seedlings at any time, once the crab seedlings released by the egg-hugging crabs reach the corresponding number, the remaining crab cages will be immediately removed, otherwise the density of crab seedlings is too large, and the probability of cannibalism will be greatly increased. Maintaining 100,000-300,000 crab seedlings per cubic body of water is a normal density range, and too high or too low seedlings are not very effective.
3. Adjust the water quality and desalinate the seedlings
The larval stage of river crabs should pay attention to maintaining good water quality, ammonia nitrogen control below 1.5mg/L, and the daily water exchange is not easy to be too large, in order to prevent the temperature difference from changing sharply and causing stress. The dissolved oxygen content in the pool water should be maintained above 4~5mg/L, the water temperature should be maintained at 20~25°C, and the salinity should be 18~25.
。 The caught crab fry can be sold to other farmers or further reared.
Fishing method: The big-eyed larvae of river crabs like to flock, and can use their phototrophot characteristics to trap and catch with a thin net of about 2 mm.
Desalination method: During the desalination period, a certain amount of fresh water is added to the breeding pond every day, so that the salinity is reduced by 2 to 3 units, and after 4 to 5 days, the salinity of the breeding pond will drop below 5, and the crab seedlings at this time will be transformed into desalinated crab seedlings.
4. Baby crab breeding: the critical period of river crab pond
The survival rate of baby crabs is very low, generally about 30%, and only 5% to 10% of the low crabs, so the cultivation of baby crabs is sloppy, which is directly related to the production of the later stage of breeding.
(1) Crab seedlings should be suspended before they descend the pond, and they should go down to the pond during the peak period of flea breeding.
(2) In order to reduce residual food, certain aquatic plants can be planted in the pond to improve the pond environment.
(3) Control a reasonable breeding density, do not blindly pursue production, quality is the strongest competitiveness.
(4) Strengthen the supply of nutrients, mainly natural food such as water fleas in the early stage, and artificial feed in the later stage.
5. Cultivation of crab species
The growth rate of river crabs varies greatly, and in the autumn and winter, some river crabs can weigh 50 to 70 grams, the largest can reach about 150 grams, and some are far from reaching the standard of buckling crabs, weighing only about 1-2 grams - even if an individual like this is raised for two more years, it is difficult to reach the commodity specifications.
Crab breeding is carried out on the basis of baby crabs, and the cultivation of this process should also pay attention to the above four aspects (the same as the main points of breeding baby crabs).
One add-on: What is a button crab?
The buckle crab is a stage of the growth and development of river crabs, also known as first-class crab species and crab seedlings. The weight of the button crab is more than 2 grams, and the body size is like a button, so it is called a button crab.
<h1 class= "pgc-h-arrow-right" > the rearing of commercial crabs: this is the final link in determining the benefits of farming</h1>
Commercial crab rearing refers to the process of raising buck crabs (crab species) into marketable specifications. At present, there are three main modes of raising commercial crabs in China, pond culture, fence culture and rice field culture. The most traditional and common pond culture is used here as an example.
The goal of pond crab farming is to achieve a size of more than 100 grams of river crabs and more than 150 grams in some parts. Compared with lakes with large water surfaces, the river crabs raised in ponds are slightly inferior in meat quality and specifications. In order to improve the quality of river crabs, the pond area is generally controlled at about 5-10 acres.
(1) Construction of pond environment.
The location of the crab pond requires sufficient water sources, convenient water discharge, an area of 3000 to 6000 square meters, a water depth of 1 to 1.2 meters, and good lighting. When the weather warms up in March every year, each mu of water surface is disinfected with 200 to 300 kg of quicklime clearing pond, and anti-escape facilities are arranged around the pond, which can be surrounded by 0.7 meters wide thick plastic film to form a protective fence.
(2) Plant aquatic weeds and stock field snails
In mid-March, it is possible to sow Elegae at the bottom of the pond, add bitter grass in April and May, and then transplant some of the black algae and goldfish algae. In the hot summer, the algae coverage rate should reach more than 50%. Before and after the Qingming Dynasty every year, 300 catties of field snails can be stocked per mu of water surface, and the field snails can not only provide sufficient biological bait for crab seedlings, but also improve water quality and remove organic metabolites.
(3) Daily feeding and management
The feed of river crabs is mainly compound feed, which can also be assisted by feeding a certain amount of green feed, and the daily feeding amount can be flexibly adjusted according to the water temperature, water quality, and eating conditions.
In the breeding process, the lack of oxygen has a great impact on river crabs, so it is necessary to fill new water regularly, which is also conducive to the growth of aquatic grasses. It is also necessary to pay attention to the prevention of escape and disease, and must not use the enemy insects when using drugs (crustacean organisms are very sensitive to the enemy insects, and a small amount can cause large-scale death).
<h1 class = "pgc-h-arrow-right" > Summary: River crab farming, promising</h1>
In short, the edible value of river crabs is very high, the market demand is large, and there are very broad prospects for breeding. At present, with the artificial breeding of river crabs and the cultivation of seedlings have made great breakthroughs, the industrial chain of river crabs has become increasingly perfect and mature.
However, there are still many deficiencies in China's research on improving the size of river crabs, breeding varieties and the best breeding environment. This is the bottleneck encountered in the further development of the river crab industry, and it is also a major difficulty in improving the economic benefits of aquaculture.
What do you think about the cultivation of river crabs? Welcome to the joint discussion in the comments section!
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