CATDOLL : CATDOLL: Why do eels grow so big?

1. Why does the eel grow so big?

rumor

The reporter learned from the Provincial Fisheries Bureau on May 20 that although there are rumors in society that there are contraceptive residues in eels, fishery research departments in Shanghai and other places have found through a large number of tests that no contraceptives were detected in either wild or farmed eels.

??Swinhoe eel is a freshwater bottom-dwelling fish with high nutritional value. Since the 1980s, the resources of wild eels have become increasingly scarce, and their artificial breeding industry has emerged and gradually developed towards high-density breeding. Experts say that eel farming is an industry with low investment and quick results, and has broad development prospects in inland areas. But since when and where, news came that someone used contraceptives to ripen and fatten eels, causing people to stay away from eels. "These guys who look like snakes but look kinder than snakes have become as scary as venomous snakes, and they bring their hormones to the people who eat them."

The head of the Provincial Fisheries Bureau introduced the biological characteristics of the eel to reporters. The eel is hermaphroditic, with both organs on the same body, so it can change sex. It is female when it is a larva, and then gradually becomes male as the female hormone shrinks. Regarding the rumor that individual businesses let the eels take contraceptives to change sex in order to make huge profits, the head said, "Most contraceptives are expensive, and even if they are used, the input and output are not proportional." Relevant experts also revealed that contraceptives are actually very complicated. They both inhibit estrogen and contain estrogen. Too much estrogen will in turn inhibit androgen. After spot checks on eels in the aquatic market, it was found that the estrogen content was within the normal range and there was nothing unusual.

The results of the feeding experiment show that if the eel eats feed with a certain concentration of norethindrone (a common contraceptive), the residue of norethindrone can be detected in its body. In order to ensure the safety of the public and guide consumption correctly, Shanghai and other places have carried out special tests on the residues of contraceptives and other sex hormone substances in eels. The results show that norethindrone was not detected in wild and farmed eels collected from the market. Experts say that this shows that the eels currently supplied on the market can be eaten with confidence.

2. Are there male and female eels?

Yes! There are many special features in the reproduction habits of the yellow eel. The reproductive glands of female yellow eels are different in size, with the right side well developed and the left side degenerated. Generally, fish can reach sexual maturity at the age of 2. The biggest feature of yellow eel reproduction is the phenomenon of "sex reversal". From the embryonic period to the first sexual maturity, it is a female individual. After spawning, the ovary gradually becomes a testis. At the second sexual maturity, sperm is discharged, and then it is a male for life. If the length of the yellow eel is divided, those with a body length of less than 22 cm are all female; around 36 cm, the number of males and females is half each; individuals over 53 cm are all male. The spawning period of the yellow eel is from April to August, and the number of eggs is relatively small, generally about 500. When spawning, the parent fish often spits foam at the entrance of the cave where they live, and the eggs are laid at the roots of aquatic plants or between the cracks of rocks near the entrance. The foam has the function of retaining the eggs. The fertilized eggs usually hatch into young fish in about 8 days. About 12 days after hatching, the young fish can actively swim and forage for food. During this period, the young fish rely on the protection of the male and female parents and the nutrition of the yolk sac.

The growth of eels is relatively slow. A one-year-old fish can grow to 20 cm, a two-year-old fish can grow to 30 cm, and a three-year-old fish can grow to 40 cm. The growth rate of artificially cultivated eels is related to whether there is enough bait. When there is enough bait, they generally grow faster than in nature.

The activity of the eel is closely related to the water temperature, and the suitable water temperature for its growth is 15-30℃. When the water temperature is below 10℃, it stops eating and enters hibernation; when the water temperature rises above 15℃, it starts to hunt normally; when the water temperature exceeds 30℃, it burrows into caves to spend the summer.

3. What are the morphological and physiological characteristics and living habits of the eel and loach?

(I) Types and distribution

Loach, commonly known as loach. There are many species, including loach, large-scaled loach, Inner Mongolian loach (E's loach), cyan loach, pseudo loach, two-color loach, etc. There are more than ten species in the world, and their appearances are almost the same. At present, loach is commonly cultivated, and a small number of large-scaled loach are cultivated.

Loaches are widely distributed naturally. Except for the Qinghai-Tibet Plateau, they are distributed in natural freshwater bodies such as rivers, ditches, paddy fields, ponds, lakes and reservoirs across the country. They are especially widely distributed in the middle and lower reaches of the Yangtze River and Pearl River basins. The unit yield is not high, but the group yield is large.

Loaches are widely distributed abroad in Japan, North Korea, Russia, India and other places along the coast of Asia.

Loach has strong vitality and high adaptability to the environment. Its food is easy to obtain. The area occupied by breeding is small, the water consumption is not large, it is easy to raise and transport, and the cost is low, the profit is large, and the effect is fast. Therefore, in addition to developing large-scale breeding, various water bodies such as rice fields, depressions, and ponds can be used to develop loach breeding according to local conditions and local materials. It can be expected that loach breeding industry can be vigorously developed in my country's aquaculture, especially in rural family sideline industries, and has a good prospect.

(IV) Living habits and characteristics of loach

1. Lifestyle

(1) Benthic loaches are warm-water bottom-dwelling fish that prefer to live in shallow waters with lots of soft mud, such as ditches, ponds, lakes, and paddy fields, or in the surface of mud with lots of humus. Generally, they rarely swim to the upper or middle layers of the water.

(2) Thermophilic loach is suitable for water temperatures of 15-30℃, and the most suitable water temperature is 25-27℃. When the water temperature exceeds 34℃ in summer, or is below 6℃ in winter, or when the water is dry during the dry season, it will dive into the mud layer 10-30 cm deep and enter a dormant state without eating or moving. During the dormant period, as long as there is water in the mud layer to moisten the skin, it can survive. This is because loach can breathe through its skin and intestines in addition to its gills.

(3) Tolerance to hypoxia and intestinal respiration is a physiological phenomenon unique to loach. The intestinal wall of loach is thin and rich in blood vessels, which can assist breathing and gas exchange. When the water temperature rises or there is a lack of oxygen in the water, loach swims vertically to the water surface to breathe air, and makes a sound of its body hitting the water surface when sinking. The swallowed air undergoes gas exchange in the intestinal tract, absorbing oxygen, and the excess waste gas and carbon dioxide produced in the intestine are discharged from the body through the anus. Loach's ability to tolerate hypoxia is far better than other farmed fish, making it suitable for high-density farming. When other farmed fish die due to lack of oxygen in intensive pond farming, loach can still move normally, so there is great potential for increased production. It will not die due to lack of oxygen during transportation.

(4) Good at escaping. Loaches are very good at escaping. There is a lot of rain in spring and summer. When the pond is full or the pond wall is washed out by water, all the loaches will escape overnight, especially when the water level rises, they will escape from the inlet and outlet of the loach pond. Therefore, when raising loaches, it is important to strengthen anti-escape management. Check whether the anti-escape facilities at the inlet and outlet are intact and whether there is any blockage. Drain the water in time to prevent loaches from escaping.

(5) Nocturnal loaches are used to eating at night, but they also eat during the day during the spawning period and the period of vigorous growth. In artificial breeding, they can also be domesticated to eat during the day. Both young loaches and adult loaches have no obvious phototaxis or photophobia reactions to light.

2. Food Habits Loach is not very picky about food. It feeds on mud, humus, and leftovers of other animals in the water. The types of bait organisms it feeds include aquatic animals and plants. When the total length of loach is 3-5 cm, it likes to eat humus, followed by small crustaceans, insects, etc. In the gastrointestinal food mass, the proportion of mud and humus is as high as about 70%, and the weight of biological bait accounts for only 30%. When the total length is 5-8 cm, it likes to eat aquatic plankton, silk earthworms, etc., and occasionally eats algae, small food debris, and tender leaves and buds of water plants. When the total length is 8-10 cm, the diet is more mixed, mainly eating debris, diatoms, rhizomes, leaves, seeds of higher aquatic plants, and large plankton, and also eats some algae and microorganisms. Under artificial breeding conditions, biological bait can be cultivated by fertilization, or low-cost commercial feed can be fed, such as rice bran, bran, bean dregs, third-grade flour, etc., in addition to snails, earthworms, silkworm pupa powder, river clam meat and poultry offal, etc. If loach is raised with other fish, it can eat the feces and residues of fish, so loach is called the cleaner in the pond.

Whether loach is eating natural feed or artificial feed, it shows a clear preference for animal feed.

Since loaches have a varied diet, a wide and abundant source of bait is very important for their rapid growth and increased production.

3. Feeding and digestion As the loach grows larger, its eating habits also increase. We often see loaches eating a lot due to their greed, especially when there is abundant food organisms, their bellies are full.

Loach belongs to the Cypriniformes, Cobitidae, Loach genus. Loach has strong vitality, reproduces quickly, and feeds on a variety of baits. It is the easiest fish to raise and can produce high yields. Loach meat is delicious, nutritious, and high in protein. It also contains fat, riboflavin, phosphorus, iron and other nutrients. In medicine, it has a certain effect on carbuncle, mumps, etc., and is also one of the main aquatic products for foreign trade export. It has become the main species for garden farming, pond farming and other small water surface farming.

1. Characteristics

The loach has a long body, slightly rounded in the front and flattened in the back. It has small eyes, a lower mouth, well-developed lips, and 5 pairs of whiskers. Its scales are small and buried under the skin. Its pectoral and caudal fins are round, with a round black spot at the base. Its body is gray-black with many irregular black spots. Its body color generally varies depending on its living environment. Its abdomen is lighter in color, and its body surface is rich in fluid.

Loach is a bottom fish, mostly living in still water and the bottom of water with soft mud. Except for special reasons, it generally does not move to the upper and middle layers of the water. The suitable water temperature for growth is 15-30℃, and the optimal growth temperature is 25-27℃. When the water temperature is below 5℃, or above 35℃ in summer, or when there is drought and little water, it will dive into the mud layer to "hibernate". As long as there is a little moisture in the soil to moisten the skin, it can maintain life.

In addition to breathing through gills and skin, loaches can also breathe through their intestines, which is a unique physiological phenomenon. Their intestinal walls are thin and rich in blood vessels, which allows for gas exchange. When there is a lack of oxygen in the water, they float to the surface, inhale air, absorb oxygen in their intestines, and then expel waste gas from their anus. Another characteristic of loaches is that they are very likely to escape. When the water in the pond rises, they will escape in large numbers as long as there are holes in the bank. Especially in the spring and summer evenings or on rainy days, when the water level rises, loaches can easily escape from the water inlet and outlet. Therefore, escape prevention is a link that deserves attention in the process of artificial breeding of loaches.

Loach is omnivorous. In the seedling stage, when the body length is less than 5 cm, it mainly feeds on cladoceran zooplankton. When the body length is 5-8 cm, it becomes omnivorous. In addition to benthic animals such as crustaceans, insect larvae, silk earthworms, etc., it also feeds on algae, higher plants, and organic residues and debris. When the body length grows to more than 8 cm, it mainly feeds on plant baits, such as diatoms and the roots, stems, leaves and seeds of higher plants. Loach is particularly greedy. In natural water bodies, it feeds mostly at night. During the spawning period and the peak growth season, it also feeds during the day. When the water temperature is 25-27℃, the appetite is the strongest and the growth is rapid. When the water temperature exceeds 30℃ or is lower than 15℃, the appetite decreases and the growth is slow.

Generally, loach can reach sexual maturity after two winters. The spawning period is from April to September, with the peak period from May to July. The suitable water temperature is 25-26℃. During spawning, the male fish tightly wraps around the female fish, pressing the female fish's abdomen to make the eggs out of the body. At the same time, the male fish releases sperm for external fertilization. The eggs are often laid in ditches, shallow water belts, paddy fields, and the roots of aquatic grass seedlings. The eggs are yellow, round in shape, and have poor adhesion.

(ii) Loach breeding

1. Natural spawning and reproduction: Put the parent loach in the spawning pond. When the water temperature rises to above 18℃, place sterilized palm leaves, willow roots, water plants, etc. in the pond as fish nests. After the loach is found to be spawning, the fish nest with eggs is transferred to the hatching pond, and a new fish nest is placed at the same time, waiting for it to spawn again. The hatching of fertilized eggs does not require high water quality and temperature. It can be hatched in general aquaculture water bodies. The optimal water temperature is 20-28℃, and the fry can hatch in about 2 days.

2. Artificial induction of spawning: When the temperature is suitable in April and May, the parent loaches are caught and the parent fish with soft abdomen, slightly pink color, and signs of spawning are selected. The body length is 12 cm, the weight is more than 20 grams, preferably about 40 grams, and the individual is induced to spawn by drugs. The dosage is 1 carp PG or LRH-A5-10 micrograms per female loach, and the dosage for male loach is halved. Then put the parent loach after induced spawning into the spawning pond and let it spawn and fertilize on its own or perform artificial insemination. Finally, put the fertilized eggs into the hatching pond for hatching. This method can obtain loach seedlings in a timely, large and planned manner.

3. Stocking and supplementary breeding: This is the simplest breeding method. That is, put an appropriate amount of mature loaches in the water body for breeding loaches, but do not stock other fish at first. In the reproductive season, place some artificial fish nests. Loaches lay eggs in the breeding pond, hatch in the original pond, and the seedlings can be cultivated in small boxes and then still raised in the original pond.

(III) Artificial breeding

1. Raising loaches in special ponds: This is generally a breeding method adopted by producers who specialize in loach breeding. It is best to have a mud bottom in the pond. If it is a cement pond, the soil should be 15-20 cm deep. The area of ​​the pond should not be too large, about one mu is appropriate, and 50,000 to 100,000 loach fry of 3-4 cm in length can be placed per mu. Before stocking, base fertilizer should be spread, mainly organic fertilizer, to cultivate a large number of zooplankton as bait for loaches. After stocking, gradually increase the feeding of feed and apply organic fertilizer. The main types of bait fed are rice bran, bean cake, bran, bean dregs, silkworm pupa powder, earthworms and food processing waste. Fertilizers include human and livestock manure, crop straw compost, etc. If the water temperature is higher than 30℃ or lower than 10℃, the amount of feed should be reduced. Generally, it can be caught after 8-10 months of breeding, and the high yield can reach 250-400 kg per mu. Each tail is about 15 cm long and weighs 10-15 grams, meeting commercial specifications.

2. Raising loaches in rice fields: Raising loaches in rice fields can use natural bait organisms such as earthworms, chironomid larvae, water fleas and weeds in the fields. By feeding a small amount of feed, good economic benefits can be obtained. Because loaches have strong vitality, they can burrow into wet mud and use their intestines and skin to breathe to maintain their lives even when the rice fields are drained and dried. Therefore, loaches are one of the ideal objects for rice field farming. The construction of rice fields is the same as raising other fish. One or several fish ponds are dug in the fields, with an area of ​​2-3 square meters and a depth of about 50 cm. The fish ponds are connected to the fish ditches, and the fish ditches are opened in the shape of "field" or "well". Fish interception facilities should be set up at the inlets and outlets to prevent escape. 15,000 to 20,000 loach seedlings with a body length of about 3 cm are released per mu of rice fields. Be careful to avoid using lime and pesticides, feed and fertilize appropriately, and catch them after harvesting rice in autumn, or continue to breed them after irrigating them, and catch them again when the fields are plowed in the spring of the following year. Generally, 30-50 kilograms of loaches can be harvested from every acre of rice field.

In addition, a certain number of loaches can be raised in the water surface where lotus, water chestnut and other aquatic plants are planted. Generally, about 1,000 loaches are raised per mu. As long as the breeding and management are proper, the income from the water surface can be increased.

3. Family loach farming: that is, using the pits and reservoirs in front of and behind the house or in the vegetable garden to raise loaches. Generally, 4,000 to 5,000 loach fry of 3 to 4 cm in length can be put into stocking per mu, or they can be mixed with eels and carps. Pay attention to feeding and fertilizing at ordinary times, such as leftovers, livestock and poultry manure, vegetable leaves, etc., and the feed should be a combination of plant and animal. At the same time, control the water quality. When loaches are found to jump out of the water and "swallow air", it indicates that there is a lack of oxygen in the water body. Fertilization should be stopped and new water should be replaced. When loaches grow to 15 to 20 cm, they can be caught and put on the market. After 8 to 10 months of breeding, the yield per mu can reach more than 100 kilograms.

4. Fishing methods

Since loaches are small and have the ability to burrow into mud, it is difficult to catch them. Based on other characteristics of loaches, the following fishing methods can be used:

Flush fishing: that is, lay a fine-mesh fishing net near the water inlet, and then release water from the water inlet. Since loaches have the habit of escaping upstream, they gather at the water inlet. Then, lift the laid net in time to catch the loaches.

Bait trapping: Place fragrant rice bran or other palatable bait in the net, and when the loach enters to look for food, lift the net at the right time to catch it. You can also put the bait into a fish basket or sack and put it on the bottom of the water, which will attract a large number of loaches. It is often put in the evening and taken out the next morning, and a certain number of loaches can be caught.

Catch in a dry pond: When the water temperature drops below 15℃ in autumn, loaches will gradually burrow into the mud at the bottom of the pond. You can catch them by draining the pond. Generally, drain the pond water first, then dig several small ditches in the mud at the bottom of the pond. The loaches will gather in the drainage ditch and be caught manually, or dig the mud containing loaches into an iron sieve and use water to flush away the mud and catch them.

(V) Temporary Care and Transportation

1. Temporary breeding: The captured loaches should be temporarily raised for 1-3 days before being sold to remove the muddy smell of the fish meat and empty the feces in the intestines. This can not only improve the edible value of the loaches, but also maintain good water quality during transportation and improve the survival rate.

The temporary rearing of loaches can be done in bamboo baskets or wooden barrels. Bamboo baskets are commonly used for temporary rearing. The bamboo baskets containing fish are placed in the water, with 1/3 of the basket exposed to the water surface to ensure that the loaches can breathe. A bamboo basket with an upper diameter of 24 cm, a bottom diameter of 65 cm, and a height of 24 cm can temporarily rear 7-8 kg of loaches in still water, and 15-20 kg in flowing water. Small floating net cages can also be used for temporary rearing.

When using wooden barrels or iron boxes for temporary breeding, put about 140 grams of loach per liter of water. At the beginning of temporary breeding, change the water at least 4-5 times a day, and after two days, change the water 2-3 times a day. Each time, change about 1/3 of the water in the barrel.

2. Transportation

The transportation methods are mostly dry transportation and nylon bag oxygenation transportation. If the transportation time is within 2-3 hours, dry transportation can be used, that is, the loach is placed in a container with an appropriate amount of soft grass, and the loach skin is kept moist for transportation. In the case of long-distance transportation, nylon bag oxygenation transportation can be used. After adding a small amount of water or ice cubes to each 60×120 cm nylon bag, put 10 kg of loach, fill it with oxygen, tie the bag tightly, and put the bag in a cardboard box or wooden box for transportation.

(VI) Disease prevention and control

During the loach breeding process, attention should be paid to disease prevention and control. Common diseases include red fin disease, saprolegniasis, bubble disease, bone curvature disease, trichodinasis, tongue cup disease, pesticide poisoning and other biological pests. The main prevention and control methods are as follows:

1. Prevention and treatment of diseases caused by microorganisms

Red fin disease: This disease is very harmful to loaches and has a high incidence rate. It is caused by bacillus. Symptoms include the fins or part of the body peeling off, turning grayish white, the muscles starting to rot, the anus turning red, and then blood spots appearing in these parts, which gradually turn dark red. In severe cases, the fins fall off, the fish stop eating, and eventually die. It is mainly prevalent in summer. Prevention and control methods: Avoid fish injuries, and use 4% salt water baths for disinfection before stocking the seedlings; when the disease occurs, chloramphenicol can be mixed into the feed for feeding.

Saprolegniasis: For loach eggs, add 400 grams of salt and 400 grams of baking soda per cubic meter of water to bathe for 1 hour. Diseased loaches can be soaked in 3% salt solution for 5-10 minutes.

Printing disease: The lesions are generally oval or round, swollen and erythematous. The affected area is mainly at the base of the caudal peduncle. It is prevalent in July and August. Treatment can be done by spraying 1 gram/cubic meter of bleaching powder or 2-4 grams/cubic meter of gallnut on the whole pond.

2. Prevention and control of diseases caused by parasites

Trichodinasis: parasites on the gills and body surface. After the disease occurs, the food intake decreases and the animals swim alone. In severe cases, the worms are densely packed and can cause death if not treated in time. It is prevalent from May to August. The preventive measure is to clean the pond with quicklime; the treatment method is to spray the entire pond with 0.7 grams of crystal trichlorfon per cubic meter of pond water.

Tongue cup worm disease: parasites on the skin or gills of loaches, and usually absorbs food from the surrounding water for nutrition, which has little effect on loaches. However, if a large number of parasites are on loach fry, it will cause breathing difficulties, and in severe cases, it will lead to the death of loach fry. It can occur all year round, but it is more common from May to August. Prevention is mainly to soak the loach seedlings in 8 grams/cubic water copper sulfate solution for 15-20 minutes before stocking. After the disease occurs, treatment is to spray the whole pond with 0.7 grams/cubic water copper sulfate and ferrous sulfate (5:2) mixture.

3. Pesticide poisoning

The residual toxins of various chemical pesticides used in farmland will pollute the water quality to varying degrees, causing loaches to be poisoned and die. This disease can easily cause great losses to loaches raised in rice fields. Therefore, in order to ensure the safety of loaches, the amount of pesticides used and the method of spraying must be strictly controlled.