Downriver migration of eels

Some fish that live in fresh water and migrate from fresh water to the ocean to spawn during their sexual maturity process migrate to the sea through rivers of varying lengths to spawn and swim to a certain spawning ground. The spawning migration down the river is also called the spawning migration down the sea. Fish of the genus Anguilla are typical representatives of migratory fish that descend into rivers, and Songjiang perch also belongs to this type.

There are many species in the genus Anguilla, with 19 species and subspecies existing in the world. Adult eels usually live in fresh water. When they are close to sexual maturity, they descend into rivers and sea in large numbers in autumn. After a long journey, they go to the depths of the ocean to reproduce. For example, the European eel crosses the Atlantic Ocean and, after a migration of 3,000-4,000 kilometers, arrives in the western Atlantic Ocean to lay eggs in the deep sea near the Bermuda Islands. The migration distance of American eels after descending from rivers into the sea is more than 2,000 kilometers. The spawning grounds of eels found in China and Japan are said to be in the western Pacific Ocean at 210-260N and 1230-1290E.

When eels migrate down rivers, they mostly move at night and are difficult to detect. Because eels have the auxiliary respiratory function of skin respiration, they can live out of water for a short period of time and can move from one lake to another through moist grass at night. After entering the sea, some changes occurred in its body shape and color. Its eyes became larger and its snout became pointed. The color of the back of the body becomes darker and the abdomen becomes silvery white, so it is called silver eel. In addition, the osmotic pressure of body fluids increases and the swim bladder becomes smaller.

Since eels live in fresh water, male and female individuals have no obvious secondary sexual characteristics, making it difficult to distinguish between male and female eels. Gonads cannot mature and reproduce in fresh water. Therefore, the reproductive habits of eels have long been regarded as very mysterious. In the past, some natural scientists made very unscientific conclusions about the reproduction of eels. Aristotle once Said: "Mature eggs and sperm have never been found in eels, so they may not have reproductive organs." He also said: "Eels are born from the ground and are a natural occurrence." This statement was supported by the famous scholar Wheeldon at the time.

Brynner believes that eels have no gender. He said: "When eels live, they rub their bodies on rocks, and the falling fragments are what give birth to the next generation." Some scholars have suggested: "Eels originated from 5 "It is produced by the dew on the early morning of the moon." or "Eels are formed from horse hair that falls in the water." or "Eels jump out from the gills of other fishes." Kyle Cross proposed an even more bizarre theory in 1862. Opinion: "The ancestor of the silver eel was a small beetle"! From the 19th century to the beginning of the 20th century, people only knew that in autumn, large groups of adult fish went to the sea. In spring, young eels or small eels about 6 to 7 centimeters long traveled up the river in groups and moved upstream. Therefore, it is natural to think that this young eel must be the descendant of the adult fish that went into the sea a few months ago. Some people think that eels reproduce in estuaries.

When eels live in fresh water, they are called yellow eels due to their yellowish body color. When they enter the sea for reproduction, their body color changes and they are called silver eels. Yellow eels inhabit various water bodies from the coast to inland. Females are usually larger than males. When they reach the age of 8-10 years, they start to stop foraging, their eyes become enlarged, their lips become thinner, their snout becomes pointed, their back color turns black, their sides change from yellow to silvery white, their internal reproductive organs develop and their digestive organs atrophy. They descend into rivers and seas in large groups in late summer and early autumn. At this time, the attraction for reproduction is very strong. Even if it usually lives in an isolated pond or lake, it can still pass through the wilderness at night from the dew-covered grassland, enter the nearby river, and smoothly descend into the sea. After a long journey, they arrive at a spawning site suitable for their reproduction. They reproduce in a sea area with a very high water temperature and suitable salinity at a depth of 400 meters. The parent fish dies immediately after reproduction.

The fertilized eggs of eels float in the sea for a period of time and hatch into transparent willow leaf-shaped larvae in spring, called leaf eels. Leaf eels have long, needle-shaped teeth that they use to catch tiny creatures for food. They soon began their long journey back to their hometown. Eel larvae and juveniles have to go through a long period of the willow eel stage and follow the sea currents to the coast of the sea where their parents descend into the river and go to sea. And begins to metamorphose into a young eel (also known as a thread eel or a glass eel). Metamorphosis occurs in autumn. During metamorphosis, it stops foraging, its needle-like teeth disappear, and its body length and height gradually shorten until it turns into a 5 or 6 cm long cylindrical thread eel. From newly hatched larvae to juvenile eels that have completed metamorphosis, it takes 2.5-3 years for European eels, 1 year for American eels, and 1 year for Chinese eels to complete metamorphosis. After metamorphosis, the young eels quickly develop a new set of small, conical teeth and are ready to begin their journey up the river.

Every spring, a large number of young eels enter rivers from the coast, and can continue to be traced upstream to areas thousands of kilometers away from the river mouth. For example, eels are found in the Jinsha River, Minjiang River and Jialing River in the upper reaches of the Yangtze River. They grow and fatten in rivers, lakes, ponds, and reservoirs, and they lie dormant during the day and emerge at night. When they reach sexual maturity, they begin to descend into rivers and seas in groups from autumn to carry out distant spawning migrations. Each eel goes through five growth and development stages in its life: ① The planktonic larvae era in the ocean; ② The coastal metamorphosis period, from willow eel to glass eel; ③ The yellow eel growth period in fresh water; ④ The silver eel that migrates down the sea Eels; ⑤ Died after laying eggs.

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