The living habits and morphological characteristics of sweetfish (the king of freshwater fish)

Ayu (Plecoglossus altivelis) belongs to the order Salmoniformes, the family Ayuidae, and the genus Ayu. Commonly known as: Qiu Sheng fish (Liaodong Peninsula), sea baby fish (West Coast of Bohai Sea), catfish (Japan), New Year fish, sweet fish, sweet fish, moon fish, sweet fish. English name: Ayu, Sweetfish.

Endangerment level: Vulnerable.

Ayu has a slender body and a small head. The snout is pointed, with the front end bent downward into a hook-shaped protrusion. The mouth is large, with a protrusion at the front end of both sides of the mandible, and a concave shape between the protrusions. When the mouth is closed, the hook of the snout matches this concavity. There are wide and flat fine teeth in the upper and lower jaws. There are teeth in the premaxilla, maxilla and tongue. There are cystic mucosal folds in the floor of the mouth. Except for the head, the whole body is densely covered with extremely fine round scales. There is a small adipose fin behind the dorsal fin, opposite to the rear end of the anal fin. The back of the body is blue-black, the sides of the body are gradually yellowish from the upper half to the lower half, the abdomen is silvery white, each fin is light yellow, the adipose fin is reddish around, and there is a group of yellow spots above the pectoral fin.

Ayu is an anadromous migratory fish that lays eggs in rivers every autumn, and the young fish hatched that year go to the sea to overwinter. Overwinter on the calm coast. In the spring of the following year, the ayu, which is about 46 mm in length, travels up from the sea to the river's food-rich areas to be fattened. At this time, it is completely transparent, and the Japanese call it "fat catfish." It can travel more than 20 kilometers in one day and can overcome considerable obstacles. Tracing up is usually divided into 3 or 4 batches, with the first batch being the largest. If there is no cold water upstream, the ayu can be traced back closer to its origin. The river where ayu enters for fattening and spawning must have steep terrain, rapid water flow, small depth, noisy water flow, water temperature below 27°C, clear water quality and high transparency, and a gravel bottom with a lot of epiphytic algae and no sediment. Attached rivers leading to the sea. However, some rivers located in wide alluvial plains, with gentle water flow, fertile soil along the coast, sandy and silt bottoms, or large tidal waves are not suitable for the growth of sweetfish, so there are no sweetfish in such rivers. perch.

The spawning grounds of sweetfish are mostly in the middle and lower reaches of rivers where the current is rapid and there are gravels. The eggs are sticky and adhere to the gravels for hatching. The spawning season is from late August to late September. Spawning occurs most frequently at night. Whenever it rains and the water temperature drops, the number of spawning fish increases. The number of eggs a sweetfish can carry is approximately 46,900 for an individual with a tail length of 17 cm and a weight of 74.6 grams, and approximately 19,500 eggs for an individual with a tail length of 15.5 cm and a weight of 36.3 grams.

The juvenile sweetfish feed on zooplankton and can be caught with a hair hook. After entering fresh water, they mainly eat diatoms, basketweed and other plants on rocks. They also feed on insects and zooplankton, so fishermen often catch them with artificial bait hooks.

Ayu is mostly migratory, but land-locked ecological groups are found in a few rivers. The life cycle of ayu is short, and it dies after the 1+ year of sexual maturity and spawning. Ayu grows quickly, generally reaching a length of 18-25 cm and a weight of about 100 grams. The meat of ayu is mellow, tender and delicious, and has nourishing medicinal value. People in southern Fujian use it as a nutritional supplement for pregnant women, and it can also treat dysentery. Ayu is rare in the world because it has a cavity full of balsam on its back, which can emit a special fragrance. Therefore, it is known as the "King of Freshwater Fish" in the international market. Lien Heng, a famous historian and poet in Taiwan, once wrote a poem: "The spring water fills the Xindian River at the beginning, and the stream is filled with water casings on the edge of the glass. The sweet fish is just three inches on the hook, and the mandarin oranges are sprinkled to listen to the oriole." It is said that as early as the Qing Dynasty, Zhejiang Fuxi sweetfish was sealed as a tribute treasure, and it has always been sold at a very high price. The ayu produced in Yandang, north and south of Zhejiang, is roasted and dried into golden yellow dried fish. It is famous both at home and abroad for its excellent color, flavor and aroma.

Ayu is native to China, North Korea, and Japan, but currently the ayu in Korea and Japan has become extinct. Our country is distributed in coastal streams in the Yellow Sea, Bohai Sea, East China Sea and other places, reaching the Liaodong Peninsula and the Western Liaoning Corridor in the north, and southern Fujian and Taiwan in the south. Ayu was once a precious economic fish in my country. In the past two decades, due to the influence of the external environment, the amount of resources has decreased sharply. Especially in the upper reaches of the river section where ayu fish spawn and are fattened, a large number of forests have been cut down and the land has been reclaimed, causing severe soil erosion and destroying the ayu fish. environment for the rivers to thrive; in addition, almost all rivers have been dammed and built with reservoirs, blocking their migration channels and changing the hydrological conditions of the streams; industrial sewage is discharged into the streams in large quantities, causing serious water pollution and destroying the original ecology. The environment; what’s more serious is that poisoning and bombing that kill a large number of young and adult ayu fish are common in the production areas and seriously damage resources. In addition, bad fishing methods such as electricity, dense nets and cormorants have caused devastating damage to the ayu. Currently, ayu resources are in the "vulnerable" stage.

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