Anadromous migration of Atlantic salmon

Some fish that live and grow in the ocean and enter rivers from the ocean to reproduce in fresh water after sexual maturity. After regular, long-distance, large-scale upstream spawning migration. Typical representatives include some fish species of the family Salmonidae and Herringidae. They forage and grow in the sea. Within a certain period of time, individuals that reach sexual maturity cluster in the ocean for long-distance migration, and enter fresh water through estuaries. They lay eggs in rivers and reproduce their offspring. The most famous examples of this category are lampreys, sturgeons, sprats, salmon, trout and rainbow trout.

Atlantic salmon is a very famous anadromous fish. It spawns in streams in the upper reaches of freshwater rivers and returns to the ocean for fattening after spawning. The young fish live in fresh water for 2-3 years, and then go to the sea, where they live for one or several years until they are sexually mature and return to their original birthplace to lay eggs. Although the spawning period of Atlantic salmon is from September to February of the following year, within a year, schools of fish approach the coast almost every month, and with the help of currents, they can be traced from the estuary into the river. To reach the upper reaches after entering the estuary, you must rely on your own swimming ability. The strength they use to complete their reproductive tasks is very powerful. In order to fly over obstacles in the river such as waterfalls and weirs, they must use extremely strong swimming ability. Ability to reach out of the water and jump over obstacles. Their behavior of "flying" over the waterfall has been praised as a spectacle for many years. They stop eating as soon as they reach fresh water, so they gradually lose weight after leaving the ocean and entering rivers. When it first enters the river, the salmon has a healthy and graceful shape and a distinctive silvery white color because the parent fish has stored sufficient nutrients in the ocean.

But after a long journey, once they got close to the spawning ground, there were significant changes, especially the external appearance. The bright silvery color disappeared and turned into a dark auburn. The male's front teeth changed, and the snout and lower jaw became prominent. The front end of the lower jaw curves upward to form a hook shape. When reaching the upper reaches of the river, the skin on the back becomes quite thick and becomes spongy, with scales buried in the middle. Red and orange spots and markings appear on the body, and black spots surrounded by white circles appear. During this period, males Salmon is also called red fish, and the black and strong mature female salmon is called black fish. Salmon undergo important changes in their muscles during their journey upstream. The muscles of salmon that have just left the ocean are solid and red when they start to swim upstream, and there is abundant fat in their tissues; but when they reach the upper reaches and are close to spawning, the fat has been exhausted due to the development of their reproductive organs, and the muscles become It is gray and dull. The process of rapid transfer of fat from muscles to reproductive organs causes sexual dimorphism in fish bodies. This is also a by-product formed during fat transfer. Because it cannot be completely excreted, it forms protrusions at the front of the upper and lower jaws or appears on the skin. stains.

Salmon spawning sites are chosen in shallow waters with gravel bottoms where the current is quite fast. Once the brood fish arrive at the spawning ground, the males and females first disperse in pairs, and then the female fish starts to work, shaking its body and tail to dig out shallow depressions and lay a few eggs in the depressions. These eggs are immediately fertilized by male fish, and the fertilized eggs sink to the bottom of the depression. Because the surface of the eggs is sticky, the eggs can adhere to the bottom of the river. Afterwards, the female covers the eggs with fine gravel. Because the water flow can quickly pass through the gaps in the fine gravel, the eggs can get sufficient oxygen. This spawning behavior is repeated every few minutes. After each spawning, the parent fish gradually swims upstream. This egg-laying action lasts for 1-2 weeks until all eggs have been ovulated and fertilization completed. The spawning bed or depression of salmon is called the "reproductive bed", and the reproductive bed of a pair of salmon can be several meters long. During the spawning period, male fish are generally very rough. If there is an intruder, the male fish will immediately chase and drive away violently. Even if they encounter a male fish of the same species, they will cause a fierce fight. The most difficult enemy to deal with in the spawning grounds is the male trout, which deposits its sperm on the eggs of the salmon when the male salmon is not around.

The spawning operation is very draining for salmon, especially males. The broodstock fish after laying eggs appear very tired, with a big head and a thin body. In a state of extreme fatigue, it is very difficult to return to the ocean after a long journey. Most individuals are either too tired, or due to disease or trauma and hunger, or are killed by water birds, river otters and other enemies. . Only a few male fish can return to the ocean and survive until the second time to spawn. But most of the female fish can still return to the ocean, get abundant food in the ocean, and return to their previous normal state. Their bright silvery color appears again, and the protruding part of their jaws is also reduced due to absorption.

There are many species of salmon, including 5 or 6 species on the Pacific coast of North America, many in Northern Europe, and 3 species in the Heilongjiang Basin of my country. They all have similar spawning habits. For some species, the journey upriver is more arduous, and some travel more than 2,000 kilometers. Therefore, after the reproduction of these salmon is completed, sometimes before it is completed, they all die. In some rivers, the dead salmon floating in the river can be several kilometers long, or piled up to several centimeters high on the river bank.

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