Why do salmon swim to their place of origin to spawn?

Common salmon belongs to the Salmonidae family. Its hometown is Heilongjiang, Wusuli River and Tumen River. Every autumn, they travel together with male and female fish and swim thousands of miles from the ocean to their birthplace - inland rivers to breed their offspring. Why is this?

It turns out that salmon were originally freshwater fish, but the living environment there has changed. The rivers have dried up and food has become less and less. So, they swam in groups to the ocean far, far away. At first, many fish died because they could not adapt to the living environment there, but a very small number of fish survived tenaciously. Over time, salmon live comfortably in the ocean and have abundant food. Ordinarily, wouldn’t it be nice to “settle” in the ocean with peace of mind? Strangely enough, they have a strong sense of "homesickness" and have to return to their birthplace - the rivers - to breed offspring every year. This has become a "family rule", so they have to go on a difficult trip every year.

The salmon hatch in fresh water. When they grow to 4 to 5 centimeters, they collectively return to the sea and live a carefree life in the ocean. After about four years, when they are sexually mature, they set off together again and return to their birthplace to lay eggs.