It turns out that salmon used to be a freshwater fish, but the living environment there has changed, the rivers have dried up and the food is decreasing. So, they swim in groups to the far, far ocean. At first, many fish died because they didn't adapt to the living environment there, but a few survived tenaciously. Over time, salmon lives comfortably in the ocean and has rich food. Supposedly, isn't it nice to settle in the ocean? Strangely, their homesickness is very strong, and they have to return to their birthplace-rivers, lakes and seas every year to reproduce, which has become a "family rule", so they have to go out for a difficult trip every year.