How does a salmon know the river in which they were born?

When salmon grow up, they go out to sea, but at the time to lay their eggs, they return to the river where they were born.

It is believed that salmon remember the smell of the river in which they were born and rely on that memory to find it.

Salmon eggs are laid in the fall and hatch in about two months.
Newborn fry have a bag full of nutrients hanging from their stomachs, and for another two months, they do not move at the bottom of the river and grow only with the nutrients in the bag.

And in the spring they start swimming and getting their own food. In early summer, when they grow to about 3-4 centimeters, fry form a flock, descend the river and head to the sea.
At this time, their body changes from a body that can live in fresh water to a body that can live in seawater, the pattern of their body is erased, and it becomes silver.

The fry that have gone out to the sea grow up to about 10 centimeters near the shore and gain strength. And finally heading offshore.
they then continue to grow as they travel through the northern Pacific Ocean, and 3-4 years later, they return to their native river to lay eggs.

When heading to Japan from the northern Pacific Ocean, they are thought to be orienting by feeling the Earth's magnetism or using the sun as a marker, but it is not yet clear.

Salmon reach the shores of Japan around autumn. Based on the memory of the smell they smelled when they were born, they find the river where they were born and go up the river to the spawning ground.

This time, on the contrary, their body structure is changed from a body that can live in seawater to a body that can live in fresh water, and the body color becomes a mixture of red, black, and yellow.

When they reach a place that the bottom is gravel in the middle stream of the river, female salmon use their tail fins to make a depression in the bottom of the river and lay eggs there.

The male ejaculates on the egg. The female then uses the tail fin again to gravel over her egg to hide it.
And they are exhausted and the salmon end their lives.

Salmon grow up to about 70 centimeters. They live on jellyfish, crabs, shrimp, etc.
Salmon lay about 3,000 eggs at a time, but only two of them can grow up.