Using the Earth's magnetic field as a compass for large distances and then orienting oneself with sight and smell when they are near the place to be reached.

The magnetic field, in fact, varies (even if only slightly) from one year to the next, but it is very good to guide them in the migrations of thousands of kilometers that take them from the waters where they live (between the Faroe Islands and Greenland) and the rivers of Europe and North America where they were born and where they return to reproduce.

Magnetic field
According to a study by Kenneth Lohmann of the University of North Carolina at Chapel Hill and his collaborators, salmon, the first time they gain the sea, would in fact be able to memorize the characteristics of the magnetic field to exploit them when, around 4 years of age, they will have to look for the place where they were born. At the beginning of 2013, a new study by Oregon State University confirmed this theory by studying the data collected over 56 years of observations of red salmon migrations (Oncorhynchus nerka) from the ocean to the Fraser river in British Columbia. The course followed by the fish around the island of Vancouver seems to be determined precisely by the perception of changes in the intensity of the magnetic field.
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