Because if they didn't die, the waters of the world would be filled with salmon. They have to die, similar to other life forms. If they didn't die, they would overpopulate the Earth.
how do salmon die
Cecil Salmon died in 1975.
William Salmon died in 1713.
Eliza Salmon died in 1849.
Isidore Salmon died in 1941.
Herman Salmon died in 1980.
Robert Salmon died in 1845.
George Salmon died in 1904.
Graham Salmon died in 1999.
Gaston Salmon died in 1917.
Nancy Salmon died in 1999.
André Salmon died in 1969.