It's a common misconception that salmon swim upstream into freshwater rivers and creeks to spawn. Spawning is actually an incidental side effect. In truth, salmon are the aquatic equivalent of salt miners. They spend a two-year tour collecting salt from the ocean and then return home ladened (and pre-seasoned) with salty goodness. Then they a protective layer of eggs over their horded collection of salt... as both cushioning and a tasty distraction for would-be thieves.