Let's take a look at freshwater eels

And, unlike most migratory fish,such as salmon,which spawn in fresh water but live their adult lives in salt water eels of the genus Anguilla migrate in the opposite direction,spawning and breeding in oceans and seas,while spending most of their intervening time in fresh water.If we were to take one such fresh water eel and follow its life story,it would be born in the middle of the North Atlantic Ocean,about a thousand miles east of Bermuda.This area, called the Sargasso Sea,forms the western part of a subtropical gyre,a giant whirlpool in the middle of the ocean.