I had the same thought. I always knew they were fish but always assumed they were local fresh water fish. I mean everyone talks about how Salmon does this incredible journey. If there was another species which did something equally incredible I should have heard about it.

Thanks for the link! A rabbit hole indeed.

These eels undergo a notable sequence of transformations before their journey back to the sea. It wasn't until the 19th century that science connected the transitions from glass eel (larval form), to elvers, to yellow eel (freshwater adult), to silver eel (ocean spawning) form as members of the same species. Salmon are less mysterious as their spawning could be observed.