From the included article-

When it’s time to mate, eels are very determined to make it to their breeding site at the Sargasso Sea. The Sargasso Sea, a two-million-square-mile span of ocean,  is the site in which all freshwater eels mate

It’s way the hell down there in the article, though. Apparently they travel to freshwater as larva.

Eels are freaking weird, man.