Eel and Freshwater eel

Eel noun - Voracious snakelike marine or freshwater fishes with smooth slimy usually scaleless skin and having a continuous vertical fin but no ventral fins.

Freshwater eel and eel are semantically related. In some cases you can replace phrase "Freshwater eel" with "Eel", this terms are similar.

Freshwater eel noun - Eels that live in fresh water as adults but return to sea to spawn; found in Europe and America; marketed both fresh and smoked.

Eel and freshwater eel are semantically related. You can use "Eel" instead a noun phrase "Freshwater eel".