eel vs freshwater eel

eel and freshwater eel both are nouns.

Word	Noun	Adjective	Verb	Adverb
eel	Yes	No	No	No
freshwater eel	Yes	No	No	No

As nouns, freshwater eel is a hyponym of eel; that is, freshwater eel is a word with a more specific, narrower meaning than eel:

eel: voracious snakelike marine or freshwater fishes with smooth slimy usually scaleless skin and having a continuous vertical fin but no ventral fins
freshwater eel: eels that live in fresh water as adults but return to sea to spawn; found in Europe and America; marketed both fresh and smoked

Other hyponyms of eel include elver, common eel, Anguilla sucklandii, tuna, moray, moray eel, conger, conger eel.

eel (noun)	freshwater eel (noun)
voracious snakelike marine or freshwater fishes with smooth slimy usually scaleless skin and having a continuous vertical fin but no ventral fins	eels that live in fresh water as adults but return to sea to spawn; found in Europe and America; marketed both fresh and smoked
the fatty flesh of eel; an elongate fish found in fresh water in Europe and America; large eels are usually smoked or pickled