smew vs fish duck

smew and fish duck both are nouns.

Word	Noun	Adjective	Verb	Adverb
smew	Yes	No	No	No
fish duck	Yes	No	No	No

As nouns, fish duck is a hypernym of smew; that is, fish duck is a word with a broader meaning than smew:

smew: smallest merganser and most expert diver; found in northern Eurasia
fish duck: large crested fish-eating diving duck having a slender hooked bill with serrated edges

Other hypernyms of smew include merganser, sawbill, sheldrake.

smew (noun)	fish duck (noun)
smallest merganser and most expert diver; found in northern Eurasia	large crested fish-eating diving duck having a slender hooked bill with serrated edges