merganser vs fish duck

merganser and fish duck both are nouns.

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

As nouns, merganser and fish duck are synonyms defined as:

merganser and fish duck: large crested fish-eating diving duck having a slender hooked bill with serrated edges

Other synonyms of merganser include sawbill, sheldrake.

merganser (noun)	fish duck (noun)
large crested fish-eating diving duck having a slender hooked bill with serrated edges	large crested fish-eating diving duck having a slender hooked bill with serrated edges