fish duck vs merganser

fish duck and merganser both are nouns.

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

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

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

Other synonyms of fish duck include sawbill, sheldrake.

fish duck (noun)	merganser (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