Mergus serrator vs fish duck

Mergus serrator and fish duck both are nouns.

Word	Noun	Adjective	Verb	Adverb
Mergus serrator	Yes	No	No	No
fish duck	Yes	No	No	No

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

Mergus serrator: widely distributed merganser of America and Europe
fish duck: large crested fish-eating diving duck having a slender hooked bill with serrated edges

Other hypernyms of Mergus serrator include merganser, sawbill, sheldrake.

Mergus serrator (noun)	fish duck (noun)
widely distributed merganser of America and Europe	large crested fish-eating diving duck having a slender hooked bill with serrated edges