Mergus merganser vs fish duck

Mergus merganser and fish duck both are nouns.

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

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

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

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

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