Mergus albellus vs fish duck

Mergus albellus and fish duck both are nouns.

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

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

Mergus albellus: 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 Mergus albellus include merganser, sawbill, sheldrake.

Mergus albellus (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