fish vs grey mullet

fish and grey mullet both are nouns.

Word	Noun	Adjective	Verb	Adverb
fish	Yes	No	Yes	No
grey mullet	Yes	No	No	No

As nouns, grey mullet is a hyponym of fish; that is, grey mullet is a word with a more specific, narrower meaning than fish:

fish (noun)	grey mullet (noun)
any of various mostly cold-blooded aquatic vertebrates usually having scales and breathing through gills	freshwater or coastal food fishes a spindle-shaped body; found worldwide
the flesh of fish used as food	highly valued lean flesh of marine or freshwater mullet

fish (verb)	grey mullet (verb)
catch or try to catch fish or shellfish
seek indirectly