fish vs net fish

fish is a noun but net fish is not a noun.

Word	Noun	Adjective	Verb	Adverb
fish	Yes	No	Yes	No
net fish	No	No	Yes	No

As verbs, net fish is a hyponym of fish; that is, net fish is a word with a more specific, narrower meaning than fish:

Other hyponyms of fish include crab, seine, scallop, scollop, rail, brail, angle, shrimp, shark, trawl, still-fish, prawn.

fish (noun)	net fish (noun)
any of various mostly cold-blooded aquatic vertebrates usually having scales and breathing through gills
the flesh of fish used as food

fish (verb)	net fish (verb)
catch or try to catch fish or shellfish	fish with nets
seek indirectly