fish fly vs fish-fly

fish fly and fish-fly both are nouns.

Word	Noun	Adjective	Verb	Adverb
fish fly	Yes	No	No	No
fish-fly	Yes	No	No	No

As nouns, fish fly and fish-fly are synonyms defined as:

fish fly and fish-fly: similar to but smaller than the dobsonfly; larvae are used as fishing bait

fish fly (noun)	fish-fly (noun)
similar to but smaller than the dobsonfly; larvae are used as fishing bait	similar to but smaller than the dobsonfly; larvae are used as fishing bait