smelt vs soft-finned fish

smelt and soft-finned fish both are nouns.

Word	Noun	Adjective	Verb	Adverb
smelt	Yes	No	Yes	No
soft-finned fish	Yes	No	No	No

As nouns, soft-finned fish is a hypernym of smelt; that is, soft-finned fish is a word with a broader meaning than smelt:

smelt: small trout-like silvery marine or freshwater food fishes of cold northern waters
soft-finned fish: any fish of the superorder Malacopterygii

Other hypernyms of smelt include malacopterygian.

smelt (noun)	soft-finned fish (noun)
small trout-like silvery marine or freshwater food fishes of cold northern waters	any fish of the superorder Malacopterygii
small cold-water silvery fish; migrate between salt and fresh water

smelt (verb)	soft-finned fish (verb)
extract (metals) by heating