by-catch vs marine creature

by-catch and marine creature both are nouns.

Word	Noun	Adjective	Verb	Adverb
by-catch	Yes	No	No	No
marine creature	Yes	No	No	No

As nouns, marine creature is a hypernym of by-catch; that is, marine creature is a word with a broader meaning than by-catch:

by-catch: unwanted marine creatures that are caught in the nets while fishing for another species
marine creature: any of numerous animals inhabiting the sea including e.g. fishes and molluscs and many mammals

Other hypernyms of by-catch include marine animal, sea animal, sea creature.

by-catch (noun)	marine creature (noun)
unwanted marine creatures that are caught in the nets while fishing for another species	any of numerous animals inhabiting the sea including e.g. fishes and molluscs and many mammals