by-catch vs marine animal

by-catch and marine animal both are nouns.

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

As nouns, marine animal is a hypernym of by-catch; that is, marine animal 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 animal: any of numerous animals inhabiting the sea including e.g. fishes and molluscs and many mammals

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

by-catch (noun)	marine animal (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