by-catch vs sea animal

by-catch and sea animal both are nouns.

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

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

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

by-catch (noun)	sea 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