sea animal vs by-catch

sea animal and by-catch both are nouns.

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

As nouns, by-catch is a hyponym of sea animal; that is, by-catch is a word with a more specific, narrower meaning than sea animal:

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

Other hyponyms of sea animal include bycatch.

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