sea animal vs bycatch

sea animal and bycatch both are nouns.

Word	Noun	Adjective	Verb	Adverb
sea animal	Yes	No	No	No
bycatch	Yes	No	No	No

As nouns, bycatch is a hyponym of sea animal; that is, bycatch 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
bycatch: unwanted marine creatures that are caught in the nets while fishing for another species

Other hyponyms of sea animal include by-catch.

sea animal (noun)	bycatch (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