sea creature vs bycatch

sea creature and bycatch both are nouns.

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

As nouns, bycatch is a hyponym of sea creature; that is, bycatch is a word with a more specific, narrower meaning than sea creature:

sea creature: 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 creature include by-catch.

sea creature (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