bycatch vs sea creature

bycatch and sea creature both are nouns.

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

As nouns, sea creature is a hypernym of bycatch; that is, sea creature is a word with a broader meaning than bycatch:

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

Other hypernyms of bycatch include marine animal, marine creature, sea animal.

bycatch (noun)	sea creature (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