by-catch vs sea creature

by-catch and sea creature both are nouns.

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

As nouns, sea creature is a hypernym of by-catch; that is, sea creature 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 creature: 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 animal.

by-catch (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