sea squab vs saltwater fish

sea squab and saltwater fish both are nouns.

Word	Noun	Adjective	Verb	Adverb
sea squab	Yes	No	No	No
saltwater fish	Yes	No	No	No

As nouns, saltwater fish is a hypernym of sea squab; that is, saltwater fish is a word with a broader meaning than sea squab:

sea squab: delicacy that is highly dangerous because of a potent nerve poison in ovaries and liver
saltwater fish: flesh of fish from the sea used as food

sea squab (noun)	saltwater fish (noun)
delicacy that is highly dangerous because of a potent nerve poison in ovaries and liver	flesh of fish from the sea used as food