Which ocean is called the "Herring Pond"?
Answer: The Atlantic
They probably called it that because there is a lot more herring fish in it than any other kind.
(According to the Second Edition of the Oxford English Dictionary, the saying is surprisingly old, with the first example of great pond being recorded in 1641 and herring-pond in 1686. Early examples are all from writers in various British North American colonies and so it’s reasonable to suppose that the expressions originated there.)