The "Fish island" description is not an exaggeration. In Mark Kurlansky's book Cod: A Biography of the Fish that Changed the World, the fisheries around North America were described in this way, with cod so abundant that one need only lower a basked into the waters to pull it up filled with fish. The water itself could hardly be seen for all the fish that were swimming in it.
We think of the deep sea as a kind of desert concealing its life beneath a veil of water, but that is an image of recent centuries. The untouched ocean was teeming with life. Historical descriptions of New England say that lobsters used to wash up into giant piles on shore after a storm. The past century or so have established in our minds a normal image of an ocean of scarcity, where nets have to be cast deeply to haul up fish, and ships have to roam across leagues to encounter whales. But the times before human exploitation had the seas filled with life to an unimaginable extent. Vast herds of whales instead of the scattered pods we see now are the remnant of a devastated population surviving after a holocaust. Migrations of fish that you could practically walk on.