The island fish

The Voyage version of the island fish goes like this:

Brendan and his monks arrive at a wooded shore growing on the back of a fish which has sustained itself for many years in a place where a river runs into the sea. They enter the wood and start cutting wood for a fire and hang up their kettle. They find a dry tree and as soon as they start cutting it down (or, as soon as the fish feels the fire), the shore disappears under water. The mariners barely manage to scramble back to their ship. Once safely there, they sing God’s praise. This was the third wonder that they saw. Brendan says: “It must have been a fish which pulled down the forest. He must have been old for all that wood to have grown on his back.” Next they are blown across the sea. They pray to God that they may soon reach land and have a rest.

-- in Strijbosch, The Seafaring Saint: Sources and Analogue of the 12th Century “Voyage of St. Brendan” (50)

The story repeats in greater detail over 3 episodes in the Navagatio and is repeated in the Vita Brendani.

The story was widely disseminated during the Middle Ages, and may root in the early Christian Greek work Physiologus, which discusses animals, plants and stones, and is infused with Christian moral teaching. Here is the account from that work:

... A certain whale in the sea is called the aspidocelon and is exceedingly large like an island, heavier than sand, and is a figure of the devil. Ignorant sailors tie their ships to the beast as to an island and plant their anchors and stakes in it. They light their cooking fires on the whale, but when he feels the heat, he urinates and plunges into the depths, sinking all the ships. You also, O man, if you fix and bind yourself to the hope of the devil, he will plunge you along with himself into hell-fire.

-- in Strijbosch, ibid 52