Peter being a novice at the Augustinian Abbey had been given the unpleasant task of flushing the monks waste into the river Wharfe, unfortunately he did not listen carefully enough to the instructions given to him. Indeed the morning was so mild and the trees so fine, by the time he had reached the river he was back in his childhood playing with his friends. Now the toilets were flushed by removing a stone and allowing the river to flush the open sewer. Peter removed the wrong stone, allowing the monks fish stock to return to the river. Peters penance, restock the fish pond.
Footnote: as there was no picture available, I made use of the name for my key. Peter Abbey