As a part of my dissertation I’ve been researching eel-rents in medieval England. I am in the process of assembling a database of eel-rents from the 10th century onward. Though eels (and probably other in-kind rents) become increasingly replaced by currency by the 13th century, there are still eel-rents on the books into the 1600s. I’m planning on mapping the eel-rents both by the origin of the eels and by their destination, with the hops of creating a good visualization of the movement of eels as currency around the country. At present, though, I only have origins mapped, along with notes and citations. You can see more here.