Eels were commonly currency:
https://www.atlasobscura.com/articles/medieval-eel-rent-map-...
replies(1):
https://www.atlasobscura.com/articles/medieval-eel-rent-map-...
https://news.ycombinator.com/item?id=35997727 ("To Pay Rent in Medieval England, Catch Some Eels (atlasobscura.com)", 42 comments)
https://news.ycombinator.com/item?id=25543802 ("Paying Medieval Taxes Using Eels (historiacartarum.org)", 14 comments)
https://news.ycombinator.com/item?id=34284363 ("English Eel-Rents: 10th-17th Centuries (historiacartarum.org)", 12 comments)
> "One enormous transaction shows that Ely Abbey, now known as Ely Cathedral, paid Thorney Abbey 26,275 eels to rent a fen (similar to a wetland),"
Your article left out a neat twist: the name "Ely" is actually derived from the word "eel"!