Eels were commonly currency:

https://www.atlasobscura.com/articles/medieval-eel-rent-map-...

Some HN threads on this topic (meaning the "Eel-Rents Project" organized by John Wyatt Greenlee),

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"!