"...and little Agatha Timms has put up half shares in her eel farm on a weeklong match." (GF7)
An eel is a type of long, slender fish (somewhat resembling a snake in appearance) that spends most of its life in freshwater but breeds in the ocean, and consequently is found in marshy areas where freshwater has access to the sea. Eels are farmed for their meat.
Eels are not commonly eaten in the U.S. but eel pie, for instance, is eaten in Britain. To take an example from one of JKR's favourite books, when in C.S. Lewis' The Silver Chair, the Marsh-wiggle Puddleglum offered Jill and Eustace eel stew but was dubious about whether humans could eat it or would enjoy it, he was merely giving one of his typical gloomy observations about life, the universe, and everything.