Eels

Eels were caught in huge numbers in English rivers and fens, chiefly in wicker traps and in weirs of wattle hurdles that funnelled them into baskets, and also with spears. They were an important food on the many fast days when meat was forbidden. Eels were also used as rent: Domesday Book records more than half a million owed each year, often counted in 'sticks' of 25.

Sources: historiacartarum.org · medieval.eu