The Celts lived in houses built on piers, eating the fish you were shitting on would wipe out your tribe due to cholera and dysentry, they imported their water from local springs. They ate locally caught seafood but not freshwater fish. Carp were introduced by the Romans however the Romans had a means to wash the fish prior to consumption. There was no need to eat something possibly detrimental due to a lack of population pressure, the Romans washed carp in fresh water for a period of days prior to consumption, the Celts didn't simply because they didn't have to.