Coded acoustic tags and an array with moored receivers were used to study the effect of a sub-sea AC power cable on migrating European eel, Anguilla anguilla (L.) in the Baltic Sea. Sixty eels were tagged and the migration speed was measured in a strait with a 130 kV AC power cable. Observed swimming speed over the ground was corrected for advection by the water current. Eel swimming speed was significantly lower around the cable than both north and south of the cable. No details on the behaviour during passage over the cable were possible and possible physiological mechanisms explaining the phenomenon are unknown. Further work is needed to understand the nature of the effect.