Behavioural responses of European silver eels(Anguilla anguilla) to the geomagnetic field

Magnetic orientation of European silver eels(Anguilla anguilla) was tested in an octagonal tank. Orientation was determined from photo-registrations of eel positions in tests performed alternately in the natural magnetic field and a field with the horizontal component rotated 180°. Tests were performed in LD 11 : 13. At a daytime light intensity of 100 lux the fish were diurnally active, while at 0.10 lux crepuscular or nocturnal activity dominated. The eels probably differed in preferred orientation, largely depending on the clockwise or anti-clockwise swimming of some of the animals. Therefore there was no preferred direction common to all eels. The orientation of single eels differed, however, significantly between the two magnetic fields, suggesting that the eels responded to the geomagnetic field.