Influence of magnetic field on zebrafish activity and orientation in a plus maze

•Zebrafish prefer to visit E–W arms of a plus maze in the unmodified GMF.•This bimodal preference remains stable over repeated trials divided by several days.•This preference changes for N–S arms upon the 90° turn of GMF horizontal component.•The change of preferred directions is accompanied by an increased locomotor activity.•Reversals of other GMF components do not change the preference of E–W directions. We describe an impact of the geomagnetic field (GMF) and its modification on zebrafish's orientation and locomotor activity in a plus maze with four arms oriented to the north, east, south and west. Zebrafish's directional preferences were bimodal in GMF: they visited two arms oriented in opposed directions (east–west) most frequently. This bimodal preference remained stable for same individuals across experiments divided by several days. When the horizontal GMF component was turned 90° clockwise, the preference accordingly shifted by 90° to arms oriented to the north and south. Other modifications of GMF (reversal of both vertical and horizontal GMF components; reversal of vertical component only; and reversal of horizontal component only) did not exert any discernible effect on the orientation of zebrafish. The 90° turn of horizontal component also resulted in a significant increase of fish's locomotor activity in comparison with the natural GMF. This increase became even more pronounced when the horizontal component was repeatedly turned by 90° and back with 1min interval between turns. Our results show that GMF and its variations should be taken into account when interpreting zebrafish's directional preferences and locomotor activity in mazes and other experimental devices.