Orientation of homing salmon (Salmo salar L.) mapped in relation to geomagnietic field.

A migration study on the homing of Atlantic salmon was carried out in waters off SW – Iceland. Data was sampled on the salmon behaviour and environment during spawning migration in sea. New type of data storage tag was used to measure the earth‘s magnetic field and the tilt (in 3 directions) and paired information on fish depth (pressure) and correspondingly the temperature experienced. By taking advantage of this new research tool we have managed in this study to bring forward new information on salmon orientation. The study involves new approach in mapping information on behaviour and positioning of free ranging fish. Information already available from the study is showing that salmon migration behaviour changes when moving from offshore to inshore waters. We conclude that this stands for changes in the orientation behaviour of the salmon due to the increased use of olfactory sense in the inshore area. Based on that we recommend to divide the spawning migration in sea into three phases in relation to the different migration (orientation) behaviour in different sea area (offshore; inshore; estuary). Parallel to this finding is the fact that the directional geomagnetic recordings sampled during the migration of the salmon from release offshore to entry to their home river showed that fixed direction all the way from offshore to the home fjord area or home estuary could not explain their migration. Therefore we make that assumption that shoreline orientation is important in finding the way once the shore has been approached. The study setup has brought to shore many interesting recordings of new parameters on behaviour and environmental patterns that will be examined further later. Keywords: Atlantic salmon, migration, homing, orientation, navigation, data storage tag