Disturbance-induced responses of VHF and satellite tagged harbour seals

The response of individual harbour seals (Phoca vitulina) to controlled and sporadic disturbances when hauled out in the Anholt seal reserve, Denmark, was studied. Sporadic disturbances from pedestrians, boats, low-flying aeroplanes and grey seals (Halichoerus grypus) were observed in or near the reserve. VHF and satellite transmitters were attached to eight harbour seals to determine post-disturbance and undisturbed behaviour during the pre-breeding period (25 April to 21 May 2008).Both disturbed and undisturbed seals mostly returned to the haul-out site from dusk and throughout the night. Only pedestrian disturbances caused significantly longer-lasting trips at sea than undisturbed trips, while the other types of disturbance did not affect trip duration. The general maximum extent and area-use of disturbed and undisturbed trips were comparable and almost all at-sea locations were within 40 km from the haul-out site. The maximum extent of post-disturbance trips, however, varied among individuals and disturbance types, and was strongly correlated with the duration of trips.Disturbed and undisturbed seals used the same areas, suggesting that these areas represent normal foraging areas. This may indicate that harbour seals reduce the cost of being disturbed by foraging after disturbances instead of waiting close to the haul-out site to resume hauling out.During the pre-breeding period the seals showed a very high site fidelity by consistently returning to the same haul-out site, even when subjected to repeated disturbances. Copyright (c) 2013 John Wiley & Sons, Ltd.