Diving depths of Northern Gannets: Acoustic observations of Sula bassana from an autonomous underwater vehicle

The Autonomous Underwater Vehicle (AUV) Auiosub-1 made observations of the sea surface with an upward-looking echosounder during fish surveys in the v icinity of Shetland and Orkney (North Sea) in July 1999. Echograms from the AUV contained vertical traces extending downwards from the sea surface tha t were caused by diving seabirds. Visual observations provided evidence tha t those seabirds were Northern Cannets Sula bassana. Analysis of trace exte nt suggests a mean dive depth of 19.7 m (n = 19, SD = 7.5). Data on gannet diving depths are sparse, but this value is somewhat deeper than that accep ted for the related Cafe Gannet (Morus capensis, mean 5.9 m) which has been used in foraging models for the Northern Gannet. These observations have i mplications for our understanding of the foraging capabilities of gannets, and the interactions of gannets with commercially targeted fish species.