Diving Depths of Northern Gannets : Acoustic Observations of Sula Bassana from an

Abstract

The Autonomous Underwater Vehicle (AUV) Autosub-1 made observations of the sea sur- face with an upward-looking echosounder during fish surveys in the vicinity of Shetland and Orkney (North Sea) in July 1999. Echograms from the AUV contained vertical traces extending downwards from the sea surface that were caused by diving seabirds. Visual observations provided evidence that those seabirds were Northern Gannets Sula bassana. Anal- ysis of trace extent 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 accepted for the related Cape Gannet (Mo- rus capensis, mean 5.9 m) which has been used in for- aging models for the Northern Gannet. These obser- vations have implications for our understanding of the foraging capabilities of gannets, and the inter- actions of gannets with commercially targeted fish species.