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The Autonomous Underwater Vehicle (AUV) Autosub-1 made observations of the sea surface 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. Analysis of trace extent suggests a mean dive depth of 19.7 m (n ϭ 19,doi:10.2307/4089816 fatcat:jtgx3g3xmnffdjs6c5tpwdy5cm