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International audience; Side scan sonar is an effective tool for the localization of benthic structures over large areas. For example, it is possible to systematically cover Loch Creran (subtidal area of 9.2 km²) in 2 days, identifying details with a 10 cm resolution. Loch Creran is one of the few world sites where the tubeworm Serpula vermicularis L. constructs extensive aggregations of calcareous tubes, attaining 3 m in width. Such biogenic reefs are a high aesthetic appeal and provide an important habitat for a diverse fauna. This study explored the utility of side scan sonar in mapping reef distribution, which is necessary in view of the threat from aquaculture, bottom fishing and mooring. Using the softwares Isis Sonar® and DelphMap®, a mosaic was constructed from all sonar tracks. Moore et al. (1998) provided the bathymetric range of 1-13 m for Loch Creran reefs, which allowed us to select the potential region of presence. To perform a supervised classification using the maximum likelihood criterion, training areas were defined around known areas of reefs. Spatial analyst and Grid extensions of the ArcGIS® software were used to map serpulid reef areas. A photo-interpretation was required to generalize the interesting areas. Regions of the loch occupied by non-aggregated serpulid worm tubes, small aggregations and sparse reefs cannot be identified due to the resolution limits of the sonar equipment. However, localized modification of reef cover, caused for instance by anthropogenic damage, can be easily delimited by side scan sonar. The method facilitates the broadscale mapping of the major reef areas within the loch. It provides a faster approach to monitoring reef distribution than observational techniques such as drop-down video and diving, although such techniques are required for groundtruthing the sonar mapping. Sonar also aids in the identification of areas where quantitative monitoring of reefs is appropriate for conservation monitoring purposes.