An assessment of the acoustic survey technique, RoxAnn, as a means of mapping seabed habitat

Abstract

RoxAnn acoustic surveys of the inner Moray Firth, undertaken in September/October 1995 and January 1996, were used to map seabed habitat on the basis of two sediment characteristics, 'roughness' (E1) and 'hardness' (E2). The traditional analytical method of fitting a 'box pattern' to E1 vs. E2 scatter plots was compared with a more objective method using False Colour Composite Image (FCCI) and cluster analysis. Although both methods produced similar maps, the latter provided greater between survey consistency. Six to seven sediment types were indicated by RoxAnn, however ordination analysis of sediment samples indicated that some of the FCCI clusters could not be separated on the basis of their particle size distributions. This may have been due to a degree of depth sensitivity, but it is also possible that RoxAnn was responding to other physical or biotic seabed features other than just particle size. After combining RoxAnn FCCI clusters where ground-truthing grab samples had shown the particle size distributions to be similar, it was evident that RoxAnn could distinguish three main sediment habitats with certainty. On this basis, the RoxAnn derived maps compared well with maps obtained from British Geological Survey data. Finally we examined the distributions of four flatfish species to determine whether these were in any way related to the different sediment habitats identified by RoxAnn.