Abstract
Malik, M. A., and Mayer, L. A. 2007. Investigation of seabed fishing impacts on benthic structure using multi-beam sonar, sidescan sonar, and video. – ICES Journal of Marine Science, 64: 1053–1065. Long, linear furrows of lengths up to several kilometres were observed during a recent high-resolution, multi-beam bathymetry survey of Jeffreys Ledge, a prominent fishing ground in the Gulf of Maine located about 50 km from Portsmouth, NH, USA. These features, which have a relief of only a few centimetres, are presumed to be caused either directly by dredging gear used in the area for scallop and clam fisheries, or indirectly through the dragging of boulders by bottom gear. Extraction of these features with very small vertical expression from a noisy data set, including several instrumental artefacts, presented a number of challenges. To enhance the detection and identification of the features, data artefacts were identified and removed selectively using spatial frequency filtering. Verification of the presence of the features was carried out with repeated multi-beam bathymetry surveys and sidescan sonar surveys. Seabed marks that were clearly detected on multi-beam and sidescan sonar records were not discernible on a subsequent video survey. The inability to see the seabed marks with video may be related to their age. The fact that with time, the textural contrasts discernible by video imagery are lost has important ramifications for the appropriateness of methodologies for quantifying gear impact. The results imply that detailed investigations of seabed impact are best done with a suite of survey tools (multi-beam bathymetry, sidescan sonar, and video) and software to integrate the disparate data sets geographically.
