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  • 1
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    Investigation of seabed fishing impacts on benthic structure using multi-beam sonar, sidescan sonar, and video (2007)
    Oxford University Press
    Details
    Publication Date: 2007-05-21
    Description: 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.
    Print ISSN: 1054-3139
    Electronic ISSN: 1095-9289
    Topics: Biology , Geosciences , Physics
    Published by Oxford University Press on behalf of International Council for the Exploration of the Sea.
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  • 2
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    Measurements of acoustic backscatter and density of captive Atlantic cod with synchronized 300-kHz multibeam and 120-kHz split-beam echosounders (2009)
    Oxford University Press
    Details
    Publication Date: 2009-04-08
    Description: Gurshin, C. W. D., Jech, J. M., Howell, W. H., Weber, T. C., and Mayer, L. A. 2009. Measurements of acoustic backscatter and density of captive Atlantic cod with synchronized 300-kHz multibeam and 120-kHz split-beam echosounders. – ICES Journal of Marine Science, 66: 1303–1309. Effective management strategies for Atlantic cod (Gadus morhua) in the Gulf of Maine require stock assessments based on accurate estimates of its abundance and distribution. If multibeam echosounders are to provide data for such estimates, the relationship between acoustic backscatter and fish biology must be better understood. Working towards this goal, a series of acoustic measurements was made using a 120 kHz, split-beam echosounder (Simrad EK60) and a 300 kHz, multibeam echosounder (Kongsberg EM3002). The transducers from both systems were fixed to a platform over a submerged 98 m3 cage made of 5 cm stretched-nylon mesh. After standard-sphere calibrations, the cage was stocked with live, mature Atlantic cod, with a mean total length of 80.7 cm (range: 51.5–105.0 cm). The echosounders synchronously collected acoustic data, while the cod were monitored with two underwater video cameras. Cod were incrementally removed from the cage to provide a time-series of acoustic backscatter at four densities (n = 128, 116, 66, and 23). Backscatter measurements of cod are compared between echosounders and over time, and the factors affecting the acoustically derived density estimates are discussed. The benefits and limitations of the EM3002 are highlighted.
    Print ISSN: 1054-3139
    Electronic ISSN: 1095-9289
    Topics: Biology , Geosciences , Physics
    Published by Oxford University Press on behalf of International Council for the Exploration of the Sea.
    Location Call Number Expected Availability
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