Quantifying and reducing the surface blind zone and the seabed dead zone using new technology

Totland, A., Johansen, G. O., Godø, O. R., Ona, E., and Torkelsen, T. 2009. Quantifying and reducing the surface blind zone and the seabed dead zone using new technology. – ICES Journal of Marine Science, 66: 1370–1376. The surface blind zone and the seabed dead zone are unobservable with hull-mounted acoustic systems. These constraints may seriously limit the effectiveness of acoustic observations in studies of ecosystem dynamics. In this paper, new technologies are used to make observations in these boundary zones, by augmenting conventional acoustic observations from transducers mounted in a retractable keel with simultaneous observations from two autonomous acoustic systems, one sampling the surface blind zone and the other the seabed dead zone. These observations to some extent overlap in depth, which allows comparisons between data collected with the vessel and the corresponding data from the two autonomous systems. The main targets of the investigation were young-of-the-year gadoids in the surface blind zone and older cod at least partly in the seabed dead zone. Species and their sizes were identified from trawl samples. The advantages and limitations of using new autonomous equipment for quantifying the acoustic backscattering within otherwise unobservable zones are discussed. The magnitudes of potential measurement biases in acoustic data collected with the vessel are also estimated for this survey.