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UBC Theses and Dissertations
The incorporation of acoustic technology into the Canadian Pacific sardine (Sardinops sagax) survey Graas, Jonathon Allan
Hydroacoustics can provide cost-effective ways of improving the accuracy of non-acoustic surveys, such as Canada’s annual nighttime Pacific sardine (Sardinops sagax) surface trawl survey. Seasonal sardine abundance in Canada is currently estimated by extrapolating average catch densities from this survey to 30 m depth over a distributional area. The primary goal of this thesis was to assess, using acoustic methods, the uncertainty associated with catch density extrapolation from the trawl survey. Nighttime bias in the acoustic data due to the near-field limits was also investigated. An alternative acoustic approach is then reviewed to direct future research. Concurrent acoustic and trawl catch data from the 2011 Canadian sardine survey were analyzed to compare the two methodologies and approximate the species’ vertical distribution within the near-surface layer at night. Acoustic observations made at 38 and 120 kHz were compared to determine if fish are present in the near-field safe range of the 38 kHz transducer. Acoustic observations indicated that sardine density was significantly greater (P<0.05) above the trawl’s foot rope path than below it. Substantial backscatter in the surface scattering layer was observed within the near-field safe range of the 38 kHz transducer. No significant linear relationships between trawl-based density and acoustic backscatter or density estimates were found. These findings have important implications as sources of uncertainty in non-acoustic and acoustic methods were identified, including catch density extrapolation and the near-surface acoustic blind-zone and/or near-field safe range. This thesis then reviews the potential of multi-beam acoustics as an alternative to single-beam. Multi-beam sonars (MBS) can sample the water column over a wide range of angles and near-surface targets can be detected. Present methodologies and hardware, however, cannot yet quantify multi-beam backscatter as abundance. Calibration procedures for MBS’s are undefined for most models. Hardware and software required for real-time interaction with acoustic gear have not been developed for many systems. Since MBS’s insonify targets from multiple angles, the conversion of multi-beam backscatter to abundance is complex. Research on the natural orientation of fish and the accurate averaging of backscatter from multiple incidence angles is needed.
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