Comparison of multi-frequency acoustic and in situ measurements of zooplankton abundances in Knight Inlet, British Columbia


  1. Trevorrow, M.V.
  2. Mackas, D.L.
  3. Benfield, M.C.
Corporate Authors
Defence R&D Canada - Atlantic, Dartmouth NS (CAN);Institute of Ocean Sciences, Sidney BC (CAN);Louisiana State Univ, Baton Rouge LA (US) Dept of Oceanography and Coastal Sciences
An investigation of midwater zooplankton aggregations in a coastal fjord was conducted in November 2002. This study focused on quantitative comparisons between a calibrated, three-frequency (38, 120, and 200 kHz) vessel-based echo-sounder, a multinet towed zooplankton sampler (BIONESS), and a high-resolution underwater camera (ZOOVIS). Daytime layers of euphausiids and amphipods near 70-90-m depth were observed in lower parts of the inlet, especially concentrated by tidal flows around a sill. Quantitative backscatter measurements of euphausiids and amphipods, combined with in situ size and abundance estimates, and using an assumed tilt-angle distribution, were in agreement with averaged fluid-cylinder scattering models produced by Stanton and Chu [ICES J. Mar. Sci. 57, 793-807, (2000)]. Acoustic measurements of physonect siphonophores in the upper inlet were found to have a strong 38-kHz scattering strength, in agreement with a damped bubble scattering model using a diameter of 0.4 mm. In relatively dense euphausiid layers, ZOOVIS abundance estimates were found to be a factor of 2 to 4 higher than the acoustic estimates, potentially due to deviations from assumed euphausiid orientation. Nocturnal near-surface euphausiid scattering exhibited a strong (15 dB) and rapid (seconds) sensitivity to vessel lights, interpreted as due to changing animal orientation.
Report Number
DRDC-ATLANTIC-SL-2004-094 — Scientific Literature
Date of publication
01 Mar 2005
Number of Pages
Reprinted from
J. Acoust. Soc. Am, vol 117, no 6, 2005, p 3574-3588
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