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Zooplankton Aggregations Near a Coastal Sill: An Examination of Echo-Sounder Data from August and September 1995 in Knight Inlet, B.C.

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Technical Report

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Defence Research Establishment Atlantic Dartmouth, Nova Scotia Canada

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This report presents a retrospective study of echo-sounder and other physical oceanographic data collected in Knight Inlet, British Columbia during the late summer of 1995. This present study was undertaken to provide oceanographic background and assist planning for focused field experiments on zooplankton aggregations which will be conducted in this same area in the autumn of 2001. High-frequency 120 and 200 kHz, uncalibrated echo-sounder data from two separate vessels is examined under both ebb and flood tide conditions, and for diurnal variations. The 1995 data revealed dramatic tidally-driven internal hydraulic flows over-top of a 65 m deep sill in Knight Inlet, created by strong, near-surface 20 m depth thermohaline stratification. In general the zooplankton populations exhibited classic diurnal migration habits, forming into relatively dense layers at 70 to 120 m depth during daytime and dispersing throughout the entire water column at night. This behavior is suggestive that the zooplankton scattering layers were dominated by Euphausiids, which as common in B.C. coastal inlets. During daytime on both ebb and flood tides there was evidence that the zooplankton layers were trapped and concentrated by the flow against the upstream side of the sill.

Subject Categories:

  • Biology
  • Acoustic Detection and Detectors

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