Long-term broadband acoustic observations of zooplankton scattering layers in Saanich Inlet, British Columbia.

Abstract

The application of broadband techniques to fish and zooplankton bioacoustics is showing potential to transform the field into one that is much more quantitative. This is because broadband techniques allow the use of the known spectra of organisms or nonbiological sources of scattering to distinguish between scatterers, allowing discrimination without the need for extensive groundtruthing. This makes it ideal for remote monitoring of fish or zooplankton assemblages, since continuous net-sampling is often not possible. An upward-looking 85–155 kHz broadband sonar has been collecting data nearly continuously on the Victoria Experimental Network Under the Sea (VENUS) mooring in Saanich Inlet, British Columbia since March 2008. Saanich Inlet is known to have large populations of euphausiids, which create a strong acoustic scattering layer that migrates from depth to the surface and back each day. The thickness, timing, strength and spectral response of this layer is examined throughout the annual cycle and the feasibility of using this type of sonar to elucidate changing zooplankton assemblages (due to growth and changing species composition) is assessed.