Abstract
Walleye pollock is a commercially and ecologically important species in the Bering Sea. Assessment of pollock stocks is conducted regularly using a combination of trawl net sampling and multiple frequency active acoustic echosounder surveys. In addition to backscatter from the species of interest, other marine life including several types of zooplankton and nekton can contribute substantial amounts of acoustic backscatter energy which can provide information regarding the abundance and distribution of these animals as well. Acoustic scattering models are used to estimate both the amount and frequency response of the acoustic energy scattered by the zooplankton, but these models require knowledge of various properties that describe the acoustically important characteristics of the zooplankton. We collected live zooplankton from the Bering Sea during June and July 2008. The morphology (spatial dimensions) and material properties (density and sound speed) of individual and groups of zooplankton (and some nekton) were measured. The taxa studied included euphausiids (Thysanoessa raschii, Thysanoessa inermis, Thysanoessa spinifera), copepods (Neocalanus sp.), amphipods (Themisto sp.), gastropod (Clione sp.), siphonophores, jellyfish, and squid. The results of our measurements with regard to acoustic scattering model development and use will also be discussed.
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