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UBC Theses and Dissertations

A study of the relationship between zooplankton and high-frequency scattering of underwater sound Pieper, Richard Edward


Quantitative volume-scattering measurements were compared to the distribution of euphausiids in Saanich Inlet, British Columbia. Scattering was recorded at 42, 107, and 200 kHz from the depths of high euphausiid concentrations, and volume-scattering coefficients were determined. No scattering from euphausiids was recorded by a 11 kHz echo-sounder. Daily variations in the volume-scattering coefficients, m(Δz), generally compared well with variations in the concentration of euphausiids. The daily behaviour of the high-frequency scattering layer., and therefore m(Δz) and the concentration of euphausiids, was influenced by moonlight and weather conditions as well as the presence or absence of an oxycline in the inlet. In the absence of an oxycline conditions would have been closer to conditions in the open ocean. Under these circumstances euphausiid concentrations and m(Δz) were low in the mornings and gradually increased throughout the day; the high-frequency scattering layer consolidated. During the evening migration, the layer became more diffuse, and the number of euphausiids per cubic metre decreased. The scattering cross-section, σ (cm²), of a euphausiid was found to increase with the average dry weight and length, and with increased frequency. For each cruise σ was approximately one order of magnitude apart at the three frequencies. The values of σ (in cm²) ranged from 4.81 x 10⁻⁵ to 5.21 x 10⁻³(200 kHz), 5.49 x 10⁻⁶ to 3.99 x 10⁻⁴ (107 kHz), and 2.30 x 10⁻⁷ to 3.67 x 10⁻⁵ (42 kHz).

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