Distributions of volume scattering observed with an 87.5-kHz sonar

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Abstract
Horizontal spatial and temporal variations of the volume backscattering coefficient were measured with a horizontally aimed 87.5-m depth. Temporal variations between 2 min and 10 h and spatial variations between 17 and 400 m could be resolved with this data set. The wavenumber spectra of the acoustic backscattering coefficient were red, indicating that large scale variations accounted for more of the horizontal variability than smaller scales. Frequency spectra were red at periods > 1 h, irregular at periods between 6 min and 1 h, and white at periods < 6 min. Elevated spectral levels between 1 to 10 cycles/h are probably due to vertically stratified organisms being lifted into and out of the sonar beam by internal waves. The shape of day spectra were similar to night spectra (both wavenumber and frequency), but total variance of the volume scattering coefficient was greater at night. This can mean that the dimensions of scatterer patchiness did not change from night to day, but the intensity of variations increased markedly at night. Mean scattering strength increased at sunset and decreased at sunrise due to the diurnal migration of plankton and nekton. Upward migration of scatterers was different than downward migration in 2 ways. First, the upward migration took only between 12-20 min to complete while the downward migration lasted .apprx. 50 min. Second, a scattering strength overshoot (i.e., a local maximum 2-4dB above ambient night scattering levels) was observed only during the upward migration. The scattering strength overshoot probably occurred because migrating midwater fish and squid are most concentrated during upward migration. These animals either disperse after finishing their migration or descended below the sonar within 12-20 min after reaching 90 m.

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