Acoustic remote sensing of benthic activity: A statistical approach

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Abstract
Acoustic backscatter at 40 kHz was recorded from a circular area of silt 75 m in radius every 0.1 d at a water depth of 91 m off the coast of northern California for 49 d in winter 1988–1989. Data were resolved into pixels that extended 7.5 m radially and 5° azimuthally. We analyzed the relatively small change in backscatter intensity and phase from a given pixel. In a time‐series analysis, this change in backscatter was decomposed into three components putatively due to large, rare, transient nekton and epibenthos; more abundant benthopelagic organisms that caused volume reverberation; and more sedentary benthos. Benthic change in backscatter was presumed due to movement of animals and other biogenic modifications of sound speed in the sediment‐water medium. Patterns of benthic activity and volume backscatter near the seabed changed abruptly twice, with the middle interval showing highest levels. During this middle interval, winter storms disturbed surficial sediments, offshore transport occurred near the seabed, and the benthic component was dominated by activity perfectly in phase with and linearly proportional to median, local, downwelling irradiance. We suspect that this circadian pattern was due to the burrowing urchin Brisaster latifrons that is known from previous studies at this site to emerge on the sediment surface at night when food resources are available there and to reburrow during the lighted part of the day. Extensive correlative physical data collected at the site as part of interdisciplinary studies (CODE and STRESS) allowed us to exclude other physical forcings as likely explanations for diurnal change in backscatter. In a spatial analysis, the transient component was removed explicitly pixel by pixel, and volume reverberation was removed implicitly to leave a benthic remainder that could be analyzed for spatial variation. Benthic activity showed distinct patchiness with coherence scales of roughly 5–10 m and negative spatial autocorrelation in activity levels near the largest spatial lag available (∼140 m). Patches did not move appreciably during the study.

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