Quantification of deep sound scattering layers using a moored upward-looking broadband split-beam echosounder

Abstract

Distinct deep sound scattering layers are prevalent throughout the ocean mesopelagic zones (200–1000 m depths). These deep sound scattering layers are often characterized by daily vertical migration, in which many of the inhabitants of the ocean mesopelagic communities migrate to shallow waters during the night to feed, and descend during the day to avoid predators. And yet little is known about the impact of mesoscale physical oceanography, such as warm core rings or Gulf stream meanders, on the patterns of daily vertical migration or on the small-scale spatial distribution, or patchiness, of the deep sound scattering layers. To address these questions, an upward-looking broadband split-beam echosounder (36 kHz–44 kHz) has been moored at 580 m depth in the slope waters south of the New England shelf break in a location in which warm core rings are a prevalent feature. Data from a 3-day test deployment are analyzed in terms of target density, spectral characterization, patchiness and timing of the migrations. The calibrated broadband spectra of individual targets are used to determine the number of swimbladdered fish in the deep sound scattering layers. Meanwhile, two other methods, including echo-counting and echo-statistics, are also applied to give target density estimation.