George R. Cutter

Single‐ and multi‐beam echosounder assessment of Antarctic krill aggregations.

Acoustic backscatter measurements from aggregations of Antarctic krill were collected over two field seasons (2006 and 2007) in the waters north of Livingston Island using two 5.5‐m inflatable vessels. One was equipped with a dual‐frequency (38‐ and 200‐kHz) single‐beam echosounder, while the other used a 200‐kHz multi‐beam system. In addition to independent survey operations, both vessels participated in joint survey operations where the two boats followed the same trackline separated by a small distance (10–40 m). These data allow us to compare how the two echosounder systems “see” the same (or very similar) krill aggregations. During 2006, some tracklines were also occupied by a large (300‐ft) vessel although vessel separation distances were larger during these operations. The combination of measurements from single‐ and multi‐beam echosounders (and nearby net tows) provides the opportunity to characterize krill aggregation size, shape, numerical density, and overall biomass. Observations of krill pred...

Characterization of pelagic scatterers using multibeam echosounder data: Echo amplitude and phase, and their variabilities and frequency spectra.

A recently‐developed statistical‐spectral approach to acoustic‐target identification (SSID) incorporates information contained in the frequency‐dependent signal amplitudes and their variances. In addition to identifying biological targets, the SSID has demonstrated utility for estimating fish aggregation densities, abundances, and behaviors, and detecting and classifying the seabed (e.g., accurate depth, within‐beam slope, hardness and roughness, and the height of the unresolved boundary region, and the so‐called dead zone). Further, the related multi‐frequency, bi‐planar, interferometric technique (MBI) allows much higher‐resolution measurements of the seabed and sub‐beam measures of slope, hardness, and roughness. Here we extend the utilities of the SSID and MBI methods, with explicit consideration of the single‐frequency signal phases and their variances, and frequency‐dependent split‐aperture phases and their variances, for improved estimations of pelagic‐target sizes, and their aggregation densities,...

Measurement of scattering directivity and behavior of fish schools using multibeam sonar

Fish dispersion and abundance are commonly estimated from volume backscattering coefficients (sv) of animals located directly beneath the survey vessel, inside a narrow transducer beamwidth. Fish density (♯/m2) is estimated from a ratio of integrated sv and a representative backscattering cross‐sectional area. Fish biomass is estimated from the product of fish density and the area sampled. Uncertainty in biomass estimates can arise from small sampling volumes, and the scattering directivities of the fish coupled with their orientations relative to the acoustic beam. Multibeam sonars can be used to increase the observational volumes, but add the complexity of a wider range of incident angles. If fish size and packing density are constant throughout a school, then the variation in sv versus aspect angle can be attributed to fish orientation. With modeling and multibeam sonar measurements, the following null hypotheses are explored: (1) The variability of sv versus roll angle is nearly uniform; (2) For measu...