Acoustic Scattering from Zooplankton and Micronekton in Relation to a Whale Feeding Site near Georges Bank and Cape Cod

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This research was part of the South Channel Ocean Productivity Experiment (SCOPEX), a multidisciplinary study to investigate the biological and physical processes associated with the very high annual springtime abundance of right whales (Eubalaena glacialis) in the Great South Channel off New England. Right whales appear to gather there in the spring because of the increased abundance of aggregations of their principal prey, the copepod Calanus finmarchicus. Observations of hydroacoustic scattering were made in relation to the hydrography, whale distributions, and other biological measurements in the vicinity of the Great South Channel during May 1986, March, April and May of 1988, and May and June of 1989. Copepods were detected (at 200 kHz) as a near-surface layer with strong diel changes. In 1989, a second frequency (120 kHz) was used to discriminate between copepod layers (which the 120 kHz detected only weakly) and other targets (which both frequencies detected). Acoustically distinct layers of zooplankton and micronekton were observed, which were often correlated in time and space with the copepod layers. Quantitative estimates derived from the acoustic data indicate that the abundance of zooplankton varied from 1–5 g wet weight m−3 to 18–25 g wet weight m−3 which correlates well with the abundances observed from MOCNESS tows. The acoustic data revealed a complex diel migration of two layers in addition to the copepods. Euphausiids (predominantly Meganyctiphanes sp.) were found in a layer above the bottom, and a mid-water layer may have been due to sand lance (Ammodytes americanus). The observed biological phenomena appeared to be related to the complex hydrography of the region. A surface thermal front existed at the northern entrance to the channel in 1988 and 1989, with colder vertically mixed water to the south and warmer stratified water to the north. A Fast Fourier Transform analysis for spectral composition and autocovariance shows (a) strong contrasts in the spectral density across one frontal feature (predominantly a salinity front) as opposed to away from the front, and (b) significant differences between those areas where a whale moved more rapidly (presumably searching for food) and where it spent more time (presumably or observably feeding). The behavior of whales, in particular the right whale, can be shown to be related to the spatial scales and abundance of their prey by the use of hydroacoustic estimates of target distribution and abundance.

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Continental Shelf Research, v. 15, issues 4-5, p. 509-537