ACOUSTIC SCATTERING FROM ZOOPLANKTON AND MICRONEKTON IN RELATION TO AWHALE FEEDING SITE NEAR GEORGES BANK AND CAPE-COD

This research was part of the South Channel Ocean Productivity Experim ent (SCO-PEX), a multidisciplinary study to investigate the biological and physical processes associated with the very high annual springtim e abundance of right whales (Eubalaena glacialis) in the Great South C hannel off New England. Right whales appear to gather there in the spr ing because of the increased abundance of aggregations of their princi pal prey, the copepod Calanus finmarchicus. Observations of hydroacous tic scattering were made in relation to the hydrography, whale distrib utions, 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-surfac e layer with strong diel changes. In 1989, a second frequency (120 kHz ) was used to discriminate between copepod layers (which the 120 kHz d etected only weakly) and other targets (which both frequencies detecte d). Acoustically distinct layers of zooplankton and micronekton were o bserved, which were often correlated in time and space with the copepo d layers. Quantitative estimates derived from the acoustic data indica te 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 abundanc es observed from MOCNESS tows. The acoustic data revealed a complex di el migration of two layers in addition to the copepods. Euphausiids (p redominantly Meganyctiphanes sp.) were found in a layer above the bott om, and a mid-water layer may have been due to sand lance (Ammodytes a mericanus). The observed biological phenomena appeared to be related t o the complex hydrogeography of the region. A surface thermal front ex isted at the northern entrance to the channel in 1988 and 1989, with c older vertically mixed water to the south and warmer stratified water to the north. A Fast Fourier Transform analysis for spectral compositi on and autocovariance shows (a) strong contrasts in the spectral densi ty across one frontal feature (predominantly a salinity front) as oppo sed to away from the front, and (b) significant differences between th ose areas where a whale moved more rapidly (presumably searching for f ood) 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 u se of hydroacoustic estimates of target distribution and abundance.