MULTIPLE-FREQUENCY ACOUSTIC BACKSCATTERING AND ZOOPLANKTON AGGREGATIONS IN THE INNER SCOTIAN SHELF BASINS

Acoustic backscatter observations at 12, 50, and 200 kHz in the inner Scotian Shelf basins reveal large populations of silver hake (Merluccius bilinearis) and its principal prey, the euphausiid Meganyctiphanes norvegica. Multi-channel sonar colour imagery facilitates separation of fish from euphausiids and delineation of their separate diurnal migration patterns. Silver hake acoustic abundances are consistent with midwater trawl sampling. Acoustic column densities for mature M. norvegica of approximately 1000/m2 exceed net sampling column densities by over 1 order of magnitude indicating strong net avoidance. Numerical simulation of euphausiid backscattering using a fluid cylinder model and natural distribution of scatterer orientations yields an approximate 20 log L target strength length dependence and little frequency domain dependence in the "geometric" scattering regime. Quantitative interpretation of euphausiid scattering in situ must account for randomness in their orientations.