Scotian Shelf euphausiid and silver hake population changes during 1984-1996 measured by multi-frequency acoustics

Abstract

A 13-year time series of integrated acoustic volume backscattering strengths from Emerald Basin on the Scotian Shelf at 12, 50, 122, and 200 kHz are examined here, together with zooplankton data from net sampling. The three highest acoustic frequencies delineate relative abundances of the euphausiid Meganyctiphanes norvegica while the 12-kHz frequency primarily reflects abundances of pelagic silver hake. Euphausiid populations derived from acoustics fluctuate by a factor of 20 combining both seasonal and inter-annual variation components. Acoustically derived euphausiid populations correlate significantly with both net-sampled euphausiid populations and acoustically derived silver hake abundances, but show no clear long-term (multi-year) trends. The correlation between Emerald Basin euphausiid and migratory pelagic fish abundances suggests the ecological influence of the Shelf Basin euphausiid stocks extends to the surrounding shelf areas. (C) 2000 International Council for the Exploration of the Sea.