JPR Advance Access originally published online on June 13, 2005
Journal of Plankton Research 2005 27(7):623-633; doi:10.1093/plankt/fbi035
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An investigation into the zooplankton composition of a prominent 38-kHz scattering layer in the North Sea
1 FRS Marine Laboratory, PO Box 101, 375 Victoria Road, Aberdeen AB11 9DB, Scotland, 2 Centre IRD de Bretagne, BP 70, 29280 Plouzané, France and 3 Gatty Marine Laboratory, University of St. Andrews, St. Andrews, FIFE KY16 8LB, Scotland
* Corresponding Author: a.mair{at}marlab.ac.uk
Received March 18, 2005; accepted in principle May 2, 2005; accepted for publication May 26, 2005; published online June 13, 2005
Communicating editor: K.J. Flynn
This study aims to determine the contribution made by zooplankton to acoustic scattering layers, which are particularly strong at 38 kHz, in the northern North Sea in summer. It uses a combination of net sampling and forward and inverse acoustic modelling of data collected at 38, 120 and 200 kHz in July 2003. Zooplankton samples were collected from regions of strong acoustic scattering in depths to 
50 m, using a undulating towed (U-tow) vehicle. Acoustic data recorded simultaneously were scrutinized to determine actual backscattering, expressed as mean volume backscattering strength (MVBS) (dB). This observed MVBS (MVBSobs) was compared with backscattering predicted by application of appropriate acoustic models (MVBSpred) to sampled densities of zooplankton. In all instances, MVBSobs was greater than MVBSpred, with the difference considerably more pronounced at 38 kHz. There was a weak correlation between MVBSpred and MVBSobs at all three frequencies, with the greatest correlation (r = 0.450, P = 0.545) at 120 kHz. A number of candidate acoustic models were inverted in order to infer the most likely type of scatterer. In most cases, scatterers with a gas inclusion were predicted by this method. Potential sources of inconsistencies between MVBSpred and MVBSobs were identified which, when considered alongside the presented forward and inverse solutions, indicate that 38 kHz scattering in particular must be due to sources other than sampled zooplankton.
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