An investigation into the zooplankton composition of a prominent 38-kHz scattering layer in the North Sea

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 backseattering, expressed as mean volume backseattering strength (MVBS) (dB). This observed MVBS (MVBS obs ) was compared with backscattering predicted by application of appropirate acoustic models (MVBS pred ) to sampled densities of zooplankton. In all instances, MVBS obs was greater than MVBS pred , with the difference considerably more pronounced at 38 kHz. There was a weak correlation between MVBS pred and MVBS obs at all three frequencies, with the greatest correlation (r = 0.450, P = 0.545) al 120 kHz. A number of candidate acoustic models were inverted in order to infer the most likely type of scatterer. In most cases, seallerers with a gas inclusion mere predicted by this method. Potential sources of inconsistencies between MVBS pred and MVBS obs were identified which, when considered alongside the presented forward and inverse solutions, indicate that 38 kHt scattering in particular must be due to sources other than. sampled zooplankton.