Volume-backscattering spectra in the North Atlantic Ocean

Abstract

Measures of acoutic volume-backscattering spectra using explosive sources were made in the eastern North Atlantic and southern Norwegian Sea (southwest and northeast of the Faeroe Islands) during August 1978. The experimental data were analyzed from 300 to 20 000 Hz with a resolution of 40 Hz. Spectra from both areas are noticeably different, particularly below 10 kHz where most volume scattering is caused by fishes possessing gas-filled swimbladders. Daytime spectra in the Norwegian Sea were dominated by a strong low-frequency peak between 1500–2000 Hz; concomitant high-resolution depth recordings indicated large concentrations of discrete targets layered between 100–350 m. The blue whiting, Micromesistius poutassou, is known to inhabit the entire area in commercial quantities during this time of year at these depths and is of the proper length (i.e., bladder size) to cause this strong peak. Daytime spectra of the southerly area exhibit features comparable with those commonly observed at midlatitudes which is attributed to swim-bladder resonance of smaller, mesopelagic fishes commonly associated with deep scattering layers.