Sound-scattering spectra of deep scattering layers in the western North Atlantic Ocean

Abstract

Sound from small explosions has been used to study the frequency-dependent characteristics of deep scattering layers in three areas of the western North Atlantic Ocean. Layers show resonant properties, the scattered sound being most intense in a narrow frequency band. The scatterers are presumed to be mainly the swimbladders of bathypelagic fishes. In layers peaking at frequencies above 5 kcps there is a systematic shift of peak frequency as layer depth changes during vertical migration. In two cases studied frequency changes as the $\text{5}\text{6}\text{th}$ power of the hydrostatic pressure. This suggests that in these cases the swimbladder simply expanded and contracted with changing pressure. In a third case frequency changes as the $\text{1}\text{2}\text{power}$ of the pressure. This suggests that the fish maintained neutral buoyancy throughout the depth migration by absorbing gas from the swimbladder as necessary. Layers peaking near or below 5 kcps are poorly resolved in depth. In one instance a direct relationship between frequency and depth has been established, but not well enough to define the relationship quantitatively. Pronounced depth and frequency migration is found in layers south of New England and south of Nova Scotia. Back-scattering coefficients were found to lie between − 64 and − 81 db re M⁻¹ in a small number of computations.