The effects of scattering-layer composition, animal size, and numerical density on the frequency response of volume backscatter

Land-associated, sound-scattering layers of mesopelagic micronekton surround the Hawaiian Islands. These animals undergo diel migrations during which they split into multiple, distinct layers that have differences in animal density, taxonomic composition, and size. A video-camera system capable of quantitatively estimating the biological constituency of the layers was combined with a four-frequency, vessel-mounted, echosounder system (38, 70, 120, and 200 kHz) to examine the effects of layer features on the frequency response of volume backscatter. Volume scattering was correlated with animal density at all frequencies, but the effects of animal length and layer composition were frequency-specific. Only scattering at 70 kHz matched the predictions of volume scattering based on the mean echo strengths and densities estimated from camera profiles, suggesting different scattering mechanisms at other frequencies. Differences in volume scattering between pairs of frequencies, however, did strongly correlate with animal length and layer composition and could be used as measures of the biological properties of layers. Applying this technique to the data shows strong partitioning of habitat by taxa and animal size in space and time, indicating the importance of competition in structuring the community.