Can a 38 and 200 kHz broadband echosounder differentiate copepods, other zooplankton, and hydrographic sources of backscatter?

Abstract

Zooplankton are a diverse group of organisms representing multiple taxonomic categories with very different acoustic scattering characteristics. While net and optical sampling methods can provide species (or higher)-level organism identification, they can not match the high spatial resolution and large sampling volume of active acoustic echosounders. A challenge in interpreting backscatter data though is when different types of animals co-occur, and exacerbating this problem is when the animals are concentrated (either actively or passively) through turbulent processes or variations in density and soundspeed which also scatter sound. Broadband backscatter data and net tows were collected in three different parts of the Northwest Atlantic (New York Bight, Cape Cod Bay, and Gulf of Maine) where copepods (and other small zooplankton) were abundant in the upper portion of the water column. The ability of a dual-frequency (38 and 200 kHz miniWBT) broadband echosounder to measure copepod abundance are discussed relative to: the use of additional frequencies, zooplankton abundance and species diversity, and physical oceanographic data.