Collaborators: Andone Lavery (WHOI), Peter Wiebe (WHOI), Timothy Stanton (WHOI)
The distribution of many pelagic animals is highly variable in space and time, stemming from a complex interaction of biological and physical processes. Quantifying patchiness in the distribution of any marine organism requires sampling tools able to resolve adequately the scales of variability. High-frequency active acoustic scattering techniques are uniquely suited to the study of zooplankton and fish distributions, as they provide remote and non-intrusive samples at high resolution and to large ranges, allowing patch structure to be quantified in fine detail: a task that is difficult to achieve using traditional net or optical sampling systems alone.
A chronic difficultly in the use of acoustics to study zooplankton, however, lies in discriminating among the various animals likely to be present and contributing to acoustic scattering measurements. With only one frequency, the problem of estimating ecologically-relevant quantities like the abundance of each animal type present is strongly complicated by differences in the scattering characteristics of the different types.
Our research group has had considerable success in the use of multi-frequency techniques, where more than one narrowband acoustic frequency is employed in order to capitalize on the fact that different kinds of animals scatter sound differently as the frequency varies. The multi-frequency approach is particularly attractive in environments like the Antarctic where the zooplankton scattering community is relative simple, and our research continues to employ multi-frequency systems.
Broadband acoustic scattering techniques, however, offer substantial improvements in species discrimination due to the ability to measure scattering relative to frequency (i.e, the scattering spectrum, or acoustic signature) over a broad and continuous frequency range. The use of such techniques is particularly appealing in ecosystems like the New England shelf/slope where the zooplankton scattering community is relatively heterogeneous and the study of weakly scattering taxa such as euphausiids using narrowband techniques can be confounded by the presence of relatively rare, but strongly scattering, animals like gas-bearing siphonophores.
These broadband techniques have been under development by Andone Lavery and Tim Stanton for the past few years, and have now reached the field-testing stage. We are thus currently conducting studies in the Gulf of Maine region in order to demonstrate the feasibility of using broadband systems for remotely discriminating among sources of scattering and making accurate and high-resolution estimates of the abundance, size, and patch structure of animals from zooplankton to fish, with the ultimate goal of applying these methods to our ongoing studies of euphausiid aggregation dynamics.