Field demonstration of broadband acoustic scattering techniques for quantifying marine mammal prey distributions

Gareth Lawson, Biology, Andone Lavery, Applied Ocean Physics and Engineering, and Peter Wiebe, Biology

Abstract

Variability, or patchiness, in the distribution of zooplankton is a key feature determining
their availability to marine mammal predators. Understanding the biological and physical
forces that underlie this patchiness is an important and fascinating question, and is crucial
to the development of predictive models of marine mammal distribution. Techniques for
quantifying patchiness in marine mammal prey fields are thus of considerable scientific
and conservation interest. High frequency acoustic systems are often used to study
zooplankton patchiness, due to their ability to sample at high resolution at a variety of
spatial and temporal scales. Traditional acoustic systems, however, often suffer from
difficulties associated with discriminating among the different animals present in mixed
species environments. Broadband acoustic sensors offer substantial improvement in the
acoustic classification of zooplankton and thereby in the quantification of patchiness in
the distribution of individual prey types. This technology has only recently emerged for
ecological applications, however, and has not been adequately field-tested.
The long-term goal of our research is to understand the bio-physical factors determining
zooplankton patchiness and availability to higher predators, especially marine mammals,
focusing on the Gulf of Maine as a model region. The specific goal of this project is to
demonstrate the power of newly-available broadband acoustic scattering techniques for
remotely discriminating among sources of scattering and making accurate and high
resolution estimates of the abundance, size, and patch structure of marine mammal prey
ranging from zooplankton to fish, and euphausiids and copepods in particular. Field tests
will be conducted on Stellwagen Bank and Wilkinson Basin in the Gulf of Maine, regions
of persistently high abundance of fish, euphausiids, copepods, and other prey species, that
are frequently visited by marine mammals. Four days of surveying will be conducted
from the R/V Tioga with a broadband acoustic scattering system. A comprehensive suite
of additional instruments, including nets and optical systems, will also be deployed to
validate broadband acoustic observations.
This is an inter-disciplinary and cross-departmental project, combining our expertise in
ocean acoustics, zooplankton sampling, and oceanography. Successful completion of the
project will provide a field-validated and cutting-edge method for the quantitative
mapping of marine mammal prey patchiness, and will thus provide leverage in seeking
external funding for future field applications.