Acoustic classification of zooplankton

Abstract

Work on the forward problem in zooplankton bioacoustics has resulted in the identification of three categories of acoustic scatterers: elastic-shelled (e.g. pteropods), fluid-like (e.g. euphausiids), and gas-bearing (e.g. siphonophores). The relationship between backscattered energy and animal biomass has been shown to vary by a factor of -19,000 across these categories, so that to make accurate estimates of zooplankton biomass from acoustic backscatter measurements of the ocean, the acoustic characteristics of the species of interest must be well-understood. This thesis describes the development of both feature based and model based classification techniques to invert broadband acoustic echoes from individual zooplankton for scatterer type, as well as for particular parameters such as animal orientation. The feature based Empirical Orthogonal Function Classifier (EOFC) discriminates scatterer types by identifying characteristic modes of variability in the echo spectra, exploiting only the inherent characteristic structure of the acoustic signatures. The model based Model Parameterisation Classifier (MPC) classifies based on correlation of observed echo spectra with simplified parameterisations of theoretical scattering models for the three classes. The Covariance Mean Variance Classifiers (CMVC) are a set of advanced model based techniques which exploit the full complexity of the theoretical models by searching the entire physical model parameter space without employing simplifying parameterisations. Three different CMVC algorithms were developed: the Integrated Score Classifier (ISC), the Pairwise Score Classifier (PSC) and the Bayesian Probability Classifier (BPC); these classifiers assign observations to a class based on similarities in covariance, mean, and variance, while accounting for model ambiguity and validity. These feature based and model based inversion techniques were successfully applied to several thousand echoes acquired from broadband (-350 kHz -750 kHz) insonifications of live zooplankton collected on Georges Bank and the Gulf of Maine to determine scatterer class. CMVC techniques were also applied to echoes from fluid-like zooplankton (Antarctic krill) to invert for angle of orientation using generic and animal-specific theoretical and empirical models. Application of these inversion techniques in situ will allow correct apportionment of backscattered energy to animal biomass, significantly improving estimates of zooplankton biomass based on acoustic surveys.

Accounting for biological and physical sources of acoustic backscatter improves estimates of zooplankton biomass

“…If aggregations of animals in a region are monospecific and of similar size, theoretical backscatter models can be used to estimate their distribution and abundance (Hewitt and Demer 2000). If more than one frequency of sound is used, then more categories and size classes of animals may be distinguished and their abundance estimated (Martin et al 1996;Brierley et al 1998). However, these approaches require that the type of the scatterers present in the water column is known.…”

Towards an acoustic‐based coupled observation and modelling system for monitoring and predicting ecosystem dynamics of the open ocean

“…If the vessel is equipped with multiple acoustic frequencies, the differences in acoustic backscatter strength among frequencies can also be used to aid partitioning (frequency response, Fig. 2a) (Martin et al 1996;Kloser et al 2002Kloser et al , 2009Korneliussen and Ona 2003;Godø et al 2009;Ressler et al 2012). There are ongoing efforts to further develop these techniques and acoustic systems operating over broad frequency ranges will further increase the potential for acoustic species identification (Stanton et al 2010).…”

Counting Critters in the Sea Using Active Acoustics

“…The use of multiple acoustic frequencies with ground truthing have allowed scientists to use acoustic backscatter data to identify and discriminate scattering from multiple size classes (Greenlaw, 1979;Kristensen and Dalen, 1986;Warren et al, 2003) or types of scatterers (Pieper et al, 1990;Martin et al, 1996) and even scattering from biological and physical sources (Warren et al, 2003). A thorough review of acoustic species identification is provided by Horne (2000).…”