ACOUSTIC CLASSIFICATION OF ZOOPLANKTON

Accurate acoustic characterization of zooplankton species is essential if reliable estimates of zooplankton biomass are to be made from acou stic backscatter measurements of the water column. Much work has recen tly been done on the forward problem, where scattering predictions hav e been made based on animal morphology. Three categories of scatterers , represented by theoretical scattering models, have been identified b y Stanton et al. (1994): gas-bearing (e.g. siphonophores), fluid-like (e.g. euphausiids) and hard elastic-shelled (e.g. pteropods). If there are consistent differences in the characteristic acoustic signatures of each of these classes of zooplankton, it should be possible to solv e the inverse problem by using acoustic backscatter data to infer math ematically the class of scatterer. In this investigation of the feasib ility of inverting acoustic data for scatterer-type, two different inv ersion techniques are applied to hundreds of pings of data collected f rom broadband ensonifications (similar to 350 kHz-750 kHz) of individu al, live zooplankton tethered and suspended in a large tank filled wit h filtered sea water. In the Model Parameterization Classifier (MPG), the theoretical models for each scatterer-type are represented as eith er straight lines with slope and intercept parameters or rectified sin usoids with frequency and phase parameters. Individual pings are class ified by comparison with these model parameterizations. The Empirical Orthogonal Function-based Classifier (EOFC) exploits the basic structu re of the frequency response (e.g. presence of a resonance structure) through decomposition of the response into empirical orthogonal functi ons. Small groups of pings are classified by comparing their dominant modes with the dominant modes representative of the three scatterer-ty pes. Preliminary results indicate that the acoustic classification of zooplankton ensonifications into categories representing distinct scat terer-types is feasible. Ultimately, it may be possible to develop in situ acoustic systems that are capable of inverting for the types of o rganisms sampled, thereby bridging the gap between acoustic backscatte r measurements and estimates of zooplankton biomass. (C) 1996 Internat ional Council for the Exploration of the Sea