Discriminant Classification of Fish and Zooplankton Backscattering at 38 and 120 kHz

Abstract

Acoustic scattering layers were evaluated for species classification by means of 38- and 120-kHz mean volume backscattering strength () collected during a 1995 acoustic–trawl survey of Pacific hake Merluccius productus off the west coasts of the United States and Canada. Scattering layers selected for analyses were shallower than 150 m and were analyzed with a −79-decibel (dB) integration threshold. Pacific hakes, euphausiids, and Pacific hake–euphausiid mixes dominated the layers. Other scatterers (unidentified, noneuphausiid, or non—Pacific hake sources) were included in the analyses. The overall mean volume backscatter difference (Δ = _{120 kHz} – _{38 kHz}) was computed for each species category, and results varied depending on the species composition of the scattering layer (i.e., Pacific hakes = −7.1 dB, euphausiids = 11.9 dB, Pacific hakes–euphausiids = 3.5 dB, and other species = 0.1 dB). Discriminant function analysis of _{120 kHz} and _{38 kHz} separated echoes originating from each of the dominant scattering layers. Backscatter was then classified into species groups with a quadratic discriminant classification model, which obtained an overall correct classification rate of 84%. The use of multiple frequencies and these analytical methods (e.g., frequency differencing and discriminant classification functions) can provide an efficient and objective means of classifying sound-scattering layers composed of different taxonomic groups.