Quantifying intraspecies variation in acoustic backscatter models

Abstract

Fisheries researches are increasing the use of frequency-, length-, and aspect-dependent backscatter models in population abundance estimates of fish and zooplankton. Model predictions are often based on measurements from a single or a limited number of organisms. The resulting model parameters are used to predict backscatter for organisms of any length in the population. At rather high frequencies (i.e., length/wavelength ratios of 1 to 20), choice of carrier frequency, swimbladder shape, and swimbladder aspect influence amplitudes of returned echoes. Morphological differences among individuals will therefore determine the variance observed in amplitudes of echo ensembles. To quantify variance in predicted backscatter within species as a function of carrier frequency, fish length, and swimbladder aspect, we combined data from digitized x rays with Kirchhoff ray-mode backscattering models of brook trout (Salvelinus fontinalis) and Atlantic code (Gadus morhua). Quantifying backscatter variances within and between species allow us to examine the applicability of multi-frequency data and the inverse approach to estimate fish length abundances. [Work supported by ONR.]