LAUSR

Abstract Non-lethal methods are sought to provide indices or measure of absolute abundance to fit fishery stock assessment models, including those computed for reef fishes. Calibrated scientific echosounders provide the foundation for stock assessments in low-diversity ecosystems but are hampered by complicated target strength responses in high diversity systems like coral and rocky reefs. Newly available broadband echosounders present advantages over narrowband predecessors including increased spatial resolution, increased signal to noise and spectral resolution; however, a better understanding of the sound scattering properties is necessary for operational use in surveys of diverse reef fish assemblages. To gain insight into the feasibility of implementing broadband techniques, we first model the frequency dependent backscatter of ecologically and commercially important species; results will aid our understanding of the backscattering as well as provide a supporting dataset to develop algorithms for classification of species/functional groups. Computed-tomography (CT) scans were performed on 149 individuals across reef fish species from the northeastern Gulf of Mexico to generate three-dimensional swim bladder models for estimation of morphometrics to input into numerical scattering models. Principal component analysis of the swim bladder morphology indicated significant variation among species; however, closely related species (confamilials) were not different. We employed boundary element method modeling to examine the frequency-dependent backscatter responses across a range of fish orientations (+/- 45°). Comparisons of frequency-dependent backscatter revealed strongest similarities between closely related species and greatest differences between more distantly related species or species groups. Patterns in the spectra have the potential to be used to classify targets toward species (or species group) for estimates of density from marine ecosystem acoustic surveys. Given that broadband echosounders are not yet commonplace, and many surveys in the region utilize narrowband echosounders, we also developed narrowband target strength – length relationships for six of the reef fish species with model fits ranging widely (r2 = 0.05 to 0.93). The potential utility of broadband echosounders to enhance ecosystem studies, or to develop fisheries-independent indices of abundance for use in stock assessment is discussed.