Modelling and measuring acoustic backscattering cross-section due to simultaneous contributions from swimbladder and fish flesh

GONZALEZ, JUAN DOMINGO; PRARIO, IGOR; BLANC, SILVIA; MADIROLAS, ADRIAN

Göthenburg

Congreso; IAHS/IAPSO/IASPEI Joint Assembly; 2013

IUGG

Recent computational improvements have enabled the superposition of swimbladder (gas-filled spheroid) and fish flesh (liquid-filled spheroid) contributions to compute fish target strength. Their corresponding backscattering cross-sections were coherently added. A prolate spheroidal scattering approach has been adopted to model scattering response by bladdered and bladder-less fish. The implemented model enables simulation of a great variety of scattering shapes in fresh water and oceans that are attainable by prolate spheroids. Moreover, the used curvilinear coordinate system leads to a separable solution for the scalar Helmholtz equation. Two different experimental conditions have been simulated with the implemented model. Measurements of acoustic backscattering target strength of Percichthys trucha at 38 kHz using encaged fish with a split-beam echo-sounder was conducted in an Argentinean lake. A Monte Carlo method was used to simulate target strength probability density functions taking into account fish behaviour through an experimental tilt-angle distribution analysis. The model was also applied for further analysis of reported maximum and average target strength dependence on fish length from gadoids and mackerel. For both experimental conditions, a very reasonable agreement was obtained when comparing measurements and simulations. Experimental evidence showed that slight tilt-angle variation can generate significant differences in backscattering target strength parameter. This feature could be simulated since fish flesh contribution was included in the total scattering response. Furthermore, incorporating the measured tilt-angle distribution improved agreement between experimental and simulated results.