An Introduction
To study these scattering properties we use anatomically-based, numeric models to quantify the relative importance of biological and physical factors that influence the amount of sound backscattered from aquatic organisms. This is important when estimating abundance or discriminating targets because interpretation of data is influenced by the amplitude and variability of backscattered sound. Experience has shown that there is no single metric such as the amplitude of a returned echo (i.e. target strength) that is unique to a species or size of fish.
Fish come in all shapes and sizes. The amount of sound that a fish reflects largely depends on whether it has a swimbladder, its size, its orientation to the sound source, and what frequency you are using to view the fish. On the left is a picture of a walleye pollock (Theragra chalcogramma) and a radiograph or x-ray of the same fish.
The dark colored organ in the radiograph is the swimbladder. Most of the returned sound from a fish is reflected by the swimbladder. Check out our radiograph gallery to see a collection of fish x-ray images.
To quantify variance in backscatter amplitudes we compare model predictions of individual or aggregations of fish to measurements from tethered fish or research survey data. Our variables of interest include species, organism length, organism aspect, organism roll, and acoustic wavelength. We invite you to explore the way sound is reflected by fish and to interact with the backscatter models and the 3-dimensional backscatter visualizations.
This research has been sponsored by the Ocean Acoustics and Optical & Biological Oceanography Divisions at the Office of Naval Research.