Applied Ocean Physics & Engineering Dept.
Woods Hole Oceanographic Institution
Woods Hole, MA 02543
Program Manager : Wendy Gabriel, NMFS/EASC
Improved estimates of fish abundance and biomass derived from fisheries acoustics surveys require fundamental understanding of sound scattering by the organisms. Acoustic scattering by fish, zooplankton, and other marine organisms is a complex function of the size, shape, morphometry, and behavior of the targets . Anatomical attributes combined with behavior of the organisms influence the estimates of organism size and number as derived from acoustic scattering survey data. Modeling of the scattering is sufficiently challenging that controlled measurements of the scattering are required. These measurements can be obtained through both in situ and laboratory experiments. This research involves laboratory measurements as a collaborative effort between WHOI and NOAA/NEFSC. The work represents an enhancement to ongoing work being funded through other sources.
In the spring of 2000 , an extensive series of measurements of target strength of live individual alewife fish was successfully completed . Using a pair of broadband transducers and computercontrolledrotator, acoustic backscattering was measured over the frequency range 40-95 kHz and over all angles of orientation in two planes of rotation inone-degree increments. The morphology of the animals was studied through a combination of dissection , physical measurements, traditional x-rays (again, in two planes), medical CT scans, and phase-contrast xrays. These data, involving twenty fish, are of very high quality.
The analysis is complete . The data have been processed in terms of their characteristics in both the time (pulse compression processing) and spectral domains. In addition, the processed data have been compared with two scattering theories--the Kirchhoff-Ray-Mode and Fourier Matching Method formulations. Through this analysis, dominant scattering mechanisms have been identified. Specifically, it was demonstrated that, in addition to the swimbladder contributing significantly to the scattering , the head and other organs also contribute significantly for angles well off normal incidence. Also, it was demonstrated that the angle of orientation of the fish could be estimated remotely from the duration of the compressed pulse provided that the length of the fish is known . Our study concludes that the Kirchhoff Ray-Mode theory works reasonably well for angles near normal incidence (dorsal). Well away from these angles, a more sophisticated model , such as the Fourier Matching Method, needs to be used, but with the addition of taking into account the other organs.
Preliminary results were reported in a special session on "Fish and Zooplankton Acoustics" at the Dec. 2000 scientific meeting of the Acoustical Society of America in Newport Beach, CA. Final results were reported in the June 2002 ICES symposium on "Acoustics in Fisheries and Aquatic Ecology" in Montpellier, France.
The following manuscripts have been submitted for consideration of publication inpeer-reviewed scientific journals:
1. Reeder, D.B. and T.K. Stanton, "Acoustic scattering by axisymmetric finite-length bodies: An extension of a 2-dimensional conformal mapping method," submitted to J. Acoust. Soc. Am.
2. Reeder, D.B., J.M. Jech, and T.K. Stanton, "Broadband acoustic backscatter and high-resolution morphology of fish: Measurement and modeling," submitted to J. Acoust. Soc. Am.
3. Stanton, T.K., D.B. Reeder, and J.M. Jech, "Inference of fish orientation from broadband acoustic echoes," submitted to ICES J. Mar. Sci.
Finally, the results in papers #1 and #2 make up much of Ben Reeder's doctoral thesis which he successfully defended in May, 2002.