David A. Demer
Scripps Inst. of Oceanogr., Univ. of California at San Diego, La Jolla, CA 92038
Linda V. Martin
Woods Hole Oceanogr. Inst., Woods Hole, MA 02543
Targets strengths of various zooplankton were measured at 200, 420, and 1 MHz and the dependence of these data on animal cross-sectional area versus volume was explored. The 420- and 1-MHz data were collected with a dual-beam sonar system and the 200-kHz data with a split-beam system. Experiments were conducted with live, but tethered, individuals in an enclosure filled with filtered seawater. The data were compared to both empirical and theoretical models of reduced target strength (RTS) versus ka (the product of wave number and equivalent cylindrical or spherical radius). Theoretical models chosen for this comparison were a volumetrically dependent high-pass bent-cylinder model and an areally dependent ray bent-cylinder model. Goodness-of-fit to these models was evaluated. The dependence of acoustic backscattering on animal volume or area was tested by fitting regression lines for TS vs ka, length, wet weight, and dry weight. The practical accuracy of each of these models was then evaluated in a demonstration of a multi-frequency inversion technique for animal size classification. [Work supported by the Hertz Foundation.]