Zooplankton target strength: volumetric or areal dependence?

Target strengths (TS) of various zooplankton were measured at 200 kHz, 420 kHz and 1 MHz and the dependence of these data on animal volume versus cross-sectional area was explored The 420 kHz and I 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, tethered individuals in an enclosure filled with filtered seawater. The data were compared to both empirical and theoretical models of reduced target strength (TS normalized by the square of the animal length) versus ka (the product of wave number and equivalent cylindrical radius). The theoretical models chosen for this comparison were two versions of a high-pass bent-cylinder model (Stanton, 1989b) that indicate TS is dependent on animal volume, and the ray bent-cylinder model (Stanton, 1993a) which implies TS is dependent on the cross-sectional area. The dependence of acoustic backscattering on animal volume or area was tested by fitting regression lines for TS versus the logs of ka, length (L), wet weight (WW) and dry weight (DW). Contrary to an empirical model derived from similar experiments (Wiebe et al., 1990), and to the high-pass models, the regressions indicated that TS is proportional to the cross-sectional area of the animal. However, neither Wiebe et al. (1990) nor this experiment directly accounted for animal orientations. Simulations using a Distorted Wave Born Approximation Model (Chu et al., 1993), indicated that animal behavior is an important factor in the scattering characteristics of zooplankton. In addition, because scattering from individual zooplankton is highly non-linear, especially in the geometric scattering region (ka>l), linear regressions of TS versus the log of ka, L, WW or DW are inappropriate and misleading.