Euphausiid Distribution Along the Western Antarctic Peninsula - (A) Development of Robust Multi-Frequency Acoustic Techniques to Identify Euphausiid Aggregations and Quantify Euphausiid Size, Abundance, and Biomass

Gareth L. Lawson, Hopkins Marine Station of Stanford University, 120 Oceanview Blvd., Pacific Grove, CA 93950

Peter H. Wiebe, Biology Department, Woods Hole Oceanographic Institution, Woods Hole, MA, 02543
Timothy K. Stanton, Department of Applied Ocean Physics and Engineering, Woods Hole Oceanographic Institution, Woods Hole, MA, 02543
Carin J. Ashjian, Biology Department, Woods Hole Oceanographic Institution, Woods Hole, MA, 02543

Methods were refined and tested for identifying the aggregations of Antarctic euphausiids (Euphausia spp.) and then estimating euphausiid size, abundance, and biomass, based on multi-frequency acoustic survey data. A threshold level of volume backscattering strength for distinguishing eu phausiid aggregations from other zooplankton was derived on the basis of published measurements of euphausiid visual acuity and estimates of the minimum density of animals over which an individual can maintain visual contact with its nearest neighbor. Differences in mean volume backscatt ering strength at 120 and 43 kHz further served to distinguish euphausiids from other sources of scattering. An inversion method was then developed to estimate simultaneously the mean length and density of euphausiids in these acoustically-identified aggregations based on measurements of mean volume backscattering strength at four frequencies (43, 120, 200, and 420 kHz). The methods were tested at certain locations within an acoustically-surveyed continental shelf region in and around Marguerite Bay, west of the Antarctic Peninsula, where independent evidence was also a vailable from net and video systems. Inversion results at these test sites were similar to net samples for estimated length, but acoustic estimates of euphausiid density exceeded those from nets by one to two orders of magnitude, likely due primarily to avoidance and to a lesser extent to differences in the volumes sampled by the two systems. In a companion study, these methods were applied to the full acoustic survey data in order to examine the distribution of euphausiids in relation to aspects of the physical and biological environment (Lawson et al., this issue).

STATUS UPDATE:

10/10/07: Received final version with U.S. GLOBEC contribution number.