Krill Distribution Along the Western Antarctic Peninsula and Associations With Environmental Features, Assessed Using Multi-Frequency Acoustic Techniques

Gareth L. Lawson, Peter H. Wiebe, Timothy K. Stanton, and Carin J. Ashjian

The distribution of krill (Euphausia spp.) in the continental shelf region in and around Marguerite Bay, west of the Antarctic Peninsula, was studied in relation to aspects of the physical and biological environment. Acoustic, video, net, and environmental data were collected during fall and winter surveys in 2001 and 2002. Methods were first refined and verified for acoustically distinguishing krill aggregations from other zooplankton, and then estimating krill length, abundance, and biomass. Krill visual acuity was used to determine a threshold level of volume backscattering strength for identifying krill aggregations. Differences in mean volume backscattering strength at 120 and 43 kHz further served to distinguish krill from other sources of scattering. Inversions of mean volume backscattering strength (43, 120, 200, and 420 kHz) in these acoustically-identified aggregations were performed to estimate the weighted mean length and density of krill. Inversion results were comparable to net samples for estimated length, but acoustic estimates of krill density exceeded those from nets by one to two orders of magnitude, likely due to net avoidance and differences in the densities of krill in the volumes sampled by the two systems. Application of these methods demonstrated strong seasonal, inter-annual, and spatial variability in krill distribution. The distribution of krill aggregations was characterized by many small aggregations closely spaced relative to one another, punctuated by much fewer aggregations of very large size that accounted for the majority of overall biomass in the region. The greatest number of aggregations was found at depths less than 100 m, but biomass was usually greatest at deeper depths. Comparable krill biomass levels were estimated for both fall surveys and for the winter of 2002, while biomass during the winter of 2001 was an order of magnitude smaller. During fall, biomass estimates vertically-integrated over the sampled portion of the water column were negatively associated with chlorophyll a concentrations, and some suggestion was observed of highest biomass in regions of lowest current magnitude and horizontal current shear. Generalized additive models indicated that high krill biomass was consistently associated with regions close to land where temperatures at depth were cooler than was available over the shelf as a whole. Krill were not associated with regions where intrusions of warm and nutrient-rich circumpolar deep water onto the shelf were present at depth, despite such intrusions being thought to be an important driver of primary productivity.

STATUS UPDATE:

06/07/06: Submission expected next week.