In situ target strength measurements of Antarctic zooplankton (Euphausia superba and Salpa thompsoni) at 120 khz and 200 khz, corroboration of scattering models, and a statistical technique for delineating species

In-situ measurements of target strength (TS) were made of Antarctic zooplankton (Euphausia superba and Salpa thompsoni) at 120 kHz and 200 kHz. Concurrently, a two meter Isaacs-Kidd midwater trawl was used to sample the zooplankton populations and animal length-frequency data were recorded. The TS and length-frequency data were combined to corroborate theoretical scattering models for both species. The individual TS measurements were collected at 120 kHz with a split-beam echosounder and a single-target detection algorithm. Because the two transducers were essentially collocated, range-bin and off-axis angles from the 120 kHz detections were used to extract the corresponding TS from the 200 kHz single-beam data. The backscattering cross-sectional areas (σbs) of salps are shown to fit a fluid sphere model [V.C. Anderson, J.Acoust. Soc. Am. 22:426-431 (1950)]; presumably, predominant scattering is from each animal's spheroidal nucleus. Consistent with the measurements of encaged krill [K.G. Foote et al., J. Acoust. Soc. Amer. 87(1):16-24 (1990) and D. Chu et al., J. Acoust. Soc. Am. 93(5):2985-2988 (1993)], the TS of in-situ krill are shown to fit a deformed cylinder model [T.K. Stanton, et al., J. Acoust. Soc. Am. 94(6):3454-3462 (1993a) and 94(6):3463-3472 (1993b)]. Utilizing these scattering models and empirically derived distributions of animal sizes, a technique is developed for acoustically delineating the two species. The method uses nonparametric Kolmogorov-Smirnov tests to evaluate cumulative distributions (CDF's) of Δσbs derived from the differences in abs measurements at two frequencies. About 15% of the tests indicated the presence of salps without krill and about 3% revealed krill without salps. The salp/no krill echograms were characterized by diffuse scattering layers which were much higher in volume backscattering strength (Sv) at 200 kHz than at 120 kHz. Conversely, the krill/no salp echograms included dense swarms with virtually equivalent Sv at the two frequencies. The ability for this method to delineate salps from krill is highly dependent on the degree to which the two CDF's differ. A simulated combination of 200 kHz and 38 kHz resulted in highly distinguishable CDF's, indicating that these frequencies may be more useful for distinguishing salps from krill. Data were collected as part of the United States Antarctic Marine Living Resources Program, near Elephant Island, Antarctica, during the Austral Summer of 1994.