A comparison of acoustic targets at South Georgia and the South Orkney Islands during a season of profound krill scarcity

Dual frequency (38 and 120 kHz) acoustic surveys of shelf-break regions in the vicinity of Willis Islands, South Georgia and Coronation Island, South Orkneys, both in the Atlantic sector of the Southern Ocean, were carried out from RRS lames Clark Ross during January 1994. Difference in signal strength between the two frequencies (δMVBS = MVBS 120 kHz - MVBS 38 kHz) was used to partition acoustic targets into three biological categories: Antarctic krill; nekton (larger than krill); and zooplankton (smaller than krill). To counter the absence of krill length-frequency data from Coronation Island a new generic target strength (TS) relationship was derived. This relationship was used in conjunction with appropriate 120 kHz data to calculate krill density estimates for both survey areas. Krill distribution in both regions was extremely patchy, resulting in a highly skewed frequency distribution of density; for example one echo-integration interval contained 104 times the mean krill density. An approximate six-fold difference in mean krill abundance was detected between the two areas, but in comparison with previously published density estimates krill densities in both regions were extremely low (1.7 and 10.7 gm-2 for Willis and Coronation Islands respectively). Gross regional differences in pelagic faunal composition were apparent from inspection of echo-charts, and were quantified in detailed analysis of acoustic signals. The water column in the vicinity of Willis Islands was characterised by a series of diffuse but continuous scattering layers visible only at 120 kHz, and 73% of acoustic targets there were classified as zooplankton. In marked contrast, the Coronation Island region was dominated by dense, discrete patches classified as larger nektonic scatterers, whereas only 12% of targets were classified as zooplankton. Simultaneous predator diet analyses carried out at Bird Island, South Georgia, confirm the profoundly low levels of krill availability, and support the acoustic observation that the Willis Island region contained large numbers of small zooplankton such as the amphipod Themisto gaudichaudii.