Acoustic targets at South Georgia and the South Orkney Islands during a season of krill scarcity
Dual frequency (38 and 120 kHz) acoustic surveys of shelf-break regions in the vicinity of the Willis Islands, South Georgia, and Coronation Island, South Orkneys, both in the Atlantic sector of the Southern Ocean, were carried out from RRS 'James Clark Ross' during January 1994. The difference in echo signal strength between the 2 frequencies [deltaMVBS (mean volume backscattering strength) = MVBS 120 kHz - MVBS 38 kHz] was used to partition acoustic targets into 3 biological categories: Antarctic krill Euphausia superba, nekton (larger than krill) and zooplankton (smaller than krill). Krill density estimates were derived for both survey areas, and the relative contribution of each class of target to overall regional pelagic biomass determined. 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 6-fold difference in mean krill abundance was detected between the 2 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 by detailed analysis of acoustic signals. The water column in the vicinity of the Willis Islands was characterised by a series of diffuse but continuous scattering layers visible only at 120 kHz, and 73% by mass of acoustic targets in this region were classified as zooplankton. In marked contrast, the Coronation Island region was dominated by dense, discrete acoustic target patches which were classified as larger, nektonic, sound scatterers (fish or squid), and only 12% of targets there were classified as zooplankton. Elsewhere, data from predator diet analyses carried out at Bird Island, South Georgia, during the same season that our acoustic measurements were made, confirm the profoundly low levels of krill availability there and support the acoustic observation that the Willis Islands region contained large numbers of small zooplankton such as the amphipod Themisto gaudichaudii. Similarly, available data on analyses of stomach contents of Chinstrap penguins nesting in a neighbouring region within an oceanographic setting similar to that of Coronation Island reveal the presence of more fish in the diet than usual. Conclusions drawn from acoustic observations as to the relative regional composition of pelagic biomass are therefore clearly supported by independent evidence gathered from predators, and the value of integrating predator/prey and acoustic studies to characterise the content of localised marine ecosystems is demonstrated.