ACOUSTIC TARGETS AT SOUTH GEORGIA AND THE SOUTH ORKNEY ISLANDS DURINGA SEASON OF KRILL SCARCITY

Dual frequency (38 and 120 kHz) acoustic surveys of shelf-break region s in the vicinity of the Willis Islands, South Georgia, and Coronation Island, South Orkneys, both in the Atlantic sector of the Southern Oc ean, were carried out from RRS 'James Clark Ross' during January 1994. The difference in echo signal strength between the 2 frequencies [del ta MVBS (mean volume backscattering strength) = MVBS 120 kHz - MVBS 38 kHz] was used to partition acoustic targets into 3 biological categor ies: Antarctic krill Euphausia superba, nekton (larger than krill) and zooplankton (smaller than krill). Krill density estimates were derive d for both survey areas, and the relative contribution of each class o f target to overall regional pelagic biomass determined. Krill distrib ution in both regions was extremely patchy, resulting in a highly skew ed frequency distribution of density; for example one echo-integration interval contained 10(4) times the mean krill density. An approximate B-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 diff erences in pelagic faunal composition were apparent ent from inspectio n of echo charts, and were quantified by detailed analysis of acoustic signals. The water column in the vicinity of the Willis Islands was c haracterised by a series of diffuse but continuous scattering layers v isible only at 120 kHz, and 73% by mass of acoustic targets in this re gion were classified as zooplankton. In marked contrast, the Coronatio n Island region was dominated by dense, discrete acoustic target patch es which were classified as larger, nektonic, sound scatterers (fish o r squid), and only 12% of targets there were classified as zooplankton . Elsewhere, data from predator diet analyses carried out at Bird Isla nd, South Georgia, during the same season that our acoustic measuremen ts were made, confirm the profoundly low levels of krill availability there and support the acoustic observation that the Willis Islands reg ion contained large numbers of small zooplankton such as the amphipod Themisto gaudichaudii. Similarly, available data on analyses of stomac h contents of Chinstrap penguins nesting in a neighbouring region with in an oceanographic setting similar to that of Coronation Island revea l 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 evidenc e gathered from predators, and the value of integrating predator/prey and acoustic studies to characterise the content of localised marine e cosystems is demonstrated.