Near-shore acoustic surveys for Antarctic krill at South Georgia, January 2004

Dual frequency (120 and 38 kHz) acoustic surveys were conducted around the Willis Islands and off the Barff Peninsula, South Georgia, from a 59 foot yacht in January 2004. The purpose of the surveys was to determine krill density near shore, in regions that have not traditionally been surveyed by large research vessels, and to survey responsively in areas where adult Macaroni penguins provisioning chicks ashore were foraging. Despite its relatively small size, the yacht (Golden Fleece) proved to be well suited as an acoustic survey platform. The echosounder transducers were deployed on a removable scaffold-tubing frame midships. With sails up the yacht had good roll stability and useable data were collected in seastate 6 with winds up to 30 knots and swells of approximately 2.5 m at survey speeds of up to 7 knots. Echo intensity data were scaled to krill density using a target strength determined from the length distribution of krill sampled from fur seal scats at Bird Island (mean krill length 53.13 mm, TS = -38.43 dB kg-1). Mean krill densities were 7.1 g m-2 off the Barff Peninsula and 5.7 g m-2 around the Willis Islands. Krill densities in both regions were low compared to time-series of observations from the past 20 years. A low density was perhaps not unexpected in line with recent cyclical patterns in density but may have been exacerbated by the presence of a large iceberg (40 miles by 30 miles) aground on the shelf edge to the north east of South Georgia that may have been blocking the influx of krill. In conclusion, acoustic surveys from small vessels enable data to be collected from regions where large vessels cannot operate safely, and offer flexibility to sample reactively on time scales that cannot always be accommodated under the scheduling constraints of multidisciplinary cruises. Combination of data from small vessels operating near shore with data collected over more extensive, off shore areas by large ocean-going research vessels will provide a more comprehensive understanding of the prey field available to krill predators.