Intra-annual variability in the abundance of Antarctic krill (Euphausia superba) at South Georgia, 2002–2005: within-year variation provides a new framework for interpreting previous ‘annual’ krill density estimates

Upward-looking acoustic Doppler current profilers (300 kHz, ADCP) and echosounders (125 kHz) were deployed on moorings on- and off-shelf to the northwest of South Georgia to measure abundance of Antarctic krill continuously between 14 October 2002 and 29 December 2005. A distinct seasonal pattern in krill abundance was detected that recurred consistently over all 3 years. Krill densities in winter were predominantly low (mean = 18.7 g m-2 SD 24.3) but rose substantially by summer in each year (mean = 89.5 g m-2 SD 64.2). A simple polynomial regression model with time as the independent variable explained 71% of the observed week-week variation. Mooring estimates of krill abundance were not statistically different (P>0.05) from estimates derived from standard ship-based krill surveys in adjacent periods suggesting that the mooring point estimates had relevance in a wider spatial context (ship surveys cover c. 100 x 100 km). Mooring data were used to explore whether high frequency temporal variation (i.e. within-year) could have led to the perceived between-year variation from previous summer surveys in the South Georgian western core box region between 1990 and 2005. Comparison of these ‘snap-shot’ ship survey estimates with the observed pattern of within-year variability showed that some of the alleged ‘year-to-year’ variation could be attributed just to sampling at different dates of year. However, there were some survey estimates that were significantly different (P<0.01) from the regression-predicted within-year variation. Years that stand out for markedly low krill abundance (i.e. densities below the range expected due intra-annual variation) were 1993/94, 1998/99 and 1999/2000. High values were observed in 2003. Moorings provide valuable data that could be important for ecosystem-based management at South Georgia because, for example, they will enable predator- prey functional responses to be explored there for the first time at appropriate temporal scales, and will enable hypotheses relating variation in krill abundance to physical oceanographic variability to be tested.