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

• Main Authors: Saunders, R.A., Brierley, A.S., Watkins, Jonathan L., Reid, Keith, Murphy, Eugene J., Enderlein, Peter, Bone, D.G.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: CCAMLR 2007
• Subjects: Marine Sciences; Zoology; Ecology and Environment; Antarctic; Antarc*; Antarctic Krill; Euphausia superba
• Online Access: http://nora.nerc.ac.uk/id/eprint/11933/; https://www.ccamlr.org/en/publications/science_journal/ccamlr-science-volume-14/ccamlr-science-volume-1427-41

Upward-looking acoustic Doppler current profilers (300 kHz) and echo sounders (125 kHz) were deployed on moorings on- and off-shelf to the northwest of South Georgia between 14 October 2002 and 29 December 2005 to measure density of Antarctic krill and environmental parameters continuously. A distinct seasonal pattern in krill density, recurring consistently over all three years, was detected. Krill densities in winter were predominantly low (mean = 18.7 g m(-2), SD = 24.3), but had risen substantially by summer in each year (mean = 89.5 g m(-2), SD = 64.2). A sinusoidal regression model (period = 52 weeks) with time as the independent variable explained 64% of the observed week-to-week variation. Estimates of krill density from moored instruments were not statistically different (P > 0.05) from estimates derived from standard ship-based krill surveys in adjacent time periods, suggesting that the point estimates from moored instruments were representative of krill density in a wider spatial context (ship surveys cover c. 100 x 100 km). Data from moored instruments were used to explore whether high-frequency temporal variation (i.e. within-year) could have led to the perceived between-year variation in krill density arising from previous summer surveys in the South Georgia 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 apparent 'year-to-year' variation could simply be attributed to sampling on different dates of the year (e.g. November cf. February). 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 density (i.e. densities below the range expected due to intra-annual variation) were 1993/94, 1998/99 and 1999/2000. Moored instruments provide valuable data that could be important for ecosystem-based management at South Georgia because, for example, ...