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, J. L., Reid, K., Murphy, E. J., Enderlein, P., Bone, D. G.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2007
• Subjects: Antarctic krill; moorings; intra-annual variability; South Georgia; CCAMLR; DOPPLER CURRENT PROFILER; TARGET-STRENGTH; INTERANNUAL VARIABILITY; TEMPORAL VARIABILITY; CHANGING ENVIRONMENT; ACOUSTIC SURVEYS; FISH STOCKS; ABUNDANCE; SEA; ECOSYSTEM; Antarctic; Antarc*; Antarctic Krill; Euphausia superba
• Online Access: https://risweb.st-andrews.ac.uk/portal/en/researchoutput/intraannual-variability-in-the-density-of-antarctic-krill-euphausia-superba-at-south-georgia-20022005-withinyear-variation-provides-a-new-framework-for-interpreting-previous-annual-estimates-of-krill-density(85516a96-f7b2-4e08-be3d-f300720c17a6).html; http://www.scopus.com/inward/record.url?scp=36249006056&partnerID=8YFLogxK

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, ...