| Summary: | 1. Euphausiids are an important component of high latitude pelagic ecosystems, but there is a paucity of information on their distribution, abundance and population processes on within-year time scales. This thesis encompasses new research into the euphausiid-ocean component of two important high latitude ecosystems (South Georgia and the Irminger Sea) on sub-annual time scales. 2. A new method for measuring abundance of Antarctic krill (Euphausia superba) continuously at South Georgia (Southern Ocean) was devised using upward-looking acoustic devices deployed on moorings. These novel platforms provide a new window of observations onto marine systems not open from conventional research vessels. At South Georgia, the moorings provided data at a high temporal resolution giving completely new insight to the function of the coupled biological-physical marine ecosystem. The use of moorings may aid ecosystem-based management at South Georgia and elsewhere. 3. Analysis of mooring data collected between October 2002 and December 2005 indicated a regular annual cycle in krill density: high in summer and low in winter. Mooring estimates of krill density were not statistically different from estimates derived from standard ship-based surveys in adjacent time periods suggesting that the mooring point estimates had relevance in a wider spatial context (c. 100 x 100 km). The results indicated that because of the sharp peaks in the biomass cycle, the exact timing of repeated ship-based acoustic surveys might be critical. Surveys that differ in their timing by only a few weeks might exhibit quite different estimates of biomass because they fall at different points of the cycle. Additionally, within this intra-annual framework, annual ship-based surveys may be able to detect differences between high and low krill years only if they differ by densities of c. 35 g per square m. in summer and c. 20 g per square m. in winter. 4. The mechanisms driving intra-annual variability in Antarctic krill density at South Georgia are likely to ...
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