CCAMLR

Central to understanding krill population dynamics is knowledge of their population structure. To examine this we used length-frequency distributions from 142 weeks of sampling (n= 23996 krill) of 3 predator species breeding at South Georgia and 12 weeks of sampling (n = 10 252 krill) from scientific nets from the same area over the summers of 1991-1997. In comparing the 5 years with both predator and net samples, despite differing selectivities and spatio-temporal circumscriptions, both were sampling the same overall krill population. Greatest similarity results from comparing net samples with samples from Antarctic fur seal and macaroni penguin combined; least temporal variation occurs in predator samples from late summer (March). From the 7-year predator time-series, within-year variation was greatest in 1991 and 1994, both years of low krill biomass at South Georgia. In both of these years large krill dominated during December but were completely replaced by small krill by February. The mean length of the March krill population showed a regular increase from 1991 to 1993, fell to a minimum in 1994 and thereafter increased steadily to 1997. Using these data in conjunction with putative size/age-group cohorts in the length-frequency distribution, we suggest that year's of high mean krill length reflect failure of small krill to recruit into the population, producing a period of low krill biomass in the following year. Similar recruitment failure in the same years is evident in krill populations in the Antarctic Peninsula region to the south, indicating large-scale events. This supports suggestions of periodic fluctuations in krill production and recruitment which may relate directly to physical phenomena such as cycles in the distribution and extent of sea-ice.

Acoustic estimates of the densities of Antarctic krill, Euphausia superba, in areas around South Georgia (SG) and Elephant Island (El) were compared for seven austral summers between 1981 and 1997. Estimated densities of krill at El were most often higher than at SG, although this may simply have been a function of differences in survey and data analysis techniques used at each site. More interestingly, the magnitudes of abundance and between-year gradients of change of abundance at each site were mirrored by those at the other location; for example, 1991 and 1994 were years of very low krill density at both SG and El. There was no apparent lag in changes in abundance at each site, and ranked between-year gradients of change in abundance at both locations were significantly correlated. These pronounced similarities suggests that densities of krill at both locations are directly linked, and may be impacted by the same gross physical and biological factors (e.g. recruitment, dispersal etc.), acting over the same temporal and spatial scales. The observed concordance also implies that the pelagic ecosystems at these widely separated sites (approximately 1500 km distant at opposite sides of the Scotia Sea) are not operating in isolation. Possible mechanisms linking krill population processes in the areas around South Georgia and Elephant Island are discussed.

The sea surface temperature dataset of Reynolds and Smith (1994) was used to describe variability around South Georgia. High levels of autocorrelation were evident in the sea surface temperature anomalies, with periodicity evident at a lag period of 4 years. To the north of the island significant autocorrelation was also evident at a lag period of 1 year; though this was restricted to an area that approximately encompassed the Georgia Basin. Crosscorrelation analyses with indices describing the El Niñio areas of the Pacific indicated that temperature fluctuations at South Georgia reflected temperature fluctuations in the Pacific. This link was separated temporally with the Pacific leading South Georgia by almost 3 years, and with the West Pacific showing the strongest correlations. These global teleconnections however, did not completely explain the variability around South Georgia as temperature anomalies also reflected the variability in the Georgia Basin. The high levels of intra annual variability at South Georgia were examined by means of Principal Component Analysis which indicated that seasonal differences between winter and summer were important. These arguments are developed to suggest that temporal variability in the onset of summer warming is potentially of great importance to the functioning of the ecosystem.

Acoustic estimates of abundance of the Antarctic krill, Euphausia superba, at South Georgia are presented for periods within ten austral summers between 1981 and 1998. This time series includes some previously published estimates arising from cruises where determination of krill density was a primary objective, and estimates derived for the first time here from acoustic data collected as a subsidiary during other studies of the pelagic ecosystem at South Georgia. Krill abundance at the island fluctuated widely from year to year over this time, ranging from ?2 to ? 150 gm-2 (wet weight). The 1982, 1991 and 1994 austral summer seasons were characterised by particularly low abundances of krill. For five of the years between 1990 and 1998 it was possible to calculate separate biomass estimates for the north-eastern and north-western ends of South Georgia. In four of these years biomass was higher to the east.

Interannual variability is a characteristic feature of the Southern Ocean ecosystem yet the relative roles of biological and physical processes in generating these fluctuations are unknown. There is now extensive evidence that there are years when there is a very low abundance of Antarctic krill (Euphausia superba) in the South Georgia area, and that the variation affects much of the ecosystem with the most obvious impacts on survival and breeding success of some of the major krill predators. The open nature of the South Georgia ecosystem means this variability has large scale relevance. Fluctuations in year class success in parts, or all, of the population across the Scotia Sea, can generate large changes in the available biomass. The ocean transport pathways maintain the large scale ecosystem structure by moving krill over large distances to areas where they are available to predator colonies. This large scale physical system shows strong spatial and temporal coherence in the patterns of the interannual and sub-decadal variability. This physical variability affects both the population dynamics of krill and the transport pathways emphasizing that both the causes and the consequences of events at South Georgia are part of much larger scale processes.