CCAMLR

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.

The assessment of environmental processes controlling variability of Antarctic krill (Euphausia superba) is an important subject to clarify the Antarctic marine ecosystem. We have assessed variability of krill recruitment and stock density with hypothesised environmental factors. We demonstrated long-term relationship between Antarctic krill recruitment in the Antarctic Peninsula area and sea-level pressure gradients across Drake Passage during 1982-1993. The years with high pressure gradients meaning strong westerly winds coincided with good levels of krill recruitment in 1981182 (austral summer season), 1987/88 and 1990/91, while the years of extremely low values meaning weak westerly winds coincided with the extremely poor krill recruitment in 1982/83, 1983/84, 1988/89, 1991192 and 1992/93. We also found significant correlations between krill stock density in the Antarctic Peninsula area and the Antarctic ozone depletion during 1977-1994 (e.g. ozone hole's area: r=-0.844, p=0.001). This implied that the mean of krill stock densities after the 1984/85 decreased drastically as compared with that before 1982/83 with abrupt ozone depletion. With a gentle-ascending gradient of ozone depletion, however, krill stock density seems to be slightly recovering year by year after 1990 indicated a minimum of it. The association of krill variability with westerlies and ozone depletion suggests that it is of critical importance to the function of the Antarctic Peninsula area ecosystem.

Salp by-catch and krill discolouration ('green' krill) caused by active feeding on phytoplankton were analysed using logbook data from Japanese krill trawlers operating near the Antarctic Peninsula. Interannual and seasonal variability of the timing, duration and intensity of salp blooms and the presence of green krill were analysed. No relationship between salp density and the proportion of green krill in the catches was evident when both salps and krill were taken together. In the Livingston Island area, the proportion of green krill was high only when salp density was extremely low. However, no clear relationship was observed in the Elephant Island area. Possible reasons for these phenomena are discussed.