CCAMLR

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.

The eco systematic studies in biology and hydrology conducted in the Atlantic sector of the South Ocean in recent years made it possible for scientists to establish a rather clear relationship among the winter and spring ice cover variations, summer water surface temperature, and "success" in the reproduction of krill (Euphausia superba) and salp (Salpa tompsoni) (SC-CAMLR-XVI4; SC-CAMLR-XVV3).
The analysis of multiannual observations ( 1977-1997) conducted by Siegel (Siegel et al., 1997) and other researches in the Antarctic Peninsula Subarea 48.1 showed good correlation between the strong year-classes of krill and severe winters when the ice-cover extended itself northwards considerably as opposed to the multiannual average level, and late thawing of ice. On the contrary, poor generations of krill were formed during the years when the winter position of the ice edge was south of the average multiannual level. The abundance of salp went up sharply in the areas of krill distribution in summer during such years of moderate winter in higher surface temperature. Salp occurred in mass constantly both in scientific and commercial fishing gear. Poor year-classes of krill and upsurges in the abundance of salp were recorded in seasons 1982-1983 and 1988-1989 (Siegel and Loeb, 1995) i.e. during the years of development of El Niño off South America (Jordan, 1991). These facts may lead one to assume that the last El Niño can also cause warming in Subarea 48.1 affecting reproductive capacity in krill.