CCAMLR

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.

Acoustic estimates of densities of Antarctic krill, Euphausia superba, at South Georgia are presented for eleven austral summers between 1981 and 1998. 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, 199 1 and 1994 austral summer seasons were characterised by particularly low abundances of krill. For five of the summers between 1990 and 1998 it was possible to calculate separate density estimates for north-eastern and north-western ends of South Georgia. In four of these years density was higher to the east.

The summer 1998 AMLR field season followed late-forming but spatially extensive winter sea ice in the Antarctic Peninsula region. Overall krill length/maturity composition reflected poor recruitment from the past two years. Unusually small juvenile krill in 1998 suggested that survival success was limited to late-spawned eggs and larvae in 1997. Delayed spawning and few larval krill in 1998 were associated with extremely high salp abundance. Sa/pa thompsoni abundance was similar to levels during the 1993 "salp year". The magnitude of this salp bloom may have resulted from a large overwintering "seed" population developed the preceding autumn. Presence of large numbers of another salp species, Ihlea racovitzai, was unique to AMLR surveys and indicated faunal input from the east.
Comparisons of January and February 1998 survey data with data from previous AMLR field seasons showed recurring patterns of species abundance relationships that allowed definition of different ecological regimes identified here as "cope pod years", "salp years" and "transition periods". The 4-5 year periodicity of "salp years" over the past fifteen years is discussed with respect to the Antarctic Circumpolar Wave (ACW).