Antarctic krill (Euphausia superba) and salps (mainly Salpa thompsoni) are major grazers in the Southern Ocean, and krill support commercial fisheries. Their density distributions have been described in the period 1926-51, while recent localized studies suggest short-term changes. To examine spatial and temporal changes over larger scales, we have combined all available scientific net sampling data from 1926 to 2003. This database shows that the productive southwest Atlantic sector contains >50% of Southern Ocean krill stocks, but here their density has declined since the 1970s. Spatially, within their habitat, summer krill density correlates positively with chlorophyll concentrations. Temporally, within the southwest Atlantic, summer krill densities correlate positively with sea-ice extent the previous winter. Summer food and the extent of winter sea ice are thus key factors in the high krill densities observed in the southwest Atlantic Ocean. Krill need the summer phytoplankton blooms of this sector, where winters of extensive sea ice mean plentiful winter food from ice algae, promoting larval recruitment and replenishing the stock. Salps, by contrast, occupy the extensive lower-productivity regions of the Southern Ocean and tolerate warmer water than krill. As krill densities decreased last century, salps appear to have increased in the southern part of their range. These changes have had profound effects within the Southern Ocean food web.

Currently, between one-third and two-thirds of marine species may be undescribed, and previous estimates of there being well over one million marine species appear highly unlikely. More species than ever before are being described annually by an increasing number of authors. If the current trend continues, most species will be discovered this century.

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Between-year variability of krill Euphausia superba year class success and recruitment during the 1977 to 1994 period are described based on data from German expeditions and U.S. Antarctic Marine Living Resources Program cruises in the Elephant Island area. The recruitment index (R,), based on the relative abundance of the l + age class, varies substantially between years, whereas it is quite sinlilar between different surveys within the same field season. The overall mean recruitment index for all years was R,,,, -0.210. Good recruitment was observed for the 1980/81, 1985/86, 1987/88, and 1990/91 year classes; exceedingly poor recruitment occurred for the 1976/77. 1982/83. 1983/84, 1988/89, 1991/92 and 1992/93 year classes. Pairwise correlations between the stock parameters, recruitment indices, and available environmental data indicate that good and poor year class success are directly and indirectly related to sea ice conditions during the preceding winter season, the timing of krlll spawning, and the occurrence of dense salp concentrations. No correlation 1s shown with upper water column temperature or krill stocklspawning stock size A concept is developed describing the interactions of various parameters leading to good or poor krill recruitment.

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