Recruitment of Antarctic krill Euphausia superba and possible causes for its variability

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.