Krill stocks in high latitudes of the Antarctic Lazarev Sea: seasonal and interannual variation in distribution, abundance and demography

The motivation of the LAKRIS project (Lazarev Krill Study) was to investigate the seasonal and interannual variability in distribution, demography, phenology and population dynamics of Antarctic krill Euphausia superba in poorly studied high-latitude areas. The core data set consists of four RMT-net-sampling surveys that were conducted in early summer (December 2005 and 2007), autumn (April 2004) and winter (July to August 2006). Supplementary data were analysed from the 1980s to view the results in a wider geographical context. The major observations are the following: (1) krill abundance and distribution showed geographical, seasonal and interannual fluctuations. Krill abundance was almost one order of magnitude lower compared to the Southwest Atlantic. (2) Krill density showed substantial differences between day and night for autumn and winter surveys. (3) A spatial gradient was observed for krill size groups, with larger sized (adult spawning) krill further south and medium-sized juveniles and immatures mainly between 60° and 67°S. (4) Data on maturity stage composition and larvae occurrence indicate an active and successful spawning in these high-latitude areas even under the conditions of sea-ice cover. At the end of the spawning season, females and males regressed in their external sexual characteristics (‘re-juvenation’) to overwintering immature stages. Major conclusions relate to a conceptual view of the spatial distribution of krill size classes and the occurrence of larvae in the context of major oceanographic features and current systems in the Lazarev Sea and adjacent ocean sectors. The Lazarev Sea obviously does not support a single self-maintaining population, but represents a complex transition zone of stocks with different origins from the Scotia Sea and the Cosmonaut Sea