Krill and zooplankton populations monitored during AMLR 2004 surveys (Subarea 48.1) with respect to the long-term Elephant Island area datasets

Net sampling during annual field season activities of the U.S. Antarctic Marine Living Resources (AMLR) Program yielded about average concentrations of postlarval krill during January and February-March surveys. Overall length-frequency distributions and maturity stage composition reflected good recruitment of the 1990/00, 2000/01 and 2001/02 year classes and little contribution from the 2002/03 season. Although little reproductive activity and unusually large proportions of immature individuals occurred in the Elephant Island Area during January, larval concentrations and maturity stage composition indicated a very early onset of seasonal spawning activity. Krill demographics during February-March indicated an influx of large spawning individuals; these along with greatly increased concentrations of larval stages suggested potentially good recruitment success from the 2003/04 year class.
The 2004 survey period was marked by a rapid and large-scale change in overall zooplankton abundance, composition and species abundance relations. This was similar to marked “transition periods” observed in 1994 and 1997 and, like those, was believed to result from hydrographic rather than biological processes. Qualitative differences between this and previous transition periods highlighted significant changes in zooplankton abundance and abundance relations, frequency of krill recruitment success and sea ice development that coincided with an oceanic-atmospheric “regime shift” affecting the Pacific Ocean basin.