Overwintering Strategies and Predation on Antarctic Krill
College of Marine Science
University of South Florida
In contrast to 2001, larval Euphausia superba were generally scarce throughout the study area in fall and winter 2002. The largest concentrations were offshelf at Sta. 1 in fall. Not all of the sea ice melted back during fall, allowing a significant biomass accumulation of sea ice biota, especially in the southern regions near George VI Sound. New sea ice also formed earlier and covered much of Marguerite Bay by the end of the cruise in late May. Juvenile and adult krill were very abundant in the back bays and fjords of Marguerite Bay and especially in Matha Strait (Crystal Sound), north of Marguerite Bay. Molting and growth rates of krill were similar to rates measured in 2001.
During winter larvae continued to be scarce, but were relatively more abundant at the northern stations on the shelf west of Adelaide Island. In contrast to 2001, larvae were less frequently observed on the undersurface of sea ice and were more common in the upper 10-30 m of the water column, and juvenile and adult krill formed large layers in some areas. Larval krill were food limited again this year and on average showed zero growth. Despite sea ice biota accumulating in ice during fall and new ice forming earlier this year, chlorophyll concentrations at the ice-water interface were relatively low and similar to that in 2001. Several alternative food sources may have supported the population at a maintenance level, including microzooplankton, benthic larvae, and detritus.
Ctenophores, which prey on larval krill, were again relatively common in the study area. Ctenophore and larval krill abundances from ROV images and net tows are being analyzed. In addition, gut contents of about 150 individuals and results of digestion and respiration experiments will be assess in relation to their predatory impact.