Search

A successful larval overwintering is a major factor determining population sizes of Antarctic krill, Euphausia superba (hereafter krill). A high population recruitment success of krill was linked to years with more extensive sea ice in the previous winter. However, the benefit of the winter sea ice cover for a successful larval development during winter remains unclear. The aim of the present study was to test the hypothesis that larval krill in pack ice regions are in better condition in terms of food supply and feeding activity than larvae from OW regions. Therefore, the condition of larval krill (furcilia (F) III–VI as well as juveniles) was investigated in open water (OW), the marginal ice zone (MIZ1+2) and the pack ice region (Ice Camp1+2) during late winter, in relation to Chlorophyll a (Chl a) and particulate organic carbon (POC) concentrations in the water column and in the sea ice. In addition, differences in the dietary intake of larval krill during the day and night were examined. Overall, krill larvae from pack ice regions were not in better condition than in OW. Although, krill larvae caught at Ice Camp1 had the largest body lengths (mean 15.69 mm) and highest dry weights (mean 4.59 mg), only the body lengths showed significant differences with larval krill from OW and stage composition was similar (few F6 and mostly juveniles). Larval krill of Ice Camp2 had the smallest body lengths (mean 7.96 mm) and dry weights (mean 0.50 mg) while stage composition ranged from F3-F6. The high amount of available food sources within the sea ice (Ice Camp1: 21.78 μg L-1 Chl a and 400.55 μg L-1 POC; Ice Camp2: 12.68 μg L-1 Chl a and 330.2 μg L-1 POC) was used only to a small extent by krill larvae, which is reflected by their lowest feeding activities and relatively empty stomachs. At Ice Camp2, larval krill was observed performing diel vertical migration (DVM). During the day larval krill was closely associated with the sea ice, whereas during the night they descended into the upper 20 m of the water column. Stomach content analyses showed that larval krill consumed diatoms and dinoflagellate primary during the day, while zooplankton and detritus predominated in the diet during the night. In pack ice regions where food abundance in the sea ice is patchy, the DVM could serve as a strategy to exploit potentially new feeding grounds.