Aspects of krill growth and condition during late winter-early spring off East Antarctica (110–130°E)

Antarctic krill, Euphausia superba, is a keystone species in the Southern Ocean. However, information on growth, diet and condition during winter and early spring is limited, hampering our understanding of these fundamental biological parameters at this important period in their lifecycle. Our study assessed diet and condition of larval and postlarval krill collected from open water and below the ice off East Antarctica (110–130ºE) in September/October 2007. Condition was assessed using lipid content, growth rates and digestive gland size; feeding history was assessed using fatty acid profiles and stomach content analysis; and a 207-day starvation study investigated the response of krill to long-term food deprivation. Potential food items (Calanus propinquus and sea-ice biota) were analysed for lipid and fatty acid composition to compare with krill samples. Krill were found to be in good condition, with mean growth rate of 0.95% per moult for postlarvae and 14.79% for larvae, and mean lipid content of 24.1% for postlarvae and 6.6% for larvae. Fatty acid profiles and stomach content analysis revealed two main feeding strategies - krill below the ice were feeding mostly on sea-ice diatoms, while those in open water were ingesting copepods and detritus. Krill below the ice had larger digestive glands than those in open water. Furciliae fatty acid profiles indicated a diet of heterotrophic flagellates and/or detritus. Postlarval krill survived 207 days of food deprivation by using body protein and lipid reserves for energy. In contrast, krill furciliae were severely depleted after just 5 days of food deprivation, indicating that they must feed continually at this time of year. Krill, copepods and sea-ice biota were all low in polyunsaturated fatty acids, indicating that krill must rely on later spring phytoplankton blooms to obtain these essential nutrients required for reproduction.