Feeding and energy budgets of Antarctic krill Euphausia superba at the onset of winter—II. Juveniles and adults

Abstract

The overwintering success of Euphausia superba is a key factor that dictates population size, but there is uncertainty over how they cope with the scarcity of pelagic food. Both nonfeeding strategies (reduced metabolism, lipid use, or shrinkage in size) and switching to other foods (carnivory, ice algae, or detritus) have been suggested. We examined these alternatives in the southwest Lazarev Sea in autumn (April 1999), when sea ice was forming and phytoplankton was at winter concentrations. Both juveniles and adults had a very high lipid content (36% and 44% of dry mass, respectively) of which Ͼ40% was phospholipid. However, their low O : N ratios suggested that these reserves were not being used. Results from gut contents analysis and large volume incubations agreed that juveniles fed mainly on phytoplankton and adults fed on small (Ͻ3 mm) copepods. This dietary difference was supported possibly by elevated concentrations of 20 : 1 and 22 : 1 fatty acids in the adults. The feeding methods also confirmed that feeding rates were low compared with those in summer. Even when acclimated to high food concentrations, clearance and ingestion rates were Ͻ30% of summer rates. Respiration and ammonium excretion rates of freshly caught krill were 60%-80% of those in summer and declined significantly during 18 d of starvation. These findings suggest both switch feeding and energy conservation strategies, with a trend of reduced and more carnivorous feeding with ontogeny. This points to a ''compromise'' strategy for postlarvae, but there are alternative explanations. First, the krill may have reduced their feeding in an autumn transition to a nonfeeding mode, and, second, some of the population may have maintained a high feeding effort whereas the remainder was not feeding.

The overwintering of Antarctic krill, Euphausia superba, from an ecophysiological perspective

“…Oxygen uptake rates of krill around the Antarctic in midlate winter have been shown to be only 30-40% of summer rates (Kawaguchi et al 1986;Quetin and Ross 1991;Torres et al 1994a;Atkinson et al 2002;Meyer et al 2010). Additional evidence of metabolic depression in winter krill was the signiWcantly lower activity of the key metabolic enzyme citrate synthase (Meyer et al 2002a) and malate dehydrogenase (Donnelly et al 2004).…”

“…However, protein metabolism may play an important role in summer. In diVerent regions of the Southern Ocean, krill showed a low O:N ratio of <15 during the feeding season from spring to autumn (Ikeda and Mitchell 1982;Atkinson et al 2002;Meyer et al 2010), suggesting that lipids are being accumulated for utilisation during winter rather than used for energy turnover, which might be covered by protein metabolism.…”

“…Completely depleted body lipids at the start of the reproductive season would aVect the onset and maintenance of reproduction in krill and hence their reproductive success (Clarke and Morris 1983;Cuzin-Roudy and Labat 1992;Quetin et al 1994). The development of external maturation characteristics during winter, the Wrst step in the reproduction cycle, seems to be fuelled preferentially by lipid reserves , highlighting the importance of a suYcient accumulation of lipids at the commencement of Lütz-Holm Bay, (Kawaguchi et al 1986) Lazarev Sea, (Atkinson et al 2002, Meyer et al 2010 winter. The amount of energy reserves at the onset of winter depends on the quantity and quality of phytoplankton during the preceding summer and autumn (Hagen et al 2001).…”

“…2b). In addition, freshly caught krill feed on natural food assemblages in autumn and winter are unable to respond to high food concentrations despite exposure to abundant food for almost 2 weeks (Atkinson et al 2002;Meyer et al 2010). The maximum feeding activity of krill was only 20% (autumn) and 14% (winter) of summer rates (Meyer et al 2010).…”

Life in the sea of plenty: Seasonal and regional comparison of physiological performance of Euphausia hanseni in the northern Benguela upwelling system

“…In general, M. norvegica preferentially displays long-term adaptations to prevailing environmental conditions rather than short-term adjustments of metabolic rates. In E. superba, respiration rates may be reduced by up to 60% during austral autumn and winter, the periods of limited food supply (Atkinson et al, 2002;Meyer et al, 2010;Torres et al, 1994). Reduced metabolic activity accompanied by reduced feeding and growth are considered adaptations to austral winter conditions in Antarctic waters.…”