Feeding and energy budgets of Antarctic krill Euphausia superba at the onset of winter—I. Furcilia III larvae

Physiological condition and feeding behavior of furcilia larvae were investigated in autumn (April 1999) in the southwestern Lazarev Sea prior to the critical overwintering period. Furcilia stage III (FIII) larvae were most abundant, so only these were used for all analyses (dry mass [DM], elemental and biochemical composition, gut content) and experiments (metabolic and ingestion rates, selective feeding behavior). Chlorophyll a (Chl a) concentrations in the mixed layer were ,0.1 mg L21. Respiration rates of freshly caught FIII larvae were between 0.4 and 1.2 ml O2 mg21 DM h21, similar to larvae fed for 7 d on high food concentrations (4 mg Chl a L21). Excretion rates ranged between 0.01 and 0.02 mg NH4 mg21 DM h21. Their atomic O : N ratio of 72 indicated that lipids were the main metabolic substrate of FIII larvae in the field. The daily C ration ranged from 0.4% at the lowest food concentration of 3 mg C L21 to 28% at the highest enriched food concentration of 216 mg C L21, whereas clearance rates decreased with increasing food concentrations. In natural seawater, 115 ml mg21 C h21, and in natural seawater enriched with ice biota, 24 ml mg21 C h21, the clearance rates on specific phytoplankton taxa revealed no significant difference across a food size range of 12–220 mm. The study suggests that during periods of low food supply in the water column, larvae have to exploit ice biota to cover their metabolic demands. Antarctic krill (Euphausia superba, hereafter ‘‘krill’’) is a key species in the Antarctic ecosystem, being a major food item for a large number of top predators such as whales, seals, and sea birds. Krill are very successful in the extreme environment of the Southern Ocean because they are capable of exploiting a food supply that is both patchy and seasonal (Quetin and Ross 1991). However, various aspects of their biology are still poorly known. Most of the available infor1 Corresponding author ([email protected]).