The characteristics of krill swarms in relation to aggregating Antarctic blue whales.

We model the presence of rare Antarctic blue whales (Balaenoptera musculus intermedia) in relation to the swarm characteristics of their main prey species, Antarctic krill (Euphausia superba). A combination of visual observations and recent advances in passive acoustic technology were used to locate Antarctic blue whales, whilst simultaneously using active underwater acoustics to characterise the distribution, size, depth, composition and density of krill swarms. Krill swarm characteristics and blue whale presence were examined at a range of spatiotemporal scales to investigate sub meso-scale (i.e., <100 km) foraging behaviour. Results suggest that at all scales, Antarctic blue whales are more likely to be detected within the vicinity of krill swarms with a higher density of krill, those found shallower in the water column, and those of greater vertical height. These findings support hypotheses that as lunge-feeders of extreme size, Antarctic blue whales target shallow, dense krill swarms to maximise their energy intake. As both Antarctic krill and blue whales play a key role in the Southern Ocean ecosystem, the nature of their predator-prey dynamics is an important consideration, not only for the recovery of this endangered species in a changing environment, but for the future management of Antarctic krill fisheries.

Diet and trophic niche of Antarctic silverfish Pleuragramma antarcticum in the Ross Sea, Antarctica.

The diet of Antarctic silverfish Pleuragramma antarcticum was evaluated by examining stomach contents of specimens collected in the Ross Sea (71°-77° S; 165°-180° E) in January to March 2008. Pleuragramma antarcticum (50-236 mm standard length, L(S)) and prey items were analysed for stable-isotopic composition of carbon and nitrogen. According to index of relative importance (I(RI) ), which incorporates frequency of occurrence, mass and number of prey items, the most important prey items were copepods (81%I(RI) over all specimens), predominantly Metridia gerlachei and Paraeuchaeta sp., with krill and fishes having low I(RI) (2·2 and 5·6%I(RI) overall). According to mass of prey (M) in stomachs, however, fishes (P. antarcticum and myctophids) and krill dominated overall diet (48 and 22%M, respectively), with copepods being a relatively minor constituent of overall diet by mass (9·9%M). Piscivory by P. antarcticum occurred mainly in the extreme south-west of the region and near the continental slope. Krill identified to species level in P. antarcticum stomachs were predominantly Euphausia superba (14·1%M) with some Euphausia crystallophorias (4·8%M). Both DistLM modelling (PRIMER-permanova+) on stomach contents (by I(RI)) and stepwise generalized linear modelling on stable isotopes showed that L(S) and location were significant predictors of P. antarcticum diet. Postlarval P. antarcticum (50-89 mm L(S)) consumed exclusively copepods. Juvenile P. antarcticum (90-151 mm L(S)) consumed predominantly krill and copepods by mass (46 and 30%M, respectively). Small adult P. antarcticum (152-178 mm L(S)) consumed krill, fishes and copepods (37, 36 and 15%M, respectively). Large adult P. antarcticum (179-236 mm L(S)) consumed predominantly fishes and krill (55 and 17%M, respectively), especially in the north (near the Ross Sea slope) and in the SW Ross Sea. Amphipods were occasionally important prey items for P. antarcticum (western Ross Sea, 39%M). General concordance between stomach contents and trophic level of P. antarcticum and prey based on δ(15) N was demonstrated. Pleuragramma antarcticum trophic level was estimated as 3·7 (postlarval fish) and 4·1 (fish aged 3+ years).