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 degrees 77 degrees S; 165 degrees 180 degrees E) in January to March 2008. Pleuragramma antarcticum (50236 mm standard length, LS) and prey items were analysed for stable-isotopic composition of carbon and nitrogen. According to index of relative importance (IRI), which incorporates frequency of occurrence, mass and number of prey items, the most important prey items were copepods (81%IRI over all specimens), predominantly Metridia gerlachei and Paraeuchaeta sp., with krill and fishes having low IRI (2.2 and 5.6%IRI 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 IRI) and stepwise generalized linear modelling on stable isotopes showed that LS and location were significant predictors of P. antarcticum diet. Postlarval P. antarcticum (5089 mm LS) consumed exclusively copepods. Juvenile P. antarcticum (90151 mm LS) consumed predominantly krill and copepods by mass (46 and 30%M, respectively). Small adult P. antarcticum (152178 mm LS) consumed krill, fishes and copepods (37, 36 and 15%M, respectively). Large adult P. antarcticum (179236 mm LS) 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 d15N was demonstrated. Pleuragramma antarcticum trophic level was estimated as 3.7 (postlarval fish) and 4.1 (fish aged 3+ years).