CCAMLR

Both Macrourus caml and M. whitsoni are probably abundant and widespread in the Ross Sea region at depths between approximately 500 and 2100 m. We present the analysis of stomach contents (including regurgitated stomach contents) from an undifferentiated mixture of 33 specimens of these species (henceforth Macrourus spp.) in the Ross Sea region of the Southern Ocean. Samples of stomach contents were obtained from the Ross Sea slope (between about 71° and 73°S) and on the Scott Island and seamount chain. Overall, amphipods were the dominant prey found in Macrourus spp. stomachs with an index of relative prey importance (IRI) of 43%. About half the amphipods were Eurythenes gryllus and about half could not be identified. Based on stable isotope results, unidentified amphipods were probably herbivorous rather than carnivorous. Other important prey were copepods (especially for fish with total length less than 30 cm), krill (mainly Euphausia superba for larger fish) and fishes (Pleuragramma antarcticum and Gymnoscopelus opisthopterus). Minor prey found in stomachs included isopods, mysids, salps and polychaetes, with rocks and fragments of coral, echinoderm and shell also found. Stomach contents data provide evidence of both pelagic and benthic feeding. Carbon and nitrogen stable-isotope analysis of 161 Macrourus spp. muscle tissue samples from the Ross Sea slope, Admiralty and Scott seamounts and two parts of the Pacific-Antarctic fracture zone (near 65°S) are presented. Nitrogen isotope results (δ15N=10.6±1.0‰, mean±standard deviation) were generally consistent with stomach contents. Carbon isotope results (δ13C = -24.7±0.8‰) were consistent with Southern Ocean residence but higher than phytoplankton values (-29.9±1.2‰). We found that Macrourus spp. in the Ross Sea region have a trophic level of 3.8±0.3, placing them well below the top of the food chain. Trophic level of Macrourus spp. was significantly affected by location and by size of fish (especially for fish less than 30 cm), whereas sex, fatness and bottom depth were not significant.

Based on mass balance modeling and mixed trophic impact analysis, cephalopods have been identified as having relatively high importance in the food-web of the Ross Sea. However, information on the trophic ecology of the cephalopod assemblage of the Ross Sea region is poor. Stable isotope signatures of nitrogen (δ¹⁵N) and carbon (δ¹³C) were determined in muscle and beak tissue from a range of cephalopod taxa sampled from the Ross Sea region. Samples were acquired through scientific trawls and from stomachs of Antarctic toothfish Dissostichus mawsoni returned from the longline fishery. Most octopods were not identified to species which limits analysis. All squid samples were identified to species, but isotopic comparisons were made at the level of genus to accommodate the uncertainty in octopod identifications. Generally, squid clustered into two distinct isotopic groups, one with depleted isotope signatures typical of feeding on relatively low trophic level prey within the Ross Sea, and the second with elevated isotopic signatures typical of feeding on relatively high trophic level prey with some suggestion of either movement northwards out of the Ross Sea or of coupling with the benthic system. In contrast, octopod genera exhibited relatively elevated and diverse isotope signatures, typical of benthic feeding where nutrients tend to be recycled repeatedly. Beak isotope values were consistently depleted compared to those in muscle tissue, whereas carbon isotope values were similar in both beak and muscle tissues. Size (and hence age) was an important factor in explaining variation in both beak and muscle tissue isotope signatures in only one species – the colossal squid Mesonychoteuthis hamiltoni: both isotope signatures were positively and significantly correlated with beak size. The utility and value of applying stable isotope analyses to cephalopod taxa from the Ross Sea is discussed.
 

The diet of Antarctic silverfish (Pleuragramma antarcticum) was evaluated by examining stomach contents of specimens collected in the Ross Sea (71°–77°S, 165°E–180°) in January–March 2008. Silverfish (50–236 mm standard length, SL) and silverfish prey items were analysed for stable isotopic composition of carbon and nitrogen. According to index of relative importance (IRI), the most important prey items of silverfish were copepods (IRI 81% over all specimens), predominantly Metridia gerlachei and Paraeuchaeta sp., with krill and fish having low IRI (IRI 2.2% and 5.6% overall). However, according to weight of prey in stomachs, fish (silverfish and lanternfish) and krill dominated diet overall (48% and 22% respectively), with copepods being a relatively minor constituent of overall diet by weight (9.9%). Piscivory by silverfish occurred mainly in the extreme south-west of the region and near the continental slope. By weight, krill identified to species level in silverfish stomachs were predominantly Euphausia superba (14.1%) with some E. crystallophorias (4.8%). were identified in silverfish stomachs. Both distLM modelling (PRIMER-permanova+) on stomach contents (by IRI) and stepwise generalised linear modelling on stable isotopes showed that SL and location were significant predictors of silverfish diet. Larval/post-larval silverfish (standard length, SL<90 mm) consumed exclusively copepods. Large adult silverfish (SL>179 mm) consumed predominantly fish by weight, especially in the north (near the Ross Sea slope) and in the SW of the Ross Sea. Juvenile silverfish (SL 96-151 mm) consumed predominantly krill by weight. Amphipods were important prey items for large adult silverfish in the west of the Ross Sea. General concordance between stomach contents and trophic level of silverfish and prey based on δ¹⁵N was demonstrated. Silverfish trophic level was 3.5 (larval/post-larval fish) and 4.0 (fish aged 3+).