Diet and trophic niche of Macrourus spp. (Gadiformes, Macrouridae) in the Ross Sea region of the Southern Ocean

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.