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    Trophic study of Ross Sea Antarctic toothfish (Dissostichus mawsoni) using carbon and nitrogen stable isotopes

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    S.J. Bury, M.H. Pinkerton, D.R. Thompson, S. Hanchet, J. Brown and I. Vorster (New Zealand)
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    This report amalgamates stable isotope analyses of fish (n=476), squid (n=50) and octopod (n=17) samples obtained from long-line fishing vessels from four CCAMLR SSRUs (88.1C, 88.1H, 88.1I and 88.1J) during two fishing seasons 2005/6 and 2006/7. The species sampled were: 6 fish: Antarctic toothfish (Dissostichus mawsoni, n= 100), Patagonian toothfish (Dissostichus eleginoides, n=8), deep sea cod/blue antimora (Antimora rostrata, n=103), icefish (Chionobathyscus dewitti, n=83), moray (or eel) cod (Muraenolepis microps, n=75), and Whitson’s grenadier (Macrourus whitsoni, n=107); 4 squid: Galiteuthis glacialis (Gg, n=3), Kondakovia longimana (Kl, n=20), Psychroteuthis glacialis (Pg, n=20) and the Colossal squid, Mesonychoteuthis hamiltoni (Mh, n=7); and 3 benthic octopods: Octopodid sp. 1 (Oct-1, n=3), Octopodid sp. 2 (Oct-2, n=5) and Cirroctopus glacialis (Cg, n=9). Length and SSRU were the most significant variables in explaining the variation of δ15N and δ13C. Positive relationships between length and δ15N indicate that, very generally, larger fish consume prey of a higher trophic level than smaller fish. There were substantial residual within-species variations in δ15N and δ13C. Dissostichus mawsoni exhibited a range of 7 ‰ (9–16 ‰) in δ15N, which is equivalent to two trophic steps. All fish, except Antimora rostrata (2.7 ‰ range) showed a d15N range greater than 3.4 ‰ spanning more than one trophic step. This implies that the diet of all species sampled was variable, or that individual species were eating a similar diet which itself varied in size and trophic status. Overall, Dissostichus mawsoni and Dissostichus eleginoides occupied a trophic level equivalent to orca (Orcinus orca) and Weddell seals (Leptonychotes weddellii). Antimora rostrata, Muraenolepis microps and Macrourus whitsoni all occupied a trophic level below them. Chionobathyscus dewitti occupied the lowest trophic level of all fish analysed. There was considerable isotopic overlap in both δ15N and δ13C for all four fish prey species. Squids, excluding Mesonychoteuthis hamiltoni were found to be at a lower trophic level than fish species sampled, whereas on average octopods occupied a similar trophic level to the four fish prey species. The squid δ13C signature was more depleted (indicating a pelagic signature) than the octopods, which were all benthic feeders. Large variations in d13C for each species (around 3 ‰ for each species) indicated a variation in source of carbon within individual species. Species with enriched d13C may be feeding further north in warmer waters or may have a stronger benthic compared to pelagic source of carbon. There was no significant difference in Dissostichus mawsoni δ15N and δ13C values between the Northern Area, Ross Sea Slope and Terra Nova Bay Trench. In contrast, all of the four potential prey species caught in the Northern Area had enriched 13C values compared to the Ross Sea Slope, most likely due to warmer temperatures to the north. Since this increased δ13C signature is not picked up by Dissostichus mawsoni, then this suggests that Dissostichus mawsoni either move between and feed equally within the Northern Area and the Ross Sea Slope, or that they predominantly feed on the Ross Sea Slope.