Stable isotope analysis of Southern Ocean fish tissue samples to investigate trophic linkages of Antarctic toothfish (Dissostichus mawsoni)

Flesh tissue samples were collected by scientific observers on-board New Zealand fishing vessels during the 2005/06 season in the Ross Sea CCAMLR Subarea 88.1 in order to investigate trophic links between toothfish and demersal fish. Muscle samples were collected from: Antarctic toothfish TOA (Dissostichus mawsoni) n=142; Patagonian toothfish TOP (D. eleginoides) n=2; Whitson’s grenadier WGR (Macrourus whitsoni) n=107; icefish CHW (Chionobathyscus dewitti) n=48; blue antimora ANT (Antimora rostrata) n=103; moray cod MRL (Muraenolepididae) n=1. Samples were lipid extracted, and analysed to determine C and N stable isotope composition. Values of ?15N and ?13C suggested that a minimum of three trophic levels exist between icefish occupying the lowest trophic level, and Antarctic toothfish occupying the highest level. Some Antarctic toothfish sampled in this study occupied a similar trophic level according to their ?15N signatures to killer whales and Weddell seals in McMurdo Sound, bluefin tuna in the Atlantic, and sperm whales from the Gulf of Mexico. There was high variance in ?15N and ?13C values for each of the species sampled, on the order of 3-4 ‰ for ?15N (which equates to one trophic level) and 4 ‰ for ?13C (suggesting multiple primary sources of organic matter). For each species where sufficient data exist (TOA, ANT, CHW, WGR), a stepwise generalised linear model was used to identify significant relationships between the two dependent variables, ?15N and ?13C, and four variables: location (SSRU), fish length, sex, and depth. Location and a positive relationship with fish length were usually the only variables identified as significant. The isotope data agrees with previous work that the diet of the Antarctic toothfish varies with location, but the spatial patterns are not clear. Positive relationships between length and ?15N indicates that larger fish consume prey of a higher trophic level than smaller fish, which may be due to ontogentic changes in diet, and/or progressive consumption of larger individuals of the same species with age. There was significant residual variance in ?15N and ?13C values for each of the species sampled. Applying typical isotope fractionation factors for one trophic level (+0.4 for ?15N, and +3.4 for ?13C) allowed us to plot “prey polygons” for SSRUs 88.1C, 88.1H and 88.1I. Antarctic toothfish generally lay outside the prey polygons implying that the isotopic composition of tissue of the predator was not explained by the isotopic composition of prey sampled in that area. This may be due to: (1) variability in isotopic ratios within species and SSRU; (2) uncertainty in trophic fractionation (in both C and N) between trophic levels; (3) missing prey items (probably Antarctic silverfish, smaller fish species, crustaceans and squid); (4) movement since formation of the muscle (number of years). The work reported here is very much a preliminary analysis of the data. We plan further analysis of the data and further sampling (including more prey species, simultaneous stomach and stable isotope analysis, and muliple tissue sampling) to investigate these factors.