Document [ to be completed by the Secretariat ] WG-EMM-08/27 Date submitted [ to be completed by the Secretariat ] 4 July 2008 Language [ to be completed by the Secretariat ] Original: English Agenda Agenda Item No(s): 7 Title Trophic study of Ross Sea Antarctic toothfish (Dissostichus mawsoni) using carbon and nitrogen stable isotopes Author(s) S.J. Bury1, M.H. Pinkerton1, D.R. Thompson1, S. Hanchet2, J. Brown1, and I. Vorster1 Affiliation(s) 1 National Institute of Water and Atmospheric Research Ltd Email: [email protected] 2 NIWA, PO Box 893, 217 Akersten Street, Nelson, New Zealand Published or accepted for Yes No ü publication elsewhere? ABSTRACT 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. SUMMARY OF FINDINGS AS RELATED TO NOMINATED AGENDA ITEMS Fishing for Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea is likely to affect 7 prey species through trophic linkages. Stable isotope data can help to elucidate these relationships. 7 Dissostichus mawsoni and Dissostichus eleginoides occupy a trophic level equivalent to orca (Orcinus orca) and Weddell seals (Leptonychotes weddellii). WG-FSA Spatial homogeneity in Dissostichus mawsoni δ15N and δ13C values between the Northern Area, Ross Sea Slope and Terra Nova Bay Trench contrasted with enriched 13C values for all four potential prey species only in the Northern Area 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. This paper is presented for consideration by CCAMLR and may contain unpublished data, analyses, and/or conclusions subject to change. Data in this paper shall not be cited or used for purposes other than the work of the CCAMLR Commission, Scientific Committee or their subsidiary bodies without the permission of the originators and/or owners of the data. 1. INTRODUCTION Antarctic toothfish (Dissostichus mawsoni) are the major finfish resource currently exploited in the Ross Sea, with an exploratory fishery operating since 1996/7 in the Convention for the Conservation of Antarctic Marine Living Resources (CCAMLR) sub-area 88.1 (the western Ross Sea) conducted by independently observed commercial longliners (Hanchet et al. 2006, 2007; Ministry of Fisheries, 2006). Information on the trophic links between Dissostichus mawsoni and both their prey and predators is currently fragmented and a more comprehensive picture is needed to assess the potential impact of the fishery on associated species (e.g. Pinkerton et al. 2006, 2007c, d). The overall aim of this study was to investigate whether stable isotope analysis of muscle tissue samples supports the hypothesis that demersal fish dominate the diet of Dissostichus mawsoni caught in the commercial fishery of the Ross Sea. We present stable isotope data from 476 fish, 50 squid and 17 octopod samples obtained from long-line fishing vessels from four CCAMLR Small Scale Research Units (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), Whitson’s grenadier (Macrourus whitsoni, n=75); 4 squid species: Galiteuthis glacialis (Gg, n=3), Kondakovia longimana (Kl, n=20), Psychroteuthis glacialis (Pg, n=20) and colossal squid, Mesonychoteuthis hamiltoni (Mh, n=7); and 3 benthic octopod species: Octopodid sp. 1 (Oct-1, n=3), Octopodid sp. 2 (Oct-2, n=5) and Cirroctopus glacialis (Cg, n=9). In total the report integrates 543 carbon and nitrogen stable isotope analyses which collectively represent 476 fish, 50 squid, and 17 octopod analyses. 1.1 Distributions and biology of Dissostichus mawsoni Dissostichus mawsoni are endemic to the Antarctic seas, and like their common prey species Chionobathyscus dewitti, Macrourus whitsoni and Muraenolepis microps, they are generally found in higher latitudes south of the Antarctic Convergence (Gon & Heemstra, 1990). In contrast, Dissostichus eleginoides are widespread throughout the southern ocean, typically extending north from the Antarctic Convergence into subantarctic waters of the Atlantic, Pacific, and Indian Oceans, whilst Antimora rostrata are even more widespread extending north to the equator in abyssal depths (Gon & Heemstra, 1990). All fish species sampled in this report are circumpolar in distribution and are known to overlap in the areas immediately to the south and north of the Antarctic Convergence, especially in the area to the north of the Ross Sea (Hanchet et al. 2003). A wide variation in fishing effort and catch of Dissostichus mawsoni has been noted between years due to annual variations in sea-ice distribution, improved knowledge of fishing grounds, and SSRU catch limits (Hanchet et al., 2007). The variability in area and depth of fishing between seasons has a large influence on length and age composition of Dissostichus mawsoni caught each year. Details of the age and length of maturity and the timing of the spawning cycle are poorly known because the areas is covered by ice for at least 6 months of the year (Sullivan et al., 2005). Dissostichus mawsoni are primarily a demersal species, but adults are neutrally buoyant and are known to migrate in the pelagic zone over deep water. They grow rapidly, reaching about 60 cm total length after five years and about 100 cm after ten years. After about 25 years, growth slows down at a length of about 150 cm. The maximum recorded age is 48 years and maximum length is 250 cm (Sullivan et al., 2005). Analyses of fish caught over the last ten years in the Ross Sea showed that generally smaller fish were caught from the southern more shallow regions, compared to deeper or more northerly locations (Dunn & Hanchet, 2007). From 1997 to 2007 the majority of Dissostichus mawsoni have been caught on the Slope (72%) compared to the northern 2 Ross Sea (22%) and on the Shelf (7%: Dunn & Hanchet, 2007). Smaller fish tend to be found in the shallower inshore regions, compared to the deeper offshore regions where only larger fish are caught (Philipps et al., 2005). Dunn et al. (2006) reported that Dissostichus mawsoni longer than 120 cm caught in the northern areas of the Ross Sea had a higher mean age-at-length than those caught on the Shelf or the Slope, i.e. large fish in the north had on average experienced a slower growth rate than similar sized fish on the slope. They suggested that the apparent differences between areas in mean age-at-length may be related to a trade-off between somatic growth and reproductive productivity.