Document [ to be completed by the Secretariat ] WG-EMM-08/43 Date submitted [ to be completed by the Secretariat ] 7 July 2008 Language [ to be completed by the Secretariat ] Original: English Agenda Agenda Item No(s): 7 Title TROPHIC OVERLAP OF WEDDELL SEALS (LEPTONYCHOTES WEDDELLI) AND ANTARCTIC TOOTHFISH (DISSOSTICHUS MAWSONI) IN THE ROSS SEA, ANTARCTICA Author(s) M.H. Pinkerton1, A. Dunn1, S.M. Hanchet2 Affiliation(s) 1 National Institute of Water and Atmospheric Research Ltd (NIWA), Private Bag 14901, Wellington, New Zealand. Email: [email protected] Telephone: +64 4 386 0369 Fax: +64 4 386 2153 2 NIWA, PO Box 893, 217 Akersten Street, Nelson, New Zealand Published or accepted for Yes No 9 publication elsewhere? If published, give details ABSTRACT We present information to investigate the significance of Antarctic toothfish as a prey item for Weddell seals in the Ross Sea. • We summarise the life history of Weddell seals to provide an overview of their use of the Ross Sea. As consumption of prey by Weddell seals (both the amount and type of prey) will vary between different life history stages at different times of the year in different areas, this is relevant to the question of whether seals predate significantly on toothfish. • There is evidence that Antarctic toothfish have lower densities near to seal breeding colonies in McMurdo Sound than further away (Testa et al. 1985). • Direct information on diet of the Weddell seals, including diver observations, animal-mounted camera information, and observations from field scientists in the McMurdo Sound region suggest that toothfish are a significant prey item for Weddell seals. • In contrast, research using seal stomach contents, vomit and scats provides no evidence that Weddell seals consume toothfish at all. Diver observations suggest that seals may feed selectively on only parts of toothfish so that otoliths and vertebrae may be under-represented in remains. • Indirect information using stable isotopes of carbon and nitrogen, even including recent analyses that have not been previously reported, remains inconclusive. We recommend further research using stable isotope analysis of blood samples from seals not at the breeding colonies, and samples of muscle or other slower-turnover tissue of seals at the breeding colonies. • Information from fatty acids or other biomarkers could potentially be used to investigate the importance of toothfish as a prey item for seals, but no results are available. • We have compared mortality of Antarctic toothfish in McMurdo Sound to consumption by Weddell seals. The estimates, although preliminary and subject to uncertainty, indicate that it is possible that toothfish comprise a substantial proportion of the diet of seals in McMurdo Sound between October and January. We conclude that while there is strong evidence that toothfish are a prey item for Weddell seals in McMurdo Sound between October and January, it is plausible but unproven that they are an important prey item. SUMMARY OF FINDINGS AS RELATED TO NOMINATED AGENDA ITEMS Agenda Item Findings We conclude that while there is strong evidence that toothfish are a prey 7 item for Weddell seals in McMurdo Sound between October and January, it is plausible but unproven that they are an important prey item. 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 1 INTRODUCTION Fisheries not only impact the target species, but also other parts of the ecosystem, both directly from by-catches, and indirectly by altering the species composition and inter-specific relationships within the ecosystem. There is presently debate over the degree to which the fishery for Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea may adversely affect the ecological viability of Weddell seals (Leptonychotes weddelli) which breed in the vicinity of McMurdo Sound in the late spring and early summer (e.g. Ainley et al. 2006; Ponganis & Stockard 2007). The fishery for Antarctic toothfish in the Ross Sea may affect Weddell seals by a number of means, including: (1) by reducing the availability of Antarctic toothfish for the Weddell seals to eat. This will be addressed by the present study where we consider evidence on the extent to which the fishery may affect Weddell seals through direct trophic overlap. (2) by affecting the ecosystem and/or habitat so that other prey items of Weddell seals are changed in abundance. It is likely that the indirect effects on the availability of other prey of Weddell seals due to the toothfish fishery will be less pronounced than the effect of direct depletion of toothfish itself if toothfish are an important prey item. It is hence important first to determine direct toothfish-seal trophic overlap. Further consideration of second-order ecosystem effects (such as the impact of the fishery on other prey items of Weddell seals) will be considered using approaches similar to the whole-ecosystem trophic model currently under ongoing development at NIWA (Pinkerton et al. 2007). (3) by affecting the ecosystem and/or habitat so that predators of Weddell seals are changed in abundance. Weddell seals have few predators in the Ross Sea so it is unlikely that the toothfish fishery will affect this to any significant extent. (4) by direct disturbance of the seals. The degree to which the fishery may disturb Weddell seals in the Ross Sea (e.g due to noise, human activity, pollution etc) is unlikely to be severe, as most fishing takes place well away from the haul out areas of the seals. No information is available to assess this issue at the present time. (5) by causing death/injury to seals during fishing itself or afterwards due to lost fishing gear. No direct mortality of Weddell seals by the toothfish fishery in the Ross Sea has been reported. As the fishery is entirely based on long-lining, there are no nets for entanglement, and depredation by Weddell seals on fish on long-lines (which could plausibly lead to death on some occasions) has not been reported over 10 years during which ~40% of all hooks hauled have been directly observed. The impact of lost fishing gear on Weddell seals in the Ross Sea is unknown. Lost fishing gear that remains on the sea-bed is unlikely to affect Weddell seals significantly because fishing takes place in waters >350 m: too deep for Weddell seals to be foraging on the bottom. It is likely therefore that the main potential impact of the fishery on Weddell seals in the Ross Sea is that toothfish become locally less available as a prey item to seals due to depletion by the fishery. The remainder of this paper addresses the question of whether it is plausible that fishing Antarctic toothfish in the Ross Sea under the current management approach could adversely impact Weddell seals. We discuss a number of pieces of evidence to quantify the importance of Antarctic toothfish as a prey item for Weddell seals in the Ross Sea. We summarise the life history of Weddell seals with particular reference to the McMurdo Sound region. Because consumption of prey by Weddell seals (both the amount and type of prey) will vary between stages of seals at different times of the year in different areas, the life history of Weddell seals in the Ross Sea is important. Next we consider direct information on diet of the Weddell seals. This includes information from diver observations in the 2 Ross Sea, animal-mounted camera information, stomach contents analysis, and investigation of sea scat remains. We then consider indirect information on Weddell seal diet, including recent information from stable isotope analysis of fish species in the Ross Sea that has not been previously reported. Finally, we compare Antarctic toothfish production near McMurdo Sound with Weddell seals consumption in the same region. This involves estimating toothfish abundance and mortality in the region, and estimating consumption of seals. Figure 1. Subareas 88.1, 88.2A and 88.2B showing the McMurdo Sound region (red). 2 Weddell seal biology and life history Weddell seals (Leptonychotes weddelli) are widespread through the Southern Ocean and occur in large numbers on fast ice, right up to the Antarctic continent, and offshore in the pack ice zone north to the Antarctic Convergence (Kooyman 1981). Weddell seals occur throughout the Ross Sea, forming breeding colonies (several hundred animals) along the coast of Victoria Land and Ross Island (Ainley 1985; Testa & Siniff 1987). Weddell seals breeding in the southern Ross Sea have been extensively studied. In McMurdo Sound, they have been studied for over 30 years (e.g. Smith 1965; Stirling 1969; Testa 1987; Schreer & Testa 1992; Burns et al. 1998, 1999; Testa & Siniff 1987; Stewart et al. 2003). An intensive study of a breeding population of Weddell seals in the Erebus Bay region of eastern McMurdo Sound started in 1968 and is ongoing (e.g. Garrott & Rotella 2008; Siniff et al. 2008; Proffitt et al. 2007). Over the 38- years of this study, over 17 000 animals have been tagged, with emphasis on maintaining and enhancing annual demographic data through the use of mark-recapture techniques (Garrott & Rotella 2008). 3 Seals begin to arrive at the breeding colonies from late September, with pups born in October. Non- breeding adults are excluded from the colonies by aggressive territorial behaviour of adult males and females with pups. Pups dive and swim within 2 weeks, and are weaned at 6 weeks. Mating occur around mid-December. During this period (October to December), breeding adults and nursing pups remain inshore (Testa et al 1985; Burns & Kooyman 2001).