CCAMLR

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.

We report on the development of a mass balanced carbon-budget trophic model of the Ross Sea with which to investigate effects of the fishery for Antarctic toothfish (Dissostichus mawsoni). A survey of the available literature provided an initial set of parameters describing the abundance (seasonal and spatial where possible), energetics (growth, reproduction, consumption), and trophic linkages (diets, predators) for major groups of biota. We also estimated the level of uncertainty on these parameters. The Ross Sea is a low primary production system, with high seasonal, spatial and interannual variability in primary production. In the relative absence of krill, Antarctic silverfish (Pleuragramma antarctica) and mesozooplankton (mainly copepods) are probably the major middle-trophic level link between primary production and the larger predators, though the role of cepahlopods in the system is poorly known. A number of demersal fish species (including Macrourus whitsoni, Bathyraja eatonii, Chionobathyscus dewitti, Antimora rostrata Chionodraco hamatus) are present, but their abundances and feeding characteristics are not well known. Toothed and baleen whales visit the Ross Sea in summer in relatively large numbers. Adelie and emperor penguins have breeding colonies along the Victoria Land coast, and petrels, skua and albatross are seasonal visitors. Weddell, crabeater, leopard and Ross seals are also present in summer, and some may stay in the region year-round. The trophic model was balanced by adjusting the initial set of parameters obtained from the literature and available datasets. We present a novel objective method of adjustment that takes into account estimates of parameter uncertainty, and appropriately handles the huge range of magnitude (>5 orders of magnitude) in trophic flows between different groups of organisms. Biomass, production rates, consumption rates and diet fractions are adjusted simultaneously. We set ecotrophic efficiency to unity for all non-primary producers. Changes to the initial set of parameters needed to obtain balance were significant: up to 62% (biomass, production, consumption), and 40% (diet fractions). The balanced model presented here has not yet been validated and should be considered a work in progress. The current version of the trophic model suggests that Antarctic toothfish have the potential to exert considerable predation pressure on some species of demersal fish. More information on demersal fish abundance is required to validate this result. Information on what the various species of demersal fish consume is needed to estimate the potential for trophic cascades due to the toothfish fishery. The significance of toothfish in the diets of predators (especially Weddell seal, type-C killer whale, sperm whale) are low in the model, but the model does not consider sub-populations of predators, or localized dependencies on toothfish as prey. More complete information on the abundances, diets, and population structures of top predators in the Ross Sea are needed to investigate these potential effects.

The authors look «inside» SSMU by the example of SOW (South Orkney West) and SGW (South Georgia West), analysing variability of krill transport and distribution characteristics in the local areas of these SSMUs. The data of the Russian repeated acoustic survey accompanied by trawl and CTD samples form the basis of this work. It is shown that the multiple replacements of the water masses in each study areas were accompanied with pulsatory pattern of krill transport, i.e. krill transported across polygon boundaries by non-uniform portions. Here not only the pulsatory pattern of the biomass value (in our case, CV biomass=57%) is important, but also the observed changes in krill aggregation patterns forming the transported biomass portions (in our case, CV swarms/mile 2 =63%). Therefore, krill biomass portions with different commercial importance were transported into the fishing grounds. The latter has been confirmed by the correspondent dynamic of the actual operational statistics of fishing fleet. The authors come to a determination that the development of krill stocks management procedures requires consideration of the temporal and spatial changes of krill transport factors in relation to dynamic of krill distribution indices and trawlers operation indices in various areas of the Scotia Sea. Simulation of such complex processes in ecosystem models (KPFM 2, SMOM, EPOC), developed for testing options of krill precautionary catch allocation among SSMUs, is possible only on the basis of the actual data describing annual and seasonal variability of krill biomass and aggregation characteristic distribution patterns in SSMUs under the impact of transport processes. Otherwise, the testing results will be insufficiently substantiated for the decision making.