CCAMLR

Large-scale transport of seawater in ocean currents may generate spatially complex population structure through the advection of life stages of marine fish species. To test this, we compared the chemistry of otolith nuclei from Patagonian toothfish (Dissostichus eleginoides), presently managed as spatially discrete populations corresponding to fishery management areas along the Antarctic Circumpolar Current (ACC), which transports water eastward around the Southern Ocean. The chemistry of otolith nuclei, laid down during early life, differed significantly between fishing areas off South America and the Antarctic and between some Antarctic areas. We also found evidence of four groups of fish with different early life chemistry: one associated with South America and three Antarctic groups showing mixing consistent with advective transport along the ACC. These results suggest that toothfish populations are structured by their physical environment; population abundance and persistence may rely on a restricted number of breeding members with access to spawning grounds, whereas fisheries may rely substantially on nonbreeding vagrants transported from fishing areas upstream.

Pronounced environmental trends across fronts suggest that the otolith chemistry of oceanic fish can resolve zones on either side, promoting application to population questions at similar spatial scales. Trace and minor elements laid down immediately prior to capture – along the edges of otoliths from Patagonian toothfish Dissostichus eleginoides – discriminated frontal zones in the Antarctic Circumpolar Current in the Southwestern Atlantic Ocean. Mean values differentiated sampling areas by up to 2.6 standard deviations, suggesting: 1) otolith Mg/Ca enrichment related to fish activity around the Burdwood Bank; 2) Mn/Ca enrichment associated with South America; 3) Sr/Ca linked to the presence of Circumpolar Deep Water; and 4) Ba/Ca to nutrient production and mixing. In the Polar Frontal Zone, meanders or eddies may account for affinities with neighbouring sampling areas, bringing water from the Subantarctic and Antarctic Zones onto the North Scotia Ridge. Moreover, fish age showed a significant relationship with depth and improved cross-validation by 14%, giving 85% classification rates to South American and Antarctic regions, and 57 to 83% to areas along the Patagonian Shelf. These results indicate that otolith chemistry reflects hydrography, detecting oceanic gradients across the slope of continental shelves and between zones separated by strong trends like fronts.

Strong contrasts in ambient isotope ratios and in diet suggest stable isotopes in the otoliths of oceanic fish can resolve water masses and geographic areas, promising a powerful multivariate approach for examining population structure and provenance. To test this, whole otoliths were taken from Patagonian toothfish (Dissostichus eleginoides) sampled off the Patagonian Shelf and South Georgia, on either side of a population boundary, and otolith δ18O and δ13C values were measured to see if they could distinguish South American-caught fish from those taken in the Antarctic. Values of δ18O and δ13C predicted capture area with 100% success, validating their use for distinguishing provenance and corroborating the prior evidence of population isolation. Values of δ18O in the otoliths reflected ambient values as well as seawater temperature: low values in Patagonian Shelf fish were consistent with exposure to Antarctic Intermediate Water (AAIW), and high values in South Georgia fish were consistent with exposure to Circumpolar Deep Water (CDW). In contrast, differences in otolith δ13C appeared to reflect diet: relative depletion of δ13C at South Georgia compared to the Patagonian Shelf were most likely linked to differences in sources of metabolic carbon, as well as δ13C in dissolved inorganic carbon (DIC) of seawater. These contrasting properties strongly suggest that stable isotopes can resolve the provenance of toothfish from Antarctic sampling areas that hitherto have been difficult to separate. These results show that, by using the chemistry recorded in otoliths, researchers can exploit biogeochemical variation in fully marine environments to examine the spatial ecology of oceanic fish.

Recruitment variability in juvenile Patagonian toothfish (Dissostichus eleginoides), a commercially important, deepwater nototheniid fish, was examined at the sub-Antarctic island of South Georgia, South Atlantic. Data from 13 demersal trawl surveys conducted over a 20-year period were analysed. Abundance of the 1+ juvenile fish cohort (13 to 15 month old dependent on survey date) was found to vary inter-annually and was found to be inversely correlated with the sea surface temperature (SST) conditions experienced by adults prior to spawning. Environmental temperatures experienced by toothfish eggs and larvae were not significantly correlated with juvenile density. The mean length of 1+ fish attained after 13-15 months was higher in years of high juvenile abundance and was significantly inversely correlated with SST in the summer prior to adult spawning. Trends in toothfish recruitment variability mirrored those previously observed in a range of krill-dependent land-based predators at South Georgia for whom non-seasonal, large-scale climatic events such as El Niño Southern Oscillation (ENSO) are considered the most likely underlying drivers of variability in breeding success. The drivers of recruitment variability in toothfish are not fully understood but a range of possible mechanisms are considered. A better understanding of recruitment variability holds great interest for fisheries managers and could be used refine forecasts of years of good or poor recruitment for the toothfish fishery at South Georgia

The Antarctic toothfish (Dissostichus mawsoni) exhibits a circumpolar distribution in coastal waters south of the Antarctic Polar Front. For a preliminary evaluation of global population structure in this species, we examined four mitochondrial regions and 13 nuclear gene fragments in samples from four CCAMLR Subareas in the Southern Ocean (Australian Antarctic Territory (Subarea 58.4.2), Ross Dependency (Subareas 88.1 and 88.2) and the South Shetland Islands (Subarea 48.1). Significant genetic differentiation within and among locations was observed for both mitochondrial and nuclear loci. The single nucleotide polymorphism (SNP) markers developed here will be useful for more extensive analyses of population structure in this species.

This paper reports on the one-day workshop held at the CCAMLR Headquarters, Hobart, Australia on 10th October 2008 to review the future of the ad hoc WG-IMAF. The workshop briefly reviewed the history of ad hoc WG-IMAF and considered the current work programme of CCAMLR with respect to the current and future role of ad hoc WG-IMAF. The workshop briefly considered future scenarios under which it might operate and the implications for its work. The workshop agreed that in future ad hoc WG-IMAF should continue to focus on the direct impacts of fishing and marine debris on seabirds and marine mammals as well as developing, an effective relationship between CCAMLR and ACAP. The workshop noted that this scope was a part of the broader contribution of CCAMLR to the conservation of these taxa. The key recommendations from the workshop to be considered by WG-IMAF are: o revised terms of reference for WG-IMAF to reflect a purpose and revised scope of the group; o in the short-medium term WG-IMAF should place a particular focus on a reduction in incidental mortality of seabirds associated with fishing in those areas of the Convention Area where this still occurs; o that the current timing and duration of meetings are appropriate but should be kept under annual review; and o medium term functions for WG-IMAF to address as possible.

CCAMLR adopted a new conservation measure in 2007 (CM 22-06) to ensure that significant adverse impacts of bottom fishing gear on Vulnerable Marine Ecosystems (VMEs) are avoided. Due to the high levels of uncertainty surrounding both the evidence of VME presence and the consequences of interaction with different types of gear, a risk management framework is proposed, similar to that which has been used successfully by IMAF to minimise the effects of longline fishing mortality on seabirds. The aim of this risk management is to avoid significant adverse impacts on VMEs from bottom fishing activities. The framework consists of four steps: (1). Risk analysis of current and proposed fishing activities; evidence of potential VMEs; scale of interactions between fishing activities and VMEs; impact of interactions on VMEs; and recovery potential of VMEs. (2) Risk evaluation. Combine information on likelihood and consequences of interactions of bottom fishing gear with VMEs and associated uncertainties from risk analysis to produce risk metrics. (3) Risk elimination or mitigation. Unacceptable levels of risk from bottom fishing activities to VMEs must be eliminated or reduced to acceptable levels through the use of management measures including, inter alia, closed areas around identified VMEs, open and closed management areas, bycatch limits for VME-forming organisms, gear modification or spatial distribution of fishing effort. (4). Review. All of the above steps should be reviewed regularly to ensure that all relevant or new information has been included, appropriate scientific research and data collection plans are in plan and that risk mitigation measures are successful in their implementation.

Tag recapture rates in new and exploratory Dissostichus spp. fisheries in the southern Indian Ocean sector of the CCAMLR area are examined. In particular, the potential for tagging programs in new and exploratory fisheries to yield sufficient data to be of use in determining TAC’s in the early stages of fishery development is considered. Scenarios are developed using a range of tag release rates, tag detection rates, natural mortality, fish movement out of the fishery, and IUU removals in order to estimate the expected numbers of tag returns. Even under “worst case” assumptions, tag recaptures are still expected to be considerably higher than currently observed in Divisions 58.4.1 and 58.4.2. If current tag recapture rates continue, tag-based assessments of stock status 58.4.1 and 58.4.2 are likely to remain uncertain in the short to medium term, and fishing should remain focussed in areas where tag releases have been concentrated until these uncertainties can be addressed.