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    An update of the ageing program for Patagonian toothfish (Dissostichus eleginoides) at the Australian Antarctic Division, including a summary of new data available for the Integrated Stock Assessment for the Heard Island and the McDonald Islands fishery (Division 58.5.2)

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    Document Number:
    WG-FSA-14/45
    Author(s):
    B.M. Farmer, E.J. Woodcock and D.C. Welsford (Australia)
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    Approved By:
    Admin Admin
    Abstract

    The Australian Antarctic Division (AAD) has recently received funding to continue an ageing program for the purposes of routine processing of Patagonian toothfish (Dissostichus eleginoides) otoliths from the Heard Island and McDonald Islands fishery. With the reinitiation of this program a number of methodological refinements have been made to those outlined by Nowara et al. (2009), particularly in relation to the laboratory processing of otoliths and quality control in the assignment of fish age. These changes have contributed to increased efficiency within the program, with a greater capacity for ageing and at a lower cost per otolith. Greatest efficiency gains have been made in the laboratory, with otoliths now set in smaller resin blocks that take much less time to section, and in quality control and reporting procedures, which are now largely automated using R.

    A key part of quality assurance in any ageing program is to minimise the effect that a reader has in assigning ages.  In the AAD program, two readers look at all otoliths, which has proved useful in helping to identify  otoliths that may be  interpreted differently by different readers, so they can be flagged, re-read, and consensus reached on the most likely age estimate.  Using the otolith weight/age relationship has also been useful for identifying otoliths that may have been aged with good precision between the two readers, but may represent systematic error in interpretation (e.g. both readers may have counted all sub-banding for a particular fish).

    This year’s stock assessment includes new age data from the 2012, 2013 and 2014 RSTSs, as well as from the commercial fishery in 2013. The recent focus by AAD to age more large fish from commercial samples has substantially increased the information available on older age classes (i.e. fish greater than 20 years) available for use in the current assessment (Ziegler et al., 2014).