Annual estimates of SBT spawning stock abundance are required to assess the status and productivity of the stock. Prior to the original CKMR study, there was no direct index of abundance of the spawning population The majority of the difference in the estimates of spawning stock status between the 2011 and 2014 stock assessments was due to the influence of the CKMR data, in combination with the 1990’s conventional tagging data. This reduced the overall uncertainty in spawning biomass and natural mortality and, therefore, identified a number of unproductive (pessimistic) parameter combinations as implausible. These were removed from the reference set of OMs (Anon 2014)

The schedule of implementation of the CCSBT MP includes a full stock assessment is scheduled for 2017. As a result of decisions by CCSBT to discontinue the Aerial Survey and move to gene-tagging, the CCSBT will initiate development of a new MP in 2017. Candidate MPs will be tested (in 2018) and a final MP selected using the OMs reconditioned in 2017 as part of the full stock assessment. The aim of this FRDC project is to provide a time-series of absolute abundance estimates of spawning potential from 2002 to 2013 for direct use in the CCSBT OMs. This abundance trajectory of the spawning potential of the stock will be independent of CPUE and catch data from the main fisheries, and will be an important input to domestic and international consideration of the stock status and performance of the rebuilding plan.

If the CKMR information is not available for the 2017 assessment, it is highly unlikely to be incorporated for testing of Candidate MPs. This would mean the CCSBT MP used to recommend future global TACs would not be “calibrated” with the most recent information on the status and recent trend in the spawning stock.

Management of SBT is greatly complicated by large uncertainties in the stock assessment. One key parameter is absolute spawning stock size, for which the only available estimates are highly uncertain and are driven entirely by fishery-derived data (e.g. Japanese CPUE, catches on the spawning ground). Hence, the management procedures being developed for SBt use relative, rather than absolute, abundance indices. Current concerns about historical over-catch, of uncertain magnitude and duration make the conventional stock aseessment even less certain. A fishery independent estimate of standing stock biomass (SSB) provides both a stand alone benchmark to compare with current catches, and a fixed reference point around which to rebuild future assessments. This cannot be done with current fishery independent approaches, such as conventional tagging. However, recent advances in genetic and statistical methods now permit a fishery-independent estimate, using identification of parent-offspring pairs in random samples of juveniles and spawners. The same approach can also provide information on age-specific fecundity and thus on appropriate definition of SSB (spawning stock biomass). This is another area of significant uncertainty for management because, as noted above, the different definitions of SSB have considerably different implications for stock projections and rebuilding times.

An absolute estimate of spawning stock biomass is particularly valuable given the estimated level of depletion of the SBT stock, and the high uncertainty about the productivity of the stock (i.e. the relationship between the parent stock and recruitment). Although this project will initially aim to provide an estimate of average SSB over 2002-2005, it will ultimately provide the methods to enable a time-series of SSB to be estimated if sampling continues. Furthermore, the statistical methods developed and applied in this project will have general applicability to a range of species.