This paper presents the Bayesian sex and age structured population stock assessment of Antarctic toothfish (Dissostichus mawsoni) in Subarea 88.2 covering SSRUs 88.2C, D, E, F, G, and H (SSRUs 88.2C–H). We examined several models including the 2013 implementation of the 2011 base case (R1), using the revised data selection method (R2), using a logistic selectivity for the northern fishery (R3), down-weighting the age data (R4), and having annual age length keys in the north (R5).
The stock assessment has strongly conflicting data: age frequency data push the biomass high, data from tags released in 2010, 2011 and 2012 suggest a much lower biomass, and data from tags released from 2004 to 2009 suggest an intermediate biomass.
The two models (R1 and R2) which estimate a higher biomass are those where the age data were more influential. However, these age data are not reliable because a single age length key has been applied. When annual age-length keys are applied there was a strong shift towards younger ages (R5) and lower biomass. The remaining two models (R4 and R5) suggest a substantially lower biomass than previously estimated, mainly due to the signal in the last three years of tag data. These are the data in which we have the highest degree of confidence for this fishery, due to improvements in tagging protocols and data collection.
We believe that there is sufficient conflicting information, and uncertainty in the age data, to warrant a decision to adopt the model which down-weights the age data in favour of the tag data as the base case. We therefore recommend that model R4 is used for providing management advice. In this model, the initial stock biomass is estimated at 6590 t (95% confidence interval 4800 – 9190 t), stock status is at 65%Bo (CI 52%–75%) and the yield based on the CCAMLR Decision Rules is 266 t. We also recommend that further ageing is carried out by fishing members involved in the fishery to allow annual full age length keys for all fisheries for future assessments.