UPDATE OF THE INTEGRATED STOCK ASSESSMENT FOR THE PATAGONIAN TOOTHFISH (DISSOSTICHUS ELEGINOIDES) FOR THE HEARD AND MCDONALD ISLANDS (DIVISION 58.5.2)

The integrated assessment of Patagonian toothfish, Dissostichus eleginoides, for the Heard and McDonald Islands (Division 58.5.2) was updated by replacing catch-at-length proportions from commercial catches with catch-at-age proportions by applying age length keys (ALKs) to gear- type/ground (i.e. sub-fishery) and year-specific length frequency (LF) data. A Poisson log-linear (contingency-type) analysis of the age-length frequency data suggested that ALKs could be pooled across sub-fisheries without significant loss of information. Pooling the ALKs in this way meant that all commercial catch data could be input to CASAL as catch-at-age proportions except for the 2009 catches. For 2006 and 2007 random stratified trawl surveys, ALKs were used to convert abundance-at-length to abundance-at-age data. Effective sample sizes for the commercial catch-at-age proportions, assuming a multinomial distribution, and the coefficient of variation (CV) for the abundance-at-age, assuming a lognormal distribution, each took into account uncertainty due to haul-level variability in catch-at-length proportions, ALK sampling error and random ageing error. CASAL allows a single ageing error matrix to be defined and applies this matrix to predictions of numbers-at-age and proportions-at-age. In other work, this matrix was found to depend on the readability score of the otoliths used for ageing, so the ageing error matrix was calculated as a weighted average of elements across nearest integer average readability scores for sub-fishery by year aged otoliths where the weights were the effective sample size for proportions-at-age. Compared to the assessment that did not incorporate catch-at-age or abundance-at-age data, the aged-based assessment dramatically lowered the CV for the recruitment series, from around 1.8 down to approximately 0.6. The other major differences to the a2-ess model of Candy and Constable (2007, CCAMLR Science 15, 1-34) were that catches and catch-at-age data for additional sub-fisheries (2 longline, 1 trawl) and pot fishing in the 2006 season were included since catches in these cases have become significant in recent years, and the CV of mean length-at-age was fixed at 0.1 and not estimated since the inclusion of age data has reduced the ability to successfully estimate this parameter. The long-term yield that satisfies the CCAMLR decision rules, using a split of the catch by sub-fishery corresponding to actual catches in 2008, was 2 565 t.