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 from WG-FSA-09/20. The main change other than updating total removals by sub-fishery and updating catch-at-age and catch-at-length proportions was the inclusion of random stratified trawl survey (RSTS) abundance-at-age for 2010 and 2011 and conversion of 2008 and 2009 RSTS abundance-at-length to abundance-at-age. This was possible due to the large number of otoliths collected and aged for each annual RSTS since 2008. Catch-at-length proportions for the commercial fisheries, both trawl and longline, were used for 2009 to 2011 since there were few fished aged for these years. A series of standardised longline CPUE data from one of the longline grounds (or subfishery) which has been consistently fished since 2003 was added for the first time. All three of the standardised CPUE series have minimal influence on the model parameter estimates due to the large degree of uncertainty about annual estimates but given this caveat they are useful in tracking observed and CASAL-estimated trends in catch rates.

A change of substantial consequence from the 2009 integrated assessment was the use of a value for natural mortality (M) of 0.155 whereas a value of 0.13 was used previously. Neither of these values of M was estimated in CASAL but the value of 0.155 was estimated externally to CASAL from catch-at-age and aged mark-recapture data as described in Candy et al. (2011). The consequence of using a higher M in the integrated assessment was a reduction in the estimate of B0 with a compensatory increase in R0. Using the same data as the 2009 assessment the reduction in B0 was 33.1% and the increase in R0 was 23.5%. For the updated assessment with M set to 0.155 the corresponding percentage change relative to the 2009 assessment were 25.8% and 36.9%, respectively. Since the trawl fishery, and to a lesser degree, the longline fishery catch relatively few fish that are at an age to begin spawning or older (i.e. mostly above age 14), the higher recruitment despite the higher M meant that a similar but slightly higher long-term yield was estimated for the updated assessment to that in 2009.

For the updated assessment, the long-term yield that satisfies the CCAMLR decision rules is 2 730 t.