Population assessment of Patagonian toothfish in Subarea 48.4 – 2011 update

In this paper we update the assessment of D. eleginoides in the northern half of Subarea 48.4 presented in 2010. The CASAL model is improved through the incorporation catch at age data, as an alternative to the catch at length data that were the only such data available in 2010. The catch at age model generates a yield of 52 tonnes based on CCAMLR decision rules.
In 2006 the northern subdivision of 48.4 was set north of Saunders Island after consideration of oceanographic and bathymetric data, and for the purpose of the experimental work leading to a stock assessment. Data collected in the second part of the experiment (2009 – 2011) show that although CPUE of D. eleginoides is very low there, Saunders Island is an area of overlap between D. eleginoides and D. mawsoni, and is the likely effective southern limit of the D. eleginoides population. It should probably be incorporated into the assessment and management of D. eleginoides in the northern half of 48.4, but to date insufficient information has been acquired, including from tagging, to establish the likely rates of exchange between Saunders and the other northern areas in 48.4.
Given current knowledge and uncertainties on both Dissostichus spp stock status, we suggest that further data on the D. eleginoides population at Saunders island should be acquired, with fishing controlled by maintenance of current north-south area boundaries, application of a D. eleginoides 52 tonne catch limit to both areas with a sub-division of 5 tonnes allocated specifically for D. eleginoides in 48.4 South. We also propose that directed fishing on D. mawsoni should not be permitted in 2011/12, but that a move-on rule to limit bycatch of D. mawsoni should apply. Some options for a move-on rule are discussed.