Theoretical considerations for estimating the density of cohorts
and mean recruitment of Patagonian toothfish (Dissostichus eleginoides) based on research trawl survey data

The current method used by the Working Group on Fish Stock Assessment (WG-FSA) for assessing the long-term yield for toothfish fisheries in CCAMLR uses estimates of density of juvenile toothfish derived from research trawl surveys as an estimate of absolute abundance of recruitment to the fishery. At the 2003 meeting of WG-FSA a number of inconsistencies were identified in the estimation of the recruitment series for Subarea 48.3. In light of this, the Scientific Committee agreed that there was an urgent need to review and revise the recruitment series for D.eleginoides in Subarea 48.3, recognizing the importance of obtaining a consistent and reliable recruitment series for review at the 2004 meeting . This paper elaborates the issues surrounding how age information should be incorporated into CMIX analyses; which age groups should be included in the estimation of recruitment, and the need for a clear set of decision rules to guide those attempting CMIX analysis. It provides an overview of the theoretical issues involved with estimating cohort densities from length density data from trawl surveys with examples for the survey series for Subarea 48.3. A checklist of issues to be considered when undertaking CMIX analyses to estimate recruitment of toothfish is provided.