Review of the use of survey data and length-at-age models in the assessment of Dissostichus eleginoides in the vicinity of Heard Island and McDonald Islands (Division 58.5.2)

This paper reviews the allocation of historical survey data to the new strata and the implications of this review to estimating the abundance of juvenile fish using CMIX. In addition, the paper examines how the latest length-at-age model for Dissostichus eleginoides (Patagonian toothfish) in 58.5.2 could affect the estimates of juvenile abundance. Model estimates were best incorporating the reassignment of hauls and the latest estimates of length at age, but quality of model fit and estimates of abundance of specific cohorts were sensitive to the number of cohorts assumed to be represented in the length density data. Assuming survey design produces unbiased estimates of length density, we suggest that best estimates of cohort abundance will be achieved when mixture analysis is informed by accurate estimates of length at age, an independent estimate of relative abundance of cohorts present in any survey year such as could be derived through an age length key, and the selectivity of the gear used in the surveys.
The consequences to the 2005 assessment of yield of toothfish of changing the time series of abundance of juvenile fish and the length-at-age model were also examined. Yield estimates were found to be most sensitive to the estimated recruitment series interacting with the estimate of mortality and the cohort abundances from CMIX, and to a lesser extent estimated fishing selectivities. A scenario equivalent to that used to recommend yield in 58.5.2 for 2005/06, incorporating the latest estimates of length at age, selectivity and the mixtures estimated from the assignment of RSTS hauls, resulted in a significant increase in estimated yield.