Modifications to the Generalised Yield Model in 2004, version GYM501E.EXE

The Generalised Yield Model is used by the WG-FSA in the assessments of Champsocephalus gunnari, Dissostichus eliginoides and Euphausia superba. Its current version is GYM501D.EXE. In the course of reviewing the methods for estimating the time series of recruitments of D. elegionoides for WG-FSA in 2004 an error in the calculation of the recruitment series from survey data was detected. This error does not affect the assessments undertaken in recent years. This paper presents the nature of the error and the manner in which it was corrected.
The recruitment series is the estimate of abundance of fish at the nominated ‘recruitment age’ (age 4 for toothfish) in each year. The first year in the time series of recruitments is determined as the Oldest Age observed in the surveys minus Recruitment Age. This was coded specifically for the case of surveys observing fish of ages one year younger than the recruitment age and greater. This calculation needed to be made general to allow for many years younger than the recruitment age. This has been corrected.
The calculation of the recruitment series relative to the first year remains unaltered and correct. A new version, GYM501E.EXE, and installation software has been made available on the AAD website and at the Secretariat.