Research Document - 2008/038

The retrospective issue with estimates of maturity for 2J3KL cod (Gadus morhua)

By M.J. Morgan

Abstract

Estimates of spawning stock biomass (SSB) for many populations are calculated by applying female maturity at age, estimated on a cohort basis, to total biomass. Over the life of a cohort, data are added for an additional age each year. This means that until the age at which the maturation process is complete, estimates are based on incomplete data for a cohort. This results in potential changes in estimates of proportion mature at age from one assessment to the next. During the 2007 assessment of northern cod, concern was raised that this might introduce a retrospective pattern in SSB. The recommendation to carry out further examination of the maturity data and model, and to carry out comparative analyses for the next assessment was addressed. Models of proportion mature at age within a cohort are refit using data from progressively fewer years and the resulting estimates compared. SSB calculated using these estimates of proportion mature at age produced with differing amounts of data and a constant matrix of population numbers at age were compared to examine the impact of the method on both retrospective and projected estimates of SSB. As expected there are some differences between estimates of proportion mature at age for Div. 2J3KL cod, depending on the age range over which data were available. The magnitude of these differences varied from cohort to cohort. The impact of these differences in estimates of proportion mature at age on estimates of SSB was relatively minor. The impact on projections was somewhat greater than the impact in retrospective analyses. In neither case were there differences in trend caused by the different maturity estimates (i.e. no cases where one set of maturity estimates resulted in an increase in SSB while another resulted in a decrease). The impact of the current method of estimating maturity at age for Div. 2J3KL cod appears to be minimal.

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