Research Document - 2012/173
Virtual population assessment (VPA) estimates of stock size and maximum sustainable yield (MSY) reference points for 3Ps cod
By P.A. Shelton and M.J. Morgan
3Ps cod has a long catch history that is not currently included in the stock assessment used for providing scientific advice to fisheries managers. Reference points based only on recent (depleted) stock history may not reliably reflect real limits of stock productivity or targets for maximizing yield. To include a longer period in the calculation of reference points an ADAPT VPA was fit to available catch and survey data going back in time as far as possible. Catch numbers at age from 1959 to 2009 were included. The survey indices were the offshore survey from 1983 to 1996 and the inshore-offshore index from 1997 to 2009. A segmented regression recruitment model was fit to the VPA estimates of SSB and R. The fitted SR model was used to estimate Fmsy and Bmsy based reference points. Long term average weights and maturities were used in this calculation while the partial recruitment vector used was the average over the period since the moratorium. Bmsy was estimated to be 91 kt of SSB and Fmsy to be 0.38. Recent SSB is estimated to have been near or below 40% Bmsy while F has been near or above Fmsy. Uncertainty about the reliability of commercial catch data has precluded applying VPA in recent assessments. However, the VPA and SR models could be used in simulation testing of feedback harvest control rules (HCRs) through a management strategy evaluation (MSE) process. Assumptions in the current VPA and stock recruit model could be varied to create the reference set of operating models. MSE does not require a single “best” assessment model.
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