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Reference Points for Eastern Georges Bank Atlantic Cod
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2014
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[PDF-1.23 MB]
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Description:A VPA model (VPA.8) that incorporates recent increasing natural mortality (M) with age is currently used to provide stock assessment advice for Eastern Georges Bank cod. This model sets M at 0.2 for all ages except for ages 6+ beginning in 1994 for which M is 0.8. In the past, a loess smoothed stock recruitment relationship (SR) in the Sissenwine-Shepherd production model yielded Fmsy =0.125, but F90%FMSY =0.11 was chosen as a fishing mortality (F) reference point due to uncertainty around the SRR and the high M. We use the VPA output from the VPA.8 model to estimate several F reference points by applying yield per recruit, spawner per recruit and production models in a Sissenwine-Shepherd approach using a number of SR fits, and use profile likelihoods to assess plausibility of Fmsy reference points. There was considerable uncertainty in the maximum likelihood point estimates for the SR and F reference points. A decision theoretic approach was used to estimate F reference points by maximising the expectation of catch by integrating across the likelihood surface of the SR parameters. Attempts to model the SR in ways that reflect apparent productivity changes did not improve the ability to estimate productivity, so the full time series of data is considered for defining F reference points. FmaxE(C), or the F that maximises the expectation of catch, which is thought to be less variable and to lessen the risk of overexploitation relative to Fmsy, was 0.097 (~0.1), and is proposed as a F reference point for the Eastern Georges Bank cod VPA.8 model.
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