Andre Punt, University of Washington

Natural mortality (M) is a key parameter in age- and size-structured methods of fish stock assessment because estimates of biomass in absolute terms and relative to reference points are sensitive to its value. M can be pre-specified based on “indirect” methods, estimated with a prior, and estimated without a prior. However, there is an absence of best practice guidelines for how to treat M within stock assessments. Five alternative broad categories of methods for treating M in stock assessments (unconstrained estimation, estimation with a prior, the “lowest plausible” and “highest plausible” values based on indirect methods, and the results of the Hamel-Amax indirect method) are compared for ten stocks in Australia’s Southern and Eastern Scalefish and Shark Fishery using likelihood profiles, retrospective analyses and hindcast skill. There is no method that performs best in all cases. The results support a best practice where estimation with a prior should be the default unless diagnostics suggest that the population dynamics or the observation model is clearly mis-specified (e.g., an estimate of M that differs markedly from the mean of a prior based on longevity information). It is also best practice to conduct sensitivity analyses and use decision tables to highlight the effects of incorrectly assumed values of M when mis-specification is suspected and M is pre-specified using a longevity-based method.