Deconstructing biomass to create an index of future overexploitation risk

Lewis Barnett1
October 24th, 2017 9:00 (PST): FSH 203

Deconstructing biomass to create an index of future overexploitation risk

Fishery management reference points focus on current population size of reproducing individuals (spawning stock biomass, SSB) and the fraction of unfished egg production that would remain after fishing at a constant rate for many years (spawning potential ratio, SPR). This perspective does not consider the effect of age- and size-structure on anything except numbers of eggs (e.g., SSB) and ignores near-term variation in SSB and recruitment (e.g., SPR) that can arise from transient fluctuations in population structure, which can contribute to the short-term risk of overexploitation. We build on recent research exploring how the age structure of fished stocks changes over time, using stock assessment model output and simple simulation of age-structured models to test the utility of candidate metrics for monitoring storage effects from age structure that buffer the influence of environmental variation on recruitment and SSB. We show how two populations with identical vital rates, exploitation rates, SSB and SPR can have striking divergence in short-term stock status because of different initial age structures. Preliminary results indicate that several common descriptors of change in age structure are influenced more by recruitment variation than the age diversity within the SSB, yet we identify a set of more robust metrics for future testing in potential application as the basis for a novel limit reference point or an additional index of assessment uncertainty.

Posted in Fisheries Think Tank.

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