Retrospective investigation of assessment uncertainty for fish stocks off southeast Australia

There is a need to provide quantitative measures of uncertainty to support fisheries management decisionmaking. A retrospective analysis of historical assessments for fish stocks off southeast Australia is conducted toquantify the extent of uncertainty associated with estimates of spawning stock biomass in absolute terms andwhen expressed relative to spawning stock biomass over a sequence of reference years. This approach toquantifying uncertainty captures more sources of uncertainty than alternative approaches, such as the estimateof the variance of terminal year spawning stock biomass from asymptotic methods, the extent to which estimatesof spawning stock biomass vary among the sensitivity tests that form part of most assessments, and conventionalretrospective analyses. By all measures, estimates of spawning stock biomass in absolute terms are much lesscertain than estimates of relative stock size (i.e. spawning stock biomass relative to a reference level), althoughapplication of most current harvest control rules rely on estimates of biomass in absolute terms. Overall, uncertaintyin estimates of spawning biomass in absolute terms can be represented as a log-scale standard error of0.37, while this standard error is 0.18 for estimates of spawning biomass in relative terms. There is considerablevariation in among-assessment uncertainty in stock assessment outputs across species groups, with, for example,higher variation for assessments of chondrichthyans compared to other species