Accounting for cohort-specific variable growth in fisheries stock assessments: A case study from south-eastern Australia

Modern statistical fishery stock assessment models rarely account for temporal variability in mean length-at-age, and almost never describe cohort-specific effects. This study employs techniques for discerning temporal change to mean length-at-age from fisheries data and introduces recently developed stock assessment methods to account for this variability. Using Stock Synthesis, a statistical catch-at-age modelling framework, a stock assessment to account for cohort-specific variability in mean length-at-age was developed for blue grenadier, Macruronus novaezelandiae, an important commercial species in Australia's Southern and Eastern Scalefish and Shark Fishery. Key outputs of a standard stock assessment model that assumes static growth were compared to those of an alternative model that accounts for observed variability in length-at-age. Comparisons show that accounting for variable growth provides a better fit to time-series data and results in significant differences to key population estimates. These differences have implications for the estimation of quantities important to management and thus to scientific advice regarding recommended catch levels.

► We identify changes to mean length-at-age for an important commercially harvested fish species.
► We use Stock Synthesis to examine alternative assessment model structures regarding growth.
► An assessment model with cohort-specific growth better fits available time-series data.
► Key model outputs and advice to management can depend on model assumptions regarding growth.