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It is challenging in fisheries stock assessment to estimate cohort dynamics from length‐based data for hard‐to‐age stocks, and existing approaches, for example, age‐structured catch‐at‐length models (ACL) are unable to account for length‐dependent processes within each cohort. Fisheries‐dependent data are usually considered the default input to stock assessment models. However, with widespread recognition of the uncertainty of fisheries‐dependent data and the increasing availability of high‐quality survey data, a new situation emerges in some fisheries where a stock assessment model based only on survey data can provide good estimation of population dynamics. We develop an age‐ and length‐structured statistical catch‐at‐length model (ALSCL) to estimate age‐based dynamics from survey catch‐at‐length data. This approach also provides a good basis to then integrate fisheries‐dependent data in the model. ALSCL can explicitly include length‐dependent mortality and growth within each cohort by simultaneously tracking the three‐dimensional dynamics across time, age, and length. We first use simulations of yellowtail flounder (Limanda ferruginea, Pleuronectidae) and bigeye tuna (Thunnus Obesus, Scombridae) to demonstrate that ALSCL outperforms ACL by providing more accurate estimates of age‐based population dynamics when length‐dependent processes are important. Next, we apply ALSCL to estimate the cohort dynamics of female yellowtail flounder on the Grand Bank off Newfoundland using survey catch‐at‐length, weight‐at‐length, and maturity‐at‐length data. We consider ALSCL as a hybrid between ACL and length‐structured stock assessment models that keeps the advantages of both, and its ability to simultaneously track age and length dynamics is an important step toward the next‐generation of fisheries stock assessment models. [ABSTRACT FROM AUTHOR]