An age- and length-structured statistical catch-at-length model for hard-to-age fisheries stocks

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.

Automated summary

Estimating cohort dynamics from length-based data for hard-to-age stocks is challenging in fisheries stock assessment. The age-structured catch-at-length models (ACL) currently used are unable to account for length-dependent processes within each cohort. However, with the increasing availability of high-quality survey data, a new situation emerges where a stock assessment model based only on survey data can provide good estimation of population dynamics. We introduce an age- and length-structured statistical catch-at-length model (ALSCL) that can estimate age-based dynamics from survey catch-at-length data, while also providing a basis to integrate fisheries-dependent data. ALSCL includes length-dependent mortality and growth within each cohort by simultaneously tracking the three-dimensional dynamics across time, age, and length. Simulation results show that ALSCL outperforms ACL in estimating age-based population dynamics when length-dependent processes are important. ALSCL is considered a hybrid model between ACL and length-structured stock assessment models, and its ability to track age and length dynamics simultaneously is an important step towards the next-generation of fisheries stock assessment models.