Progress report on development of a spatially explicit operating model for tropical tuna populations

We present planning for and progress towards the development of a spatially explicit population model of yellowfin tuna in the Indian Ocean. SPM (Spatial Population Model) is software that captures the dynamics of spatial heterogeneity of a population along with age structure, movement and reproductive stage transition in a holistic framework. Using this software, spatially explicit age-structured models will be developed for the yellowfin tuna (Thunnus albacares) population in the Indian Ocean and used as operating models to evaluate the performance of stock assessments. The models will be generalised Bayesian population models, optimised by fitting to fishery observations. Movement is parameterised using preference functions based on spatially discrete environmental layers. The shapes of the preference functions will be established through iterative model testing with the parameters defining the preference functions estimated within each model. The spatial structure of the models divides the Indian Ocean region into five-degree cells. The underlying spatial distribution of the population was either restricted to the western tropical area, or to the entire Indian Ocean. Estimates of movement rates will be compared with the results of tagging studies, and fits to the other observations explored (size, CPUE, reproductive development). These operating models will then be used to investigate potential biases of the current Stock Synthesis assessment.