Evaluating the Effect of Atlantic Bluefin Tuna Movement on the Perception of Stock Units

Atlantic bluefin tuna is currently managed as separate eastern and western stocks. However, tagging and otolith chemistry patterns indicate stock mixing. We developed a simulation model to explore the consequences of bluefin tuna population structure and movement on stock productivity and composition. The analytical framework is a stochastic, age-structured, overlap model that is seasonally and spatially-explicit. The operating model emulates the unique vital rates, recruitment dynamics, and movement (informed by tagging and otolith chemistry data) of eastern and western spawning populations. Alternate model settings were considered, including alternate estimates of movement rates, assumptions of maturity and recruitment regime. Simulations indicate considerable mixed stock composition of the mature tuna biomass and yield in the western and central Atlantic. The modeled spatial and temporal distribution and relative abundance of eastern and western populations in the Atlantic is sensitive to assumptions of age at maturity and population movement, because these features place fish in areas where they are exposed to different fishing mortalities. Simulation results such as these can be used to inform the appropriate configurations for spatially-explicit stock assessments and can form the foundation for evaluating alternative management scenarios in the context of fish movement.