Incorporating Stock Mixing in the Assessment and Long-Term Expectations of Atlantic Bluefin Tuna

Atlantic bluefin tuna is currently managed as separate eastern and western stocks. However, tagging and otolith chemistry patterns indicate extensive stock mixing. The first goal of this study is to use otolith derived stock composition information to revise data inputs (catch and catch rate time series) for eastern and western bluefin stocks and to incorporate this revised information into the current stock assessment framework for each stock.  The second goal is to develop a more complicated two-stock, spatially-explicit age-structured operating model for bluefin tuna that incorporates stock mixing for the purpose of long-term projections. The data, model estimates and 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, recruitment regime, and population movement, because these alternative processes expose fish to different fishing mortalities. This research allows us to evaluate how central assumptions of stock mixing and underlying productivity affect stock assessment and management advice on sustainable harvest for each bluefin tuna stock.