Evaluating the Impacts of Stock Structure Uncertainty on Sailfish Stock Assessment in the Atlantic Ocean

The ability to accurately delineate stock structure is a major pre-requisite of stock assessments. Despite its importance, there is usually some level of uncertainty associated with the stock structure assumption. The magnitude of this uncertainty and its effect on the stock assessment results has not been thoroughly investigated in many exploited species. In this study, simulation analyses were used to quantify the effects of stock structure uncertainty on species with limited available information, using the Atlantic sailfish (Istiophorus platypterus) as a case-study. An age-structured operating model was developed to simulate abundance, catch and effort data, assuming the three possible stock structure scenarios suggested in the 2009 ICCAT (International Commission for the Conservation of Atlantic Tunas) Atlantic sailfish assessment session: 1) single stock and 2) east vs. west stocks. Stock assessments of the simulated data were performed under alternative structure scenarios using the software ASPIC: A Stock-Production Model Incorporating Covariates (ver.5) and Auxiliary Programs. Changes in the reference points estimates (MSY, B/B_msy and F/F_msy) were evaluated. These estimates provide insight on how and to what extent stock structure uncertainties impact the estimates of management benchmarks for Atlantic sailfish.