Improving Stock Assessments and Management Advice for Bluefin Tunas and Other Highly Migratory Species

For years bluefin tuna has been the poster child for overfishing and poor management. However, recent improvements in data collection, catch monitoring and international cooperation are providing an opportunity to reverse the perception of a fishery that is doomed to collapse. Stock assessments are conducted routinely to monitor the abundance and productivity of exploited fish stocks so managers can determine how many fish can be sustainably harvested each year. Should a stock be declared overfished or under-going overfishing, the science behind stock assessments also equip managers with the knowledge necessary to make decisions about what short-term and long-term management measures should be taken to help reverse these trends. in that light, the goal of my research has been to use newly available age data to improve the quality and reliability of assessments for Atlantic bluefin tuna by reducing uncertainty about the data and methods used to infer growth and age composition. A secondary goal has been to provide managers with the knowledge necessary to implement effective stock rebuilding programs for Pacific bluefin tuna. Chapter 2 is focused on cohort slicing, a method routinely used in the Atlantic bluefin tuna assessment to estimate catch-at-age from catch-at-size information. This chapter explores how errors in cohort sliced catch-at-age data can bias estimates of total mortality rate derived from catch curve analysis. Recommendations are provided concerning the appropriate mortality estimator and plus group to use depending on the parameters characterizing the stock. Chapter 3 provides updated growth estimates for western Atlantic bluefin tuna, which were adopted in 2017 as the basis for defining growth in the assessment. Chapter 4 provides an overview of the theory behind age-length keys with particular emphasis on the assumptions that govern each method and provides notes of caution concerning their applications to real data. Chapter 5 evaluates through simulation the relative performance of different methods for estimating age composition of western Atlantic bluefin tuna catches and applies the best performing technique, the combined forward-inverse age-length key, to actual western Atlantic bluefin tuna data. Chapter 6 moves over to the Pacific and focuses on evaluating the potential impacts of different minimum size regulations on the stock of Pacific bluefin tuna and explores ways in which to minimize short-term pain to the industry while still achieving long-term yield and conservation goals. Overall, this work has contributed major improvements to the stock assessment process of Atlantic bluefin tuna and implications of this work resonate beyond the bluefin tuna world to other highly migratory species faced with similar problems