Date of Award

Summer 8-6-2025

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Marine Biology

First Committee Advisor

Lisa Kerr

Second Committee Member

Walt Golet

Third Committee Member

Michelle Staudinger

Additional Committee Members

Kohma Arai

Abstract

Atlantic bluefin tuna (Thunnus thynnus, BFT) are managed as two separate stocks (east and west) with a management boundary in the central Atlantic (45º west meridian). The Mediterranean Sea is the primary spawning ground of the eastern population, and the Gulf of Mexico is considered the main spawning ground for the western population. However, there is evidence of extensive stock mixing across the management boundary. The classification of individuals to their natal origin to better understand stock mixing is a management priority. Otolith stable isotope chemistry (δ13C and δ18O) has been used as an effective tool for BFT stock identification.

Alternative methods of natal origin assignment of BFT caught in the U.S. fishery in the Gulf of Maine were assessed. Specifically, the performance of multiple individual classifiers were compared to an ensemble model for natal area of origin classification using two baselines (i.e., adult-spawner and yearling baselines). The ensemble model consisted of both statistical and machine learning classifiers, with the results of the classifiers weighted by a Bayesian model averaging framework. Classification model performance depended on the baseline used, with models trained on the adult-spawner baseline outperforming models based on the yearling baseline. Classifiers performed similarly using the adult-spawner baseline while varying more when using the yearling baseline. The yearling-based ensemble model yielded improvements in performance compared to several of the individual classifiers.

The Gulf of Maine is a key foraging area and is the primary location of U.S. BFT landings. Understanding stock composition and mixing in this region is a priority for improving the assessment and management of BFT. BFT otoliths collected from over a decade of fishery landings in the Gulf of Maine were classified to their natal origin based on their stable isotope composition. This database consisted of > 3,500 samples with stable isotope data from BFT collected through collaboration with industry in partnership with UMaine’s Pelagic Fisheries Lab. The assignment results were used to characterize BFT stock composition and trends in the U.S. fishery in the Gulf of Maine. The Gulf of Maine is a key foraging area and is the primary location of U.S. BFT landings. Understanding stock composition and mixing in this region is a priority for improving the assessment and management of BFT. The majority of fish caught in the Gulf of Maine from 2010 to 2020 were of eastern origin. A greater proportion of younger and smaller fish were of eastern origin, while older and larger individuals were more likely to be of western origin. We also demonstrate how stock composition results can be used to modify key fishery-dependent indices of abundance that inform the stock assessment. This thesis explored the utility of several novel approaches to natal origin assignment, while characterizing thousands of samples from over a decade of BFT landings in the Gulf of Maine.

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