Semester of Graduation



Master of Science (MS)


Oceanography & Coastal Sciences

Document Type



Highly migratory fishes (e.g., tunas, billfishes, and sharks) play a significant role in the structure and function of open-ocean ecosystems and a better understanding of the food web dynamics that support their populations is essential to improve management and conservation. Here, I use complementary approaches (stomach contents, DNA barcoding, & stable isotopes) to examine seasonal variability in the feeding ecology of sub-adult (70 – 100 cm; n = 371) and adult (100 – 160 cm; n = 206) yellowfin tuna (Thunnus albacares) from the northern Gulf of Mexico over a one-year period (April 2019 – March 2020). Stomach contents were represented by 114 unique prey taxa that varied by season and size class. Carangids, ommastrephid squids, exocoetids, and hyperiid amphipods were among the most abundant and frequently encountered prey. Major seasonal differences were characterized by ommastrephid squids and exocoetids in the spring, juvenile fishes (carangids & scombrids) in the summer, coastal fishes during the fall, and an increased consumption of planktonic prey (hyperiid amphipods & salps) in the winter. Furthermore, seasonal variability in bulk (white muscle) stable isotope values (ẟ13C, ẟ15N, & ẟ34S) of yellowfin tuna was also observed. Sub-adult δ13C values were highest during the spring and lowest in the fall, while adult δ13C values were relatively consistent. Seasonal trends in δ15N and δ34S were noted in both size classes and found to be inversely related, with low δ15N (high δ34S) values during late summer/early fall and high δ15N (low δ34S) values during late winter/early spring. Lastly, the relative contribution of various sources (coastal fishes, oceanic fishes & squids, and planktonic prey) to the diets of yellowfin tuna were seasonally distinct and differed between size classes. Oceanic prey sources were more influential to adult yellowfin tuna, while planktonic prey sources were more influential to sub-adults. These findings highlight the complex food web dynamics supporting an opportunistic predator in an oceanic ecosystem and represents a critical step to better understanding seasonal cycles of prey availability and resource utilization in marine ecosystems.

Committee Chair

Dance, Michael A.