Seasonal variation in the feeding ecology of yellowfin tuna (Thunnus albacares) from the northern Gulf of Mexico

Author

Mitchell Shay Lovell, Louisiana State University and Agricultural and Mechanical CollegeFollow

Semester of Graduation

Summer

Degree

Master of Science (MS)

Department

Oceanography & Coastal Sciences

Document Type

Thesis

Abstract

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.

Recommended Citation

Lovell, Mitchell Shay, "Seasonal variation in the feeding ecology of yellowfin tuna (Thunnus albacares) from the northern Gulf of Mexico" (2021). LSU Master's Theses. 5398.
https://repository.lsu.edu/gradschool_theses/5398

Committee Chair

Dance, Michael A.

DOI

10.31390/gradschool_theses.5398