Date of Award
Program or Major
Doctor of Philosophy
Maryellen M Lutcavage
Atlantic bluefin tuna (ABFT; Thunnus thynnus) are important top predators in pelagic ecosystems. Understanding their diet and movements is necessary for proper management, but existing methods are costly and logistically challenging. Naturally occurring stable isotopes have been used to study diet and movement in many aquatic ecosystems and offer a novel approach for studying these large pelagic fishes.
Lipids, which have lower carbon isotope values than protein, can affect isotope analyses of bulk tissue. Both chemical and mathematical corrections were used to address lipid effects on ABFT and the prey. All chemical extraction methods altered nitrogen isotope values. Mathematical corrections closely replicated chemically extracted carbon values while preserving nitrogen isotope and C:N values and were applied to all ABFT samples.
Combined stomach content and stable isotope analyses were performed on ABFT in coastal forage grounds, revealing a diet based on aggregations of small fish and crustaceans. The largest prey components of adult ABFT were Atlantic herring (Clupea harengus) and sand lance ( Ammodytes spp.) in the Gulf of Maine and Atlantic menhaden ( Brevoortia tyrannus) and swimming crabs (Portunus spp.) in the Mid-Atlantic Bight. Juvenile bluefin tuna fed at a lower trophic level than adult conspecifics due to higher dietary proportions of crustaceans and smaller fishes. Diet composition of ABFT in the western Atlantic was similar to historical observations while juveniles in the Bay of Biscay fed on higher proportions of crustacean prey relative to past observations.
Isotope gradients detected between shelf and open ocean forage grounds were used to infer movements of adult ABFT into Gulf of Mexico spawning grounds and to estimate residency in Gulf of Maine forage grounds in relation to results from tagging experiments. Residency and arrival times varied among individuals in the Gulf of Maine, with late season arrivals having reduced body condition similar to early season fish. Most fish in the Gulf of Mexico had previously fed on shelf forage grounds, demonstrating connectivity between productive coastal waters and this western Atlantic spawning area. Results provide important trophic and migratory information for use in ecosystem-based management strategies and validations for future isotope studies of large pelagic fishes.
Logan, John, "Tracking diet and movement of Atlantic bluefin tuna (Thunnus thynnus) using carbon and nitrogen stable isotopes" (2009). Doctoral Dissertations. 516.