Abstract
The Atlantic bluefin tuna is an ecologically and commercially important species, acting as an apex predator and supporting a valuable fishery. Current management of the western stock of this species recognizes the Slope Sea off the Northeast USA as a spawning site, albeit one of unknown persistence and importance, supporting the need for studies into the use of this region by spawning adults as well as larval behavior and success. Stable isotope analysis using tissues of Atlantic bluefin tuna larvae captured in the Slope Sea was performed to measure δ¹³C, δ¹⁵N, and the C:N content ratio. The Slope Sea larvae were found to have a significantly more enriched δ¹³C signature compared to their Mediterranean Sea and Gulf of Mexico counterparts based on prior literature. Their δ¹⁵N, an indicator of trophic position, overlapped with Atlantic bluefin tuna larvae of the Mediterranean Sea and eastern Gulf of Mexico, but was significantly more enriched than those in the western Gulf of Mexico. The C:N ratio of the Slope Sea larvae indicates a healthy body condition. A secondary ion mass spectrometer (SIMS) ion microprobe approach was used to measure δ¹⁸O signatures in the otoliths of Atlantic bluefin tuna larvae from the Slope Sea, Gulf of Mexico, and Mediterranean Sea. This is the first study that uses SIMS ion microprobe technology to analyze larval otoliths of any species. Using this method, it was determined that the variable conditions of the Slope Sea result in a wide range of δ¹⁸O otolith signatures that both overlap with the Gulf of Mexico larvae and also include more depleted δ¹⁸O signatures. Otoliths from five adult specimens captured in the Gulf of Maine were analyzed using the same SIMS ion microprobe method to generate a δ¹⁸O transect from core to edge. This provides a temperature and salinity-based map of their migratory history from spawning to capture. This study found that western adults frequent the Slope Sea region during the summer while aged three to five years. The results of this work provide justification for more extensive development of spawning site baselines and additional analyses of adult Atlantic bluefin tuna to improve understanding of stock structure. The methods outlined can be a useful tool for the same applications in other fisheries.