[All-postdocs] Bioseminar CLARK 507 - Alanna Mnich

[Ana Velez]

Woods Hole Oceanographic Institution
Biology Department Seminar

Thursday, December 11, 2025 - 12:00 pm
Clark 507

Alanna Mnich
Postdoctoral Researcher, University of Massachusetts, Dartmouth

Isotopic Signatures in Otoliths of Atlantic Bluefin Tuna Using Secondary Ion Mass Spectrometry
The Atlantic bluefin tuna (ABFT) is an ecologically and commercially important species, acting as an apex predator and supporting a valuable fishery. Current management defines ABFT as two separate stocks, constituted by an eastern stock that spawns in the Mediterranean Sea and a western stock that spawns in the Gulf of Mexico, that differ in their respective biomasses. A recent discovery of ABFT larvae in the Slope Sea off the northeastern coast of the United States has led to questions about spawning in this region and implications for management based on the current two-stock hypothesis. Using known trends in isotope fractionation, predicted isoscapes of the Atlantic Ocean, and modern developments in analytical tools, fundamentals in chemical oceanography can be employed to test hypotheses surrounding ABFT stock identification. For this work, a secondary ion mass spectrometer (SIMS) ion microprobe approach was used to measure δ18O signatures in the otoliths of ABFT larvae from the Slope Sea and Gulf of Mexico to establish western stock spawning site signatures. Regional baseline signatures were used in a case study of identification of adult ABFT to natal origin, made possible by fishing industry collaboration. Adult ABFT otoliths were analyzed by SIMS to generate δ18O signature transects from core to edge, providing a temperature and salinity-based map of their migratory history from spawning to capture. The results of this work provide evidence of ABFT spawning in the Slope Sea and the novel methods outlined can be a useful tool for the same applications in other fisheries.

Part of the talk (maybe 1/3 of it) will include information from a method developed that was just published over the weekend. We have attached that PDF here for those interested.

Please contact Clarissa Karthauser (ckarthauser at whoi.edu<mailto:ckarthauser at whoi.edu>) if you'd want to have lunch with Alanna after the seminar, or scheduling a separate meeting.

Biology Seminar Web Page<https://www.whoi.edu/what-we-do/understand/departments-centers-labs/bio/bio-highlights-events/>

For questions, contact:

Ana Maria Velez
Biology Pre-Award Administrative Associate
Ph: 508-289-2334
[signature_1005462319]

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