Like a detective using DNA testing and fingerprint dusting to figure out “whodunnit,” MIT-WHOI joint program student Ciara Willis is using stable isotope analysis to track the flow of carbon through the marine food web. Working with Braun, Arostegui, and her advisor, biologist Simon Thorrold, Willis is examining the stomach contents of the fish caught on the longliner, as well as from smaller fish she collected from deep-sea nets on the R/V Bigelow during the same research cruise. While some of the stomachs contain identifiable fish, the samples actually reveal more at the molecular level: changes in the carbon isotopes, or molecular weight, indicate the protein source–zooplankton, squid, or other fish–and at what depth.
“You can use these unique patterns of carbon isotopes across all the essential amino acids to fingerprint different food sources, which helps us quantify how much of the swordfish and the tuna’s diet is coming from the twilight zone versus surface waters,” Willis said. “And then from there we can figure out what this means for management. How can we be respectful of existing tuna and swordfish fisheries while also theoretically allowing for responsible twilight zone fisheries?”
For Willis and Braun, being able to connect the dots between large predators, their prey, and behavior in the deep ocean is essential to inform policy decisions.
“We can say, ‘Oh, there’s probably about this much biomass down there,’ but if we want to extract that resource, what impacts might that have? The short answer is we have absolutely no clue,” said Braun. “And what if something commercially valuable like swordfish–that a lot of people’s livelihood depends on–are relying on that resource for food? What would the impacts be if we harvested fifty percent of it? That’s what we need to figure out.”
