From ocean to analyser: measuring mercury in sevengill and tope sharks

In our last blog post, we introduced our Save Our Seas Foundation project investigating chemical contamination in broadnose sevengill and tope sharks along the U.S. West Coast. This work brings together scientists from Oregon State University, UC Davis, and Clemson University to better understand how pollutants like mercury (Hg) and per- and polyfluoroalkyl substances (PFAS) accumulate in shark tissues, and what that means for their health and conservation.

Now that field collections are underway, we’re giving you a behind-the-scenes look at what happens after the samples are collected. Once Dr. Taylor Chapple, Jessica Schulte, and Ethan Personius (Oregon State University) and Dr. Meghan Holst (UC Davis) collect blood and tissue samples from sharks, they ship them to our lab at Clemson for contaminant analysis. That’s where graduate student Mel Walker takes the lead.

Mel uses a technique called thermal combustion atomic absorption spectrophotometry to directly measure total mercury concentrations in blood and tissue samples. This method is ideal for wildlife toxicology research because it requires minimal sample prep and delivers accurate, reliable results.

This process allows us to get quick and accurate readings of how much mercury is in a shark’s blood or tissue. That’s important because mercury is a toxic metal that can accumulate in sharks over time, with particularly harmful effects on sensitive tissues like the brain, liver, and reproductive organs. Measuring mercury levels helps us assess the potential health risks these animals face in the wild.

Mel in the lab at Clemson University analysing blood samples for mercury. Photo © Kylie Rock

Here’s how it works:

1. First, the frozen samples are thawed and a small amount (about the size of a grain of rice) is placed into tiny ceramic containers.
2. These are loaded into a machine that heats them to 800°C (that’s nearly 1,500°F!)—hot enough to turn the mercury into a gas.
3. The gas is trapped by a gold filter, while other combustion byproducts are flushed away
4. The gold then gets heated to release the mercury vapour.
5. Finally, the mercury passes through a beam of light, and we measure how much light it absorbs. The more light it blocks, the more mercury was in the original sample.

(Left) Analytical balance where we weigh the small pieces of tissue or blood, approximately 5-10 mg. (Top Middle) A tray of weighed blood samples to be loaded into the (Bottom Middle) NIC MA-3000 mercury analyser. (Right) Autosampler that holds 10 trays of 10 samples. Photos © Kylie Rock

We’re excited to share that we’ve now received all of the samples for this part of the project from Oregon State University, and Mel has completed the mercury analysis for each one. She recently presented these preliminary results at the 2025 Society of Toxicology and Carolinas Society of Environmental Toxicology and Chemistry annual meetings, where fellow scientists were eager to learn more about how chemical pollution is affecting marine predators like sharks. It was a great opportunity to connect with others working in environmental toxicology and to share the progress of this collaborative, coast-to-coast project.

(Left) Mel with her poster. (Right) Mel presenting her poster to fellow scientists at the 2025 Society of Toxicology conference in Orlando, FL. Photos © Kylie Rock

Meanwhile, Dr. Meghan Holst is still busy sampling in San Francisco Bay aboard her new research vessel. The project experienced a slight delay due to the vessel’s late availability, but we expect those samples to arrive at Clemson this fall. Once they do, Mel will begin analyzing them for both mercury and PFAS. To measure PFAS, she’ll use advanced techniques, including high performance liquid chromatography coupled with mass spectrometry (HPLC-MS), which allow us to detect even trace levels of these persistent chemicals in shark tissues.

(Top Left) Dr. Meghan Holst fishing aboard her new research vessel in San Francisco Bay. (Right) The research team restraining a shark while ensuring continuous flow of oxygenated water over its gills during sampling. (Bottom Left) Dr. Holst collecting a blood sample. Photos © Nicole Stauffer

This project is a true team effort, combining expert fieldwork from the West Coast with cutting-edge lab analysis on the East Coast. Together, we’re working to better understand how pollution affects sharks, and how we can protect these incredible animals for generations to come. You can learn more about this project and other ongoing shark research in the Repro-Eco Tox Lab by checking out a recent video we made—[Click Here]. Stay tuned for more updates as we dive deeper into the science!