The survey methods may be the same, but the science has evolved — and so have the stakes.
Since 1972, researchers from the UNC Institute of Marine Sciences have used baited longlines to capture and release sharks to gather data critical to understanding the ancient predators. The baseline established in 1972 relied on visual observation of the sharks’ size and species. While size and species type are still important, today’s Carolina scientists are studying isotopes from tissue samples and the molecular makeup of cells for clues about breeding, eating habits and other behaviors and even wider ecosystem trends and warning signs.
Using the latest research methods, from a small sample of a shark’s tissue, a scientist can learn about the health and abundance of the shark’s principal prey and, in turn, which species are declining or emerging in the ecosystem. Those variances often indicate habitat loss, over-fishing or even pollution affecting a key species in the food chain. By leveraging the largest and longest-running collection of data on shark populations in the U.S. with new research techniques, future researchers can use sharks as a litmus test for ocean health and climate change.
Fishing for data
Professor Frank Schwartz officially started the shark survey in 1972, although he unofficially began collecting data on sharks off the coast of North Carolina in 1965. What’s most remarkable about the program, however, is how consistent it has remained over nearly a half-century. The researchers gather survey samples from the same location using the same gear — including the same type of boat – meaning that ostensibly the data is comparable across time. The composition of shark species in each survey is the only changing variable — sharpnose and blacknose are now caught more frequently than other species, in contrast to the overall diversity of past data.
The survey takes place on a 48-foot research vessel, the Capricorn, at two locations off the coast of Shackleford Banks in Carteret County, North Carolina. Once every two weeks in the spring, summer and fall, the Capricorn’s captain navigates to fixed stations, between one and seven miles out to sea, while a crew of faculty, staff and graduate students deploy a trawling net out behind the boat to catch small bait fish. The crew members then bait up to 100 hooks on short lines, called gangions, and attach those at intervals to the mainline, which they trail behind the boat for almost a mile. After exactly one hour, they reel in the mainline and, one by one, researchers identify, measure and record information about the day’s catch.
The goal is to gather all the necessary data quickly to avoid harming the sharks, so all faculty, staff and graduate students aboard work simultaneously. They identify each shark by species and then record the length from the end of the snout to the tip of the tail. Depending on the researchers aboard that day, different biological samples are also taken — a small piece of tissue, a blood draw or a swab of the shark’s digestive track. Finally, they tag the shark, in case it is ever caught again by another fisherman or group, and release it back into the water. The entire process takes less than two minutes per shark. Forty-eight years of practice makes perfect.
Old survey, new research
Ph.D. candidate Jeff Plumlee in the environment, ecology and energy program in the College of Arts & Sciences studies how sharks fit into the food web by analyzing what molecules make up their cellular structure, using the concept “you are what you eat.”
“I collect a small tissue sample from the sharks and look at the stable isotopes that make up those tissues, and calculate the proportion of naturally occurring carbon, nitrogen or sulfur isotopes that make up the shark,” Plumlee said. “I can then identify what food sources most closely resemble the signatures we see within the sharks’ tissues, which gives us an idea of what the sharks’ primary food sources are and helps us understand what other species are dietarily important within the food web.”
Plumlee isn’t alone in his research; graduate student Savannah Ryburn, also of the E3P program, is trying to answer the same research question but through a different technique.
“I swab each shark’s intestinal tract for fecal matter and use DNA analysis to break down the different living organisms that make up its diet,” said Ryburn. She and Plumlee can later compare the results of their respective studies and check if they jibe.
Ryburn’s research is well-suited to the shark survey, but she originally planned on conducting her study almost 3,000 miles away at the Galapagos Science Center. The travel required for her research this summer became impossible during the COVID-19 pandemic, and when she and her advisor, Professor John Bruno of the Department of Biology in the College of Arts & Sciences, searched for a local alternative, they found the perfect fit for her at the IMS.
Both Plumlee and Ryburn’s research provides new ways for ecologists to map out how sharks fulfill different roles in a variety of ecosystems. Sharks are highly migratory and travel long distances, even traversing oceans depending on the season, and their diets can vary widely depending on location and time. When sharks co-occur, there is a partitioning of resources among different species of sharks that is based on their dietary preferences and life stage. However, how and when sharks change their diets is still not fully understood by scientists. The new methods used by the researchers at IMS to track sharks’ diets and molecular makeup will help researchers understand diet specialization among, and potentially even within, shark species.
The importance of tracking different sharks’ diets is also based on the species’ role as “ecosystem engineers” that regulate the diversity and distribution of prey. That regulation is essential to maintaining the precarious balance in estuaries, shallow bodies of water where fresh and saltwater meet, which serve as essential habitat for animals that make up a large portion of the coastal food web. Sharks, small fish and even shellfish all live in estuaries for at least part of their life cycles, and many live in the brackish water for their entire life cycle.