Old shipwrecks are pretty cool, so it's no surprise why scuba divers like to visit and photograph the hundreds of sunken ships in the “Graveyard of the Atlantic” scattered along the coast of North Carolina. They also like to take snaps of the local wildlife, particularly the sand tiger sharks that hang out there, too. But amid the clutter of thousands of photographs captured over years and years comes a clearer picture about the sharks, and a new understanding of the species: the female sharks have favorite wrecks they return to year after year, a finding that could help in shark conservation.

According to the new study in the journal Ecology, in 2016 and 2017, a citizen-scientist and professional underwater photographer named Tanya Houppermans took two images of a female sand tiger shark swimming around the wreck of the ship Aeolus. She uploaded the photos, taken ten months apart, to an online sand tiger shark conservation project called Spot A Shark USA. This led researchers to look back into the image collection. Using each shark's unique pattern of brown spots for identification, they found images of six female sharks returning to the same wrecks between 1 and 72 months apart. This “site fidelity” suggests that the wrecks may be an important area for the docile predators, which can reach up to 10.5 feet in length.

“Our finding reveals that shipwrecks are potentially critical habitat for sand tiger sharks,” lead author Avery Paxton of Duke University tells George Dvorsky at Earther. “We do not know the exact reason that female sand tigers are returning to the same shipwrecks over time, but our multi-institutional team is conducting additional research to hopefully solve this puzzle.”

The finding could also help researchers produce better estimates on the population of the species, also known as the gray nurse shark. According to a press release, fishing pressure in the 1980s and 1990s reduced its numbers about 75 percent before the species gained legal protectection from practices like shark finning in U.S. waters. Decades later the shark, which reproduces slowly, is still listed as vulnerable on the international endangered species list.

Figuring out whether the species is recovering is difficult. In the paper, the researchers write that because some shark species roam over wide areas that often cross various jurisdictions, counting and protecting them is difficult. Some sharks, including great whites and tiger sharks, however, show signs of site fidelity, meaning they occasionally return to certain areas. That allows researchers to keep tabs on the returning sharks to get a a sense of how they are faring and gives them good candidates for habitat conservation areas.

Knowing that the sand tiger sharks return to specific ships means conservationists can try to get a handle on their numbers and designate critical habitat among the wrecks. Spanish galleons, Civil War steamers and merchant ships of all vintages are strung up and down North Carolina's 300 miles of coast, with over 1,000 in the waters of the Outer Banks alone. The geography of the area, including shifting sand, merging ocean currents and a lack of natural harbors all contribute to the creation of the "graveyard." The images are also helping researchers understand what specific wrecks, like the Aeolus, a Navy cable repair ship from the 1940s was sunk in 1988 to create an artificial reef, the sharks prefer.

“Having photographic evidence that these wrecks form an important habitat the sharks return to from time to time gives us a focal point for ongoing research so we can better understand how the species is faring,” Paxton says in the release.

Many questions still remain about the sharks. For instance, the photographs don’t show any male sharks returning, so it’s unknown whether they show site fidelity to the wrecks as well. And then there’s the question about what the sand tigers are actually doing hanging around the sunken ships. Hap Fatzinger, director of the North Carolina Aquarium, which runs the Find a Shark site and co-author of the study, tells Jonathan Carey at Atlas Obscura he thinks the ships provide many benefits for the sharks. “Further research will better determine how valuable these shipwrecks are,” he says. “We feel they are providing a critical habitat for this species to rest from long seasonal migrations, provide opportunities for overwintering, and also create an oasis for mating and reproduction.”

Hopefully, the team will learn more about the toothy ship-lovers soon. Fatzinger tells Dvorsky his aquarium is launching a public awareness campaign to get more divers to the wrecks to photograph the sharks and get them excited about protecting the species.