For years, scientists and policymakers have debated whether putting parts of the sea off-limits to fishing actually benefits such wide-roving predators as sharks. Now, thanks to some dead sardines in front of an underwater camera, they have proof.

A team of scientists from the United States and Belize picked four ocean locations to survey over five years, from 2005 to 2010. In each spot they put a waterproof video camera on the seafloor in front of a small bait cage — contraptions they nicknamed “chum cams.” Then they counted how many sharks showed up on film.

The results — reef sharks are more common in areas where fishing is restricted — were published online Thursday in the journal PLoS ONE. The findings are significant because policymakers in the United States and overseas are now debating whether to create more protected areas known as marine reserves. Although scientists have proved in the past that many sedentary marine species benefit from putting certain parts of the ocean off-limits to fishing, there is less documentation that this benefits large, roving predators such as sharks.

Using both acoustic monitoring and chum cams, the team showed that Caribbean reef sharks in Belize showed up more often in Glover’s Reef and Caye Caulker Marine Reserves than in two other areas where fishing is not restricted.

Twenty-nine percent of chum cam deployments videotaped at least one reef shark in those reserves, compared with 8 percent of deployments on fished sites. Caye Caulker is entirely off-limits to fishing, while Glover’s Reef has a ban on longline and gill net fishing but allows other kinds of fishing in certain areas.

Mark Bond, a doctoral candidate at Stony Brook University and the paper’s lead author, said a combination of two factors accounts for why they found more sharks inside the protected areas: “the fact that there’s more of their food in the reserves, and the fact that there’s no fishing.”

Edward Brooks, program manager for the shark research and conservation program at the Cape Eleuthera Institute, called it “a pretty novel study” for its non-invasive method. Normally, scientists track sharks by catching them on a line and inserting either a radio or satellite tag to monitor them.

Brooks, who reviewed the paper and has deployed chum cams in the Atlantic, noted that although many governments are exploring ways to reduce fishing pressure on sharks, “it doesn’t work if there isn’t any fish for the sharks to eat. Creating a marine reserve is a much more effective way to undertake conservation measures than to just stop fishing sharks.”

Demian Chapman, the research project’s leader and assistant director of science for the Institute for Ocean Conservation Science at Stony Brook, said he was inspired to deploy chum cams because he had seen how researchers studying big cats had used camera traps to document their elusive subjects.

“In Belize there’s a lot of research on jaguars,” Chapman said in an interview. “I saw a TV show about counting jaguars, and they were using these camera traps, and I thought this would be applied to sharks. I’d been searching for a way to survey these animals without catching them.”

Chapman said the video footage also made it easier for him and his colleagues to show citizens in Belize how marine reserves could have an impact. “I wanted to show the people of Belize what I was seeing: There are a lot of sharks in some places, and not so many sharks in some other areas.”

The chum cams inspired intense frustration among the sharks because they could not get at the dead fish tucked inside a small metal trap within a larger cage. Sharks took different approaches in their attempts to extract food, Bond recalled. “Caribbean reef sharks bang into the cage trying to get the fish, while some nurse sharks turn upside down to get a better suction” angle, he said, adding that they would eventually give up and swim away. “It’s quite amusing.”

Although using baited cameras introduced bias into the survey, because the scent of the baited cage lured sharks into the camera’s field of view, the researchers found that bias was identical for both the reserve and non-reserve areas. They also measured environmental data on each deployment such as the oxygen, salinity, visibility and current levels in the ocean and found these factors did not make a difference in how many sharks they counted.

The fact that the reef sharks showed fidelity to the reserve, the group found, shows that policymakers can use marine reserves as a way to protect these animals from pressures such as global demand for shark fins.

Caribbean reef sharks, which live in parts of the western Atlantic Ocean ranging from Bermuda to southern Brazil, are listed as “near threatened” with extinction by the International Union for the Conservation of Nature.