Whale sharks occasionally gather in large groups to feast on plankton and fish eggs. (iStockphoto)

Whale sharks, a threatened species that can grow as big as a bus, have become so wildly popular with tourists that scientists, environmentalists and even eco-­tourism operators are calling for new limits on human contact.

The massive polka-dotted fish roam the world’s warm oceans as solitary creatures. But they occasionally gather in large groups, or aggregations, to feast on everything from plankton to fish eggs. As the aggregation sites have become known, tourists have flocked to them, with tour operators from Mexico to the Maldives selling opportunities to swim “with the world’s biggest shark.” The slow-moving whale sharks are filter-feeders and pose no danger to humans. They are found in all of the world’s temperate seas, though scientists are unsure how many exist, where they breed or where they give birth.

“Suddenly everyone has this on their bucket list,” said Brent Stewart, a scientist at San Diego’s Hubbs-SeaWorld Research Institute, who has studied whale sharks in half a dozen countries. “People are willing to pay money for this kind of eco-tourism, and now you see these unintended consequences. We have seen a frenzy in all these areas.”

In the Philippines, some boat captains have begun hand-feeding the big sharks to keep them nearby for paying tourists. That has horrified scientists, who fear it will interrupt the sharks’ migratory behavior. Some have signed a petition demanding the practice be banned, and Philippines fisheries officials recently began looking into the hand-feeding. In Kenya, some tour operators have proposed corralling the big sharks in fenced-off lagoons and hauling in paying tourists to swim with the trapped fish.

Nowhere, perhaps, are the tourism-vs.-sharks problems more apparent that off Mexico’s Yucatan Peninsula, where one of the largest aggregations occurs. The site is near Cancun, one of the Caribbean’s top tourist sites. Millions of vacationers are within a 45-minute boat ride of the area of open ocean where, from May through September, the big sharks congregate by the hundreds near Isla Mujeres.

This gathering spot, or “afuera,” was little known until three years ago, when scientists and some tour operators discovered the site. Newspaper articles, television reports and YouTube videos broadcast the news. Tourists began arriving by the tens of thousands and a new eco-tourism industry was born.

There are now more than 200 permits for whale shark tour boats — most carry eight to 10 people — in the Cancun area, according to Mexican officials. Because the afuera (Spanish for “outside”) occurs outside national park boundaries, it is not always clear which government agency is responsible for the well-being of the animals. Complicating the problems are that dozens of boats without permits often join the fray.

The Mexican government has established rules about how many boats can approach a shark and how many swimmers can snorkel near the animal. But scientists say that when the sharks are few and the people are many, those rules often go out the window.

Many scientists and even some tour operators say the Yucatan has bme a sort of Wild West of eco-tourism, a good idea and good intentions run amok. “I feel a huge responsibility, because we really helped promote the place,” said Eli Martinez, who runs Texas-based Shark Diver magazine. “Now we have to ask, ‘Are we doing a good thing or a bad thing?’ ”

Paco Remolina, director of the Whale Shark Biosphere Reserve in Mexico, said the government is trying to determine how much human interaction the Yucatan whale sharks can handle. Initial, rough projections suggest about 160 boats would be the maximum, Remolina said.

Eco-tourism generally is viewed as a very good thing for sharks, which are under threat from commercial and sport fisheries worldwide. Some shark species have shrunk by 90 percent. Done right, eco-tourism can create cascading benefits for such creatures as whale sharks. It pumps money into local economies while protecting a species from being killed for food or sport.

But Australian filmmaker Stephen Van Mil of Animal Media, whose documentary “Big Fish” is set to premiere next year, said eco-tourism must be properly managed — as it is, he said, off Australia’s Ningaloo Reef. There, whale shark tour permits are carefully restricted.

Van Mil was shocked when he visited the Yucatan and saw “hundreds of boats in the water, severe competition for the same sharks. . . . Some of the eco-tourists this kind of operation attracts are not the genuine conservationists,” Van Mil said. “Some of these people are more like bungee jumpers in the ocean.”

Andy Murch, a professional shark photographer who leads shark-themed eco-tours, arrived at the afuera this summer and encountered hundreds of whale sharks, as he’d hoped. He also came face to face with the unexpected.

Murch estimated that half of the sharks he saw on his first day had been injured or were scarred from what appeared to be collisions with boats or boat propellers. He has posted photographs of some on the Web site www.elasmodiver.com.

“Fin cuts, back scars, parts of fins missing,” Murch e-mailed in late July.

Rafael de la Parra, a Mexican biologist who co-authored a scientific paper on the afuera and gave the event its name, estimated that fewer than 25 percent of the sharks show signs of injury. But, like Murch, he thinks there are too many tour permits, too many boats and too many people descending on the sharks in Mexico.

“We have to consider options to reduce the pressure on the whole ecosystem,” de la Parra said.“It is unsustainable.”

Tharpe is the editor of PolitiFact Georgia for the Atlanta Journal-Constitution. He has written extensively about whale shark research off the Yucatan Peninsula.